Nanosecond fiber laser of high-order chaotic pulse and generation method

Abstract

The invention relates to a nanosecond fiber laser and method for high-order chaotic pulses. A fiber ring cavity of the laser comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, an analyzer, a second polarization controller, an output coupler, a dispersion control unit, a fiber isolator and a band-pass filter which are connected in sequence. 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 optical fiber and is stimulated to radiate a long-wave-band gain pulse; under the combined action of the first polarization controller, the polarization analyzer and the second polarization controller, an equivalent saturable absorption effect is generated by utilizing nonlinear polarization rotation generated when gain pulses are propagated in an optical fiber, and large anomalous dispersion is introduced to realize generation of nanosecond pulses; through the band-pass filter, nanosecond pulses are allowed to be generated, and meanwhile, the nonlinear increasing speed of propagation and accumulation of the nanosecond pulses in the cavity is controlled, so that order-by-order generation of high-order chaotic pulses is realized. According to the invention, reliable output of high-order chaotic pulses 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 nanosecond fiber laser of high-order chaotic pulses and a generation method thereof. Background Art

[0002] Chaos refers to a nonlinear, irregular, and unpredictable state that a system enters during its dynamic evolution. Its characteristics include: sensitive dependence on initial conditions, meaning that even small perturbations can lead to significant differences in the system's final output; aperiodicity, meaning that the output appears chaotic but is not completely random, following deterministic nonlinear dynamics; and fractal structure, meaning that the attractors of chaotic systems often exhibit fractal geometric properties, such as the Lorentz attractor or other strange attractors. Chaotic states are not only important in applications such as optical communication encryption but also provide a research platform for nonlinear dynamics and complex systems theory.

[0003] A chaotic fiber laser is a fiber laser system operating in a chaotic state, whose output exhibits unpredictable, non-periodic, or turbulent behavior, in stark contrast to stable continuous wave or mode-locked operating modes. Fiber lasers use optical fibers doped with rare earth elements (such as erbium and ytterbium) as gain media and have advantages such as high efficiency, excellent beam quality, and design flexibility. When the laser operates under conditions such as high pump power, nonlinear interactions, feedback, or saturable absorption, the laser output may enter a chaotic state, manifesting as irregular temporal dynamics such as intensity spikes, broad spectral characteristics, and high sensitivity to initial conditions. Chaotic fiber lasers have important application value in secure communications, chaotic synchronization experiments, and optical nonlinear dynamics research.

[0004] When saturable absorption exists in a fiber laser, mode-locked pulse output is easily achieved. As the pump power increases, due to the increase in intracavity nonlinearity and the presence of periodic confinement, the mode-locked pulse may undergo a periodic bifurcation-to-chaotic evolution, generating chaotic pulses. In the time domain, chaotic pulses appear as non-periodic signals with irregular fluctuations in both peak power and pulse width. In the frequency domain, their spectrum exhibits a wide continuous spectrum characteristic. Chaotic pulses exhibit unique potential in applications. For example, in secure communications, their extreme sensitivity to initial conditions can be exploited to achieve information encryption, or in random number generation, high-quality random sequences can be obtained by sampling their fluctuations. Chaotic pulses are not only an important vehicle for studying nonlinear optics and complex systems, but also provide new possibilities for the development of novel laser technologies. Existing research on chaotic pulses has only addressed primary / fundamental chaotic pulses. For example, the chaotic pulses output by an ytterbium-doped mode-locked fiber laser begin in a stable mode-locked state and, as the pump power increases, exhibit period-doubling bifurcations (periods two and four), chaos, and finally period three. Researchers have not discussed the effects of increasing the pump power. The chaotic pulses output by an erbium-doped mode-locked fiber laser also begin in a stable mode-locked state and, as the pump power increases, exhibit period two, period four, and chaos. Researchers have also not discussed the effects of increasing the pump power. Chaotic pulse output based on the evolution of breathing solitons similarly stops at the initial appearance of a chaotic pulse with increasing pump power. Chaos theory has predicted the existence of higher-order chaotic regions, which have higher levels of chaos or wider ranges of variation. Through specially designed fiber lasers, it is possible to obtain higher-order chaotic pulses. Generating high-order chaotic pulses in a fiber laser will help enhance our understanding of nonlinear dynamics in fiber lasers. Furthermore, since the degree of chaos directly corresponds to the randomness of random numbers, it also has great application and research value. Therefore, a nanosecond fiber laser and method for generating high-order chaotic pulses are urgently needed. Summary of the Invention

[0005] In response to the problems existing in the above-mentioned prior art, the present invention provides a nanosecond fiber laser and method for high-order chaotic pulses. The laser has a simple structure and low manufacturing cost, and can achieve reliable output of high-order chaotic pulses. The method has a simple implementation process and low implementation cost, and can enable the fiber laser to operate on a path from periodic bifurcation to chaos, and then achieve high-order chaotic pulse output by increasing pumping.

[0006] To achieve the above-mentioned object, the present invention provides a nanosecond fiber laser with high-order chaotic pulses, comprising a pump source and a fiber ring cavity; the fiber ring cavity comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, an analyzer, a second polarization controller, an output coupler, a dispersion control unit, a fiber isolator, and a bandpass filter, which are sequentially arranged along the ring direction;

[0007] The pump port of the wavelength division multiplexer is connected to the output port of the pump source through an optical fiber; one end of the erbium-doped optical fiber is connected to the common port of the wavelength division multiplexer; the input end of the first polarization controller is connected to the other end of the erbium-doped optical fiber; the input end of the polarization analyzer is connected to the output end of the first polarization controller; the input end of the second polarization controller is connected to the output end of the polarization analyzer; the input port of the output coupler is connected to the output end of the second polarization controller, and its energy output port is used to output high-order chaotic pulse laser; the input end of the dispersion control unit is connected to the signal output port of the output coupler through a fiber collimator; the input end of the fiber isolator is connected to the output end of the dispersion control unit through another fiber collimator; the input end of the bandpass filter is connected to the output end of the fiber isolator, and the output end thereof is connected to the signal port of the wavelength division multiplexer;

[0008] The erbium-doped fiber is a single-mode fiber with anomalous dispersion in the 1550nm band; the pigtails of the wavelength division multiplexer, the first polarization controller, the analyzer, the second polarization controller, the output coupler, the dispersion control unit, the fiber isolator, and the bandpass filter are all single-mode fibers with anomalous dispersion in the 1550nm band; the dispersion control unit is composed of a grating pair, a prism pair, a chirped fiber Bragg grating, a spatial light modulator, or a grating pair with a telescope system, and is used to provide anomalous large dispersion. The anomalous dispersion value provided by the anomalous dispersion control unit makes the overall dispersion value of the fiber laser less than -10 5 ps 2 The bandpass filter has an operating center wavelength of 1550nm and a 3dB bandwidth greater than 0.04nm and less than 0.24nm. While allowing the generation of nanosecond pulses, it suppresses the generation of background continuous waves and controls the increasing speed of the nonlinear accumulation of nanosecond pulses propagating in the fiber ring cavity, thereby realizing the step-by-step generation of high-order chaotic pulses. The first polarization controller, the analyzer, and the second polarization controller work together to rotate the nonlinear polarization so that the fiber laser operates in the equivalent saturable absorption region, and the fiber laser operates on the path from periodic bifurcation to chaos, thereby realizing the output of high-order chaotic pulses by increasing the pump.

[0009] As a preference, the output coupler is a fiber coupler with an output energy ratio of 20:80.

[0010] As a preference, the total length of the optical fiber portion in the optical fiber laser is less than 10 m.

[0011] As a preference, the pump source is a single-mode fiber-coupled semiconductor laser with a central wavelength of 976 nm or 1480 nm, and its output pigtail is a single-mode fiber with anomalous dispersion in the 1550 nm band, with an output power greater than 400 mW.

[0012] As a preference, the operating wavelength of the wavelength division multiplexer is 980 / 1550 nm or 1480 / 1550 nm, and its output pigtail is a single-mode optical fiber with anomalous dispersion in the 1550 nm band.

[0013] 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 4 meters, or the product of the absorption coefficient and the length of the erbium-doped optical fiber is greater than 40 dB.

[0014] Preferably, the first polarization controller is a three-coil rotating polarization controller or a squeeze-type polarization controller; the analyzer is a fiber analyzer; and the second polarization controller is a three-coil rotating polarization controller or a squeeze-type polarization controller.

[0015] As a preference, the optical fiber isolator is an isolator with a central wavelength of 1550 nm.

[0016] As a preference, the dispersion control unit is composed of a grating pair, a prism pair, a chirped fiber Bragg grating, a spatial light modulator, or a grating pair with a telescope system.

[0017] In the present invention, the pump light emitted by the pump source can be coupled into the resonant cavity by setting a wavelength division multiplexer. By setting an erbium-doped 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, 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 passing through the analyzer can be limited. By setting a second polarization controller on the output side of the analyzer, 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 set in sequence can work together to utilize the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber to produce an equivalent saturable absorption effect, thereby achieving pulse mode locking. The anomalous dispersion value provided by the dispersion control unit makes the overall dispersion value of the fiber laser less than -10 5 ps 2Because the fiber laser operates 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. Combined with the increase in pump power, the nonlinearity accumulated by the nanosecond pulses propagating in the fiber ring cavity gradually increases. After exceeding a certain threshold, the evolution from periodic bifurcation to chaos occurs, and then high-order chaotic pulse output is achieved by increasing the pump. The fiber laser of the present invention utilizes erbium-doped fiber with anomalous dispersion and passive fiber with anomalous dispersion. Operating the fiber laser in the anomalous dispersion region and introducing a 0.08nm bandpass filter are key to achieving high-order chaotic pulse output. By introducing greater anomalous dispersion, the pulse width reaches nanoseconds while maintaining a shorter cavity length. Compared to picosecond or femtosecond pulses, the rate of nonlinear enhancement accumulated by nanosecond pulses propagating within the cavity can be slowed down with increasing pump power. The introduction of a narrowband bandpass filter effectively suppresses the generation of background continuous waves, facilitating the generation of nanosecond pulses while also preventing their influence on chaotic pulse generation and limiting the rate of nonlinear enhancement accumulated by nanosecond pulses propagating within the cavity. Increasing the pump power allows for the generation of higher-order chaotic pulses in a step-by-step manner. The provision of a fiber isolator restricts the laser to unidirectional operation.

[0018] The laser has a simple structure and low manufacturing cost, and can realize the reliable output of high-order chaotic pulses.

[0019] The present invention also provides a method for generating high-order chaotic pulses, using a nanosecond fiber laser for high-order chaotic pulses, comprising the following steps:

[0020] 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;

[0021] Step 2: Use erbium-doped fiber to absorb continuous pump light and stimulate radiation to emit long-wavelength gain pulses. The generated gain pulses oscillate in the fiber laser cavity.

[0022] Step 3: Adjust the first polarization controller and the second polarization controller to make the fiber laser work on the path from periodic bifurcation to chaos. At the same time, through the joint action of the first polarization controller, the analyzer and the second polarization controller, the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is utilized to produce an equivalent saturable absorption effect, thereby achieving pulse mode locking; at the same time, the gain pulse is output to the dispersion control unit through the analyzer. The anomalous dispersion value provided by the dispersion control unit makes the overall dispersion value of the fiber laser less than -10 5 ps 2Because the fiber laser operates 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 meet the near-conversion limit. By introducing a large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining a short cavity length of the fiber laser.

[0023] Step 4: Increase the pump power of the pump source. With the increase of pump power, the nonlinearity accumulated by the nanosecond pulse propagation in the fiber ring cavity gradually increases. After exceeding a certain threshold, the evolution from periodic bifurcation to chaos occurs, and then high-order chaotic pulse output is achieved by increasing the pump. At the same time, the narrowband channel of the bandpass filter is used to suppress the generation of background continuous waves. While allowing the generation of nanosecond pulses, the increasing speed of the nanosecond pulse propagation accumulation in the fiber ring cavity is controlled to achieve the step-by-step generation of high-order chaotic pulses. Finally, the high-order chaotic pulses are output through the low-energy output port of the output coupler.

[0024] The present invention provides a method for generating high-order chaotic pulse output. Based on a pulse shaping technique determined by the balance between anomalous dispersion and nonlinear effects that satisfy the nonlinear Schrödinger equation, the method utilizes nonlinear polarization rotation mode locking to enable the laser to operate in the equivalent saturable absorption region. By operating the laser in a large anomalous dispersion region, nanosecond pulses are output that meet the near-transformation limit. As the pump power increases, the nonlinearity accumulated by the nanosecond pulses in the cavity gradually increases. After exceeding a certain threshold, a periodic bifurcation-to-chaotic evolution path emerges, generating chaotic pulses. By introducing large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining a short cavity length in the fiber laser. Compared to picosecond or femtosecond pulses, the rate of nonlinear enhancement accumulated by the nanosecond pulses in the cavity with increasing pump power can be relatively slow. Subsequently, a narrowband filter is introduced to suppress the generation of background continuous waves, which not only facilitates the generation of nanosecond pulses, but also avoids the influence of background continuous waves on the generation of chaotic pulses, while also limiting the rate of nonlinear enhancement accumulated by the nanosecond pulses in the cavity. In fiber lasers, the periodic bifurcation of nanosecond pulses into chaotic evolution is realized, and then the step-by-step generation of high-order chaotic pulses is achieved by increasing the pump.

[0025] The method has a simple implementation process and low implementation cost. It can enable the fiber laser to operate on a path from periodic bifurcation to chaos, and then achieve high-order chaotic pulse output by increasing pumping. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of the structure of the fiber laser in the present invention;

[0027] Figure 2 A time domain diagram of a near-transformation-limited nanosecond pulse output by a numerically simulated laser according to an embodiment of the present invention;

[0028] Figure 3 A near-transformation-limited nanosecond pulse spectrum diagram of a numerically simulated laser output according to an embodiment of the present invention;

[0029] Figure 4 A bifurcation diagram of the output pulse peak power obtained by increasing the small signal gain coefficient of the numerical simulation laser output according to an embodiment of the present invention, in which chaotic regions of various orders are marked;

[0030] Figure 5 A timing diagram of a fundamental chaotic pulse output by a numerically simulated laser according to an embodiment of the present invention;

[0031] Figure 6 A timing diagram of a second-order chaotic pulse output by a numerically simulated laser according to an embodiment of the present invention;

[0032] Figure 7 A timing diagram of a third-order chaotic pulse output by a numerically simulated laser according to an embodiment of the present invention;

[0033] Figure 8 A timing diagram of a fourth-order chaotic pulse output by a numerically simulated laser according to an embodiment of the present invention;

[0034] 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. Second polarization controller, 7. Output coupler, 8. Dispersion control unit, 9. Fiber isolator, 10. Bandpass filter, 11. Fiber ring cavity. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to the accompanying drawings.

[0036] like Figures 1 to 8 As shown, the present invention provides a nanosecond fiber laser with high-order chaotic pulses, comprising a pump source 1 and a fiber ring cavity 11; the fiber ring cavity 11 comprises a wavelength division multiplexer 2, an erbium-doped fiber 3, a first polarization controller 4, an analyzer 5, a second polarization controller 6, an output coupler 7, a dispersion control unit 8, a fiber isolator 9, and a bandpass filter 10, which are sequentially arranged along the ring direction;

[0037] The pump port 2a of the wavelength division multiplexer 2 is connected to the output port of the pump source 1 through an optical fiber; one end of the erbium-doped optical fiber 3 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; the input end of the second polarization controller 6 is connected to the output end of the polarization analyzer 5; the input port of the output coupler 7 is connected to the output end of the second polarization controller 6, and its energy output port is used to output high-order chaotic pulse laser; the input end of the dispersion control unit 8 is connected to the signal output port of the output coupler 7 through a fiber collimator; the input end of the fiber isolator 9 is connected to the output end of the dispersion control unit 8 through another fiber collimator; the input end of the bandpass filter 10 is connected to the output end of the fiber isolator 9, and the output end thereof is connected to the signal port 2b of the wavelength division multiplexer 2;

[0038] The erbium-doped fiber 3 is a single-mode fiber with anomalous dispersion in the 1550nm band; the pigtails of the wavelength division multiplexer 2, the first polarization controller 4, the analyzer 5, the second polarization controller 6, the output coupler 7, the dispersion control unit 8, the fiber isolator 9, and the bandpass filter 10 are all single-mode fibers with anomalous dispersion in the 1550nm band; the dispersion control unit 8 is composed of a grating pair, a prism pair, a chirped fiber Bragg grating, a spatial light modulator, or a grating pair with a telescope system, and is used to provide anomalous large dispersion. 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 ; The operating center wavelength of the bandpass filter 10 is 1550nm, and the 3dB bandwidth is greater than 0.04nm and less than 0.24nm. While allowing the generation of nanosecond pulses, it suppresses the generation of background continuous waves and controls the nonlinear increase rate of the accumulated nanosecond pulses propagating in the fiber ring cavity 11, thereby realizing the step-by-step generation of high-order chaotic pulses; preferably, the pigtail type of the bandpass filter 10 is Corning MetroCor; the first polarization controller 4, the analyzer 5 and the second polarization controller 6 work together to make the fiber laser operate in the equivalent saturable absorption region through nonlinear polarization rotation, so that the fiber laser operates on the path from periodic bifurcation to chaos, thereby realizing the output of high-order chaotic pulses by increasing the pump.

[0039] As a preferred embodiment, the output coupler 7 can be a fiber coupler with an output energy ratio of 20:80, and its pigtail type is Corning MetroCor. The output coupler 7 is used to output the high-order chaotic pulses generated in the cavity, and the output energy ratio of the output coupler 7 can be less than or equal to 20%.

[0040] Preferably, the total length of the optical fiber portion in the optical fiber laser is less than 10 m. Preferably, the total length of the optical fiber portion is 7 m.

[0041] Preferably, the pump source 1 is a single-mode fiber-coupled semiconductor laser with a central wavelength of 976 nm or 1480 nm. The output fiber is a single-mode fiber with anomalous dispersion in the 1550 nm band, and the output power is greater than 400 mW. Preferably, the output fiber of the pump source 1 is a Corning MetroCor.

[0042] As a preference, the operating wavelength of the wavelength division multiplexer 2 is 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode optical fiber with anomalous dispersion in the 1550nm band, which is used to couple the pump light into the resonant cavity, and its pigtail is Corning MetroCor.

[0043] Preferably, the erbium-doped fiber 3 has an absorption coefficient greater than 10 dB / m at 1530 nm and a length greater than 4 meters, or the product of the absorption coefficient and length of the erbium-doped fiber 3 is greater than 40 dB. Preferably, the erbium-doped fiber 3 is model ED1013-A, purchased from Yangtze Fiber Corporation, and has an absorption peak of 10-15 dB / m at 1529 nm.

[0044] As a preferred embodiment, the function of the first polarization controller 4 is to adjust the polarization and loss of the light pulse in the resonant cavity. It is a three-coil rotating polarization controller or a squeezed polarization controller, and its pigtail is Corning MetroCor; the function of the polarization analyzer 5 is to limit the polarization direction of the light pulse passing through the analyzer. It is a fiber polarization analyzer, and its pigtail type is Corning MetroCor; the function of the second polarization controller 6 is to adjust the polarization and loss of the light pulse in the resonant cavity. It is a three-coil rotating polarization controller or a squeezed polarization controller, and its pigtail is Corning MetroCor.

[0045] As a preference, the optical fiber isolator 9 is an isolator with a central wavelength of 1550 nm, which is used to limit the unidirectional operation of the laser, and its pigtail type is Corning MetroCor.

[0046] Preferably, the dispersion control unit 8 is composed of a grating pair, a prism pair, a chirped fiber Bragg grating, a spatial light modulator, or a grating pair with a telescope system. Preferably, the dispersion control unit 8 is composed of a pair of transmission gratings, which are used to introduce large anomalous dispersion, and the fiber pigtail type is Corning MetroCor.

[0047] In the present invention, the pump light emitted by the pump source can be coupled into the resonant cavity by setting a wavelength division multiplexer. By setting an erbium-doped 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, 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 passing through the analyzer can be limited. By setting a second polarization controller on the output side of the analyzer, 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 set in sequence can work together to utilize the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber to produce an equivalent saturable absorption effect, thereby achieving pulse mode locking. 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 operates 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. Combined with the increase in pump power, the nonlinearity accumulated by the nanosecond pulses propagating in the fiber ring cavity gradually increases. After exceeding a certain threshold, the evolution from periodic bifurcation to chaos occurs, and then high-order chaotic pulse output is achieved by increasing the pump. The fiber laser of the present invention utilizes erbium-doped fiber with anomalous dispersion and passive fiber with anomalous dispersion. Operating the fiber laser in the anomalous dispersion region and introducing a 0.08nm bandpass filter are key to achieving high-order chaotic pulse output. By introducing greater anomalous dispersion, the pulse width reaches nanoseconds while maintaining a shorter cavity length. Compared to picosecond or femtosecond pulses, the rate of nonlinear enhancement accumulated by nanosecond pulses propagating within the cavity can be slowed down with increasing pump power. The introduction of a narrowband bandpass filter effectively suppresses the generation of background continuous waves, facilitating the generation of nanosecond pulses while also preventing their influence on chaotic pulse generation and limiting the rate of nonlinear enhancement accumulated by nanosecond pulses propagating within the cavity. Increasing the pump power allows for the generation of higher-order chaotic pulses in a step-by-step manner. The provision of a fiber isolator restricts the laser to unidirectional operation.

[0048] The laser has a simple structure and low manufacturing cost, and can realize the reliable output of high-order chaotic pulses.

[0049] The present invention also provides a method for generating high-order chaotic pulses, using a nanosecond fiber laser for high-order chaotic pulses, comprising the following steps:

[0050] 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;

[0051] Step 2: Utilize the erbium-doped fiber 3 to absorb the continuous pump light and stimulate the emission of long-wavelength gain pulses, which oscillate in the fiber laser cavity.

[0052] Step 3: Adjust the first polarization controller 4 and the second polarization controller 5 so that the fiber laser operates on the path from periodic bifurcation to chaos. At the same time, the first polarization controller 4, the analyzer 5 and the second polarization controller 6 act together as a mode-locking starting device, and utilize the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber to produce an equivalent saturable absorption effect, thereby achieving pulse mode locking; at the same time, the gain pulse is output to the dispersion control unit 8 through the analyzer 5. The anomalous dispersion value provided by the dispersion control unit 8 makes the overall dispersion value of the fiber laser less than -10 5 ps 2 Because the fiber laser operates 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 meet the near-conversion limit. By introducing a large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining a short cavity length of the fiber laser.

[0053] Step 4: Increase the pump power of the pump source 1. As the pump power increases, the nonlinearity of the nanosecond pulse propagating and accumulating in the fiber ring cavity 11 gradually increases. After exceeding a certain threshold, a periodic bifurcation to chaos evolution occurs, and then high-order chaotic pulse output is achieved by increasing the pump. At the same time, the narrowband channel of the bandpass filter 10 is used to suppress the generation of background continuous waves, and while allowing the generation of nanosecond pulses, the increasing speed of the nanosecond pulse propagating and accumulating in the fiber ring cavity 11 is controlled to achieve the step-by-step generation of high-order chaotic pulses. Finally, the high-order chaotic pulses are output through the low-energy output port of the output coupler 7.

[0054] In order to effectively verify the effect of the fiber laser in the present invention in generating chaotic pulses, numerical simulations were performed. The time domain diagram and spectrum diagram of the nanosecond pulses output by the numerical simulation laser are shown in Figure 2. Figure 2 and 3 shown. Figure 4 The bifurcation diagram of the output pulse peak power obtained with the increase of the small signal gain coefficient is given, and the region of small signal gain coefficient corresponding to the high-order chaotic pulse is marked in the figure. Figures 5 to 8 The timing diagrams of chaotic pulses at the center of each order chaotic region are given.

[0055] The present invention provides a method for generating high-order chaotic pulse output. Based on a pulse shaping technique determined by the balance between anomalous dispersion and nonlinear effects that satisfy the nonlinear Schrödinger equation, the method utilizes nonlinear polarization rotation mode locking to enable the laser to operate in the equivalent saturable absorption region. By operating the laser in a large anomalous dispersion region, nanosecond pulses are output that meet the near-transformation limit. As the pump power increases, the nonlinearity accumulated by the nanosecond pulses in the cavity gradually increases. After exceeding a certain threshold, a periodic bifurcation-to-chaotic evolution path emerges, generating chaotic pulses. By introducing large anomalous dispersion, the pulse width reaches the nanosecond level while maintaining a short cavity length in the fiber laser. Compared to picosecond or femtosecond pulses, the rate of nonlinear enhancement accumulated by the nanosecond pulses in the cavity with increasing pump power can be relatively slow. Subsequently, a narrowband filter is introduced to suppress the generation of background continuous waves, which not only facilitates the generation of nanosecond pulses, but also avoids the influence of background continuous waves on the generation of chaotic pulses, while also limiting the rate of nonlinear enhancement accumulated by the nanosecond pulses in the cavity. In fiber lasers, the periodic bifurcation of nanosecond pulses into chaotic evolution is realized, and then the step-by-step generation of high-order chaotic pulses is achieved by increasing the pump.

[0056] The method has a simple implementation process and low implementation cost. It can enable the fiber laser to operate on a path from periodic bifurcation to chaos, and then achieve high-order chaotic pulse output by increasing pumping.

Claims

1. A nanosecond fiber laser with high-order chaotic pulses, comprising a pump source (1), characterized in that: It also includes an optical fiber ring cavity (11); the optical fiber ring cavity (11) includes a wavelength division multiplexer (2), an erbium-doped optical fiber (3), a first polarization controller (4), an analyzer (5), a second polarization controller (6), an output coupler (7), a dispersion control unit (8), an optical fiber isolator (9), and a bandpass filter (10) arranged in sequence along the ring direction; The pump port (2a) of the wavelength division multiplexer (2) is connected to the output port of the pump source (1) via an optical fiber; one end of the erbium-doped optical fiber (3) 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); the input end of the second polarization controller (6) is connected to the output end of the polarization analyzer (5); the input port of the output coupler (7) is connected to the output end of the polarization analyzer (5); The optical fiber isolator (9) is connected to the output end of the dispersion control unit (8) through a fiber collimator. The input end of the optical fiber isolator (9) is connected to the output end of the dispersion control unit (8) through another fiber collimator. The input end of the bandpass filter (10) is connected to the output end of the optical fiber isolator (9), and the output end of the bandpass filter (10) is connected to the signal port (2b) of the wavelength division multiplexer (2). The erbium-doped fiber (3) is a single-mode fiber with anomalous dispersion in the 1550 nm band; the pigtails of the wavelength division multiplexer (2), the first polarization controller (4), the analyzer (5), the second polarization controller (6), the output coupler (7), the dispersion control unit (8), the fiber isolator (9), and the bandpass filter (10) are all single-mode fibers with anomalous dispersion in the 1550 nm band; the dispersion control unit (8) is composed of a grating pair, a prism pair, a chirped fiber Bragg grating, a spatial light modulator, or a grating pair with a telescope system, and is used to provide anomalous large dispersion, and the anomalous dispersion value provided by the anomalous dispersion value makes the overall dispersion value of the fiber laser less than -10 5 ps 2 The bandpass filter (10) has an operating center wavelength of 1550 nm and a 3dB bandwidth greater than 0.04 nm and less than 0.24 nm. While allowing the generation of nanosecond pulses, it suppresses the generation of background continuous waves and controls the nonlinear increase rate of the nanosecond pulses propagating and accumulating in the fiber ring cavity (11), thereby achieving the step-by-step generation of high-order chaotic pulses. The first polarization controller (4), the analyzer (5) and the second polarization controller (6) work together to make the fiber laser work in the equivalent saturable absorption region through nonlinear polarization rotation, so that the fiber laser works on a path from periodic bifurcation to chaos, thereby achieving the output of high-order chaotic pulses by increasing pumping.

2. The high-order chaotic pulse nanosecond fiber laser according to claim 1, characterized in that: The output coupler (7) adopts an optical fiber coupler with an output energy ratio of 20:

80.

3. The high-order chaotic pulse nanosecond fiber laser according to claim 2, characterized in that: The total length of the optical fiber part in the optical fiber laser is less than 10m.

4. The high-order chaotic pulse nanosecond fiber laser according to claim 3, characterized in that: The pump source (1) is a single-mode fiber-coupled semiconductor laser, the center wavelength of which is located at 976nm or 1480nm, and the output fiber is a single-mode fiber with anomalous dispersion in the 1550nm band, and the output power is greater than 400mW.

5. The high-order chaotic pulse nanosecond fiber laser according to claim 4, characterized in that: The operating wavelength of the wavelength division multiplexer (2) is 980 / 1550nm or 1480 / 1550nm, and its output pigtail is a single-mode optical fiber with anomalous dispersion in the 1550nm band.

6. The high-order chaotic pulse nanosecond fiber laser according to claim 5, characterized in that: The absorption coefficient of the erbium-doped optical fiber (3) at 1530 nm is greater than 10 dB / m, and the length is greater than 4 meters, or the product of the absorption coefficient and the length of the erbium-doped optical fiber (3) is greater than 40 dB.

7. The high-order chaotic pulse nanosecond fiber laser according to claim 1, characterized in that: The first polarization controller (4) is a three-coil rotating polarization controller or a squeeze-type polarization controller; the analyzer (5) is a fiber analyzer; and the second polarization controller (6) is a three-coil rotating polarization controller or a squeeze-type polarization controller.

8. The high-order chaotic pulse nanosecond fiber laser according to claim 1, characterized in that: The optical fiber isolator (9) is an isolator with a central wavelength of 1550 nm.

9. The high-order chaotic pulse nanosecond fiber laser according to claim 1, characterized in that: The dispersion control unit (8) is composed of a grating pair, a prism pair, a chirped Bragg fiber grating, a spatial light modulator, or a grating pair with a telescope system.

10. A method for generating high-order chaotic pulses, using a nanosecond fiber laser for generating high-order chaotic pulses according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: Using a pump source (1) to provide continuous pump light, and coupling the continuous pump light into the fiber laser through a wavelength division multiplexer (2); Step 2: using the erbium-doped fiber (3) to absorb the pump continuous light and stimulate the radiation to emit a long-wavelength gain pulse, and the generated gain pulse oscillates in the fiber laser cavity; Step 3: Regulate the first polarization controller (4) and the second polarization controller (5) so that the fiber laser operates on a path from periodic bifurcation to chaos. At the same time, through the joint action of the first polarization controller (4), the analyzer (5) and the second polarization controller (6), the nonlinear polarization rotation generated when the gain pulse propagates in the optical fiber is utilized to generate an equivalent saturable absorption effect, thereby achieving pulse mode locking. At the same time, the gain pulse is output to the dispersion control unit (8) through the analyzer (5). The anomalous dispersion value provided by the dispersion control unit (8) makes the overall dispersion value of the fiber laser less than -10 5 ps 2 Because the fiber laser operates 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 of the nanosecond pulse propagating and accumulating in the optical fiber ring cavity (11) gradually increases. After exceeding a certain threshold, a periodic bifurcation to chaos evolution occurs, and then high-order chaotic pulse output is achieved by increasing the pump power. At the same time, the narrowband channel of the bandpass filter (10) is used to suppress the generation of background continuous waves, and while allowing the generation of nanosecond pulses, the increasing speed of the nanosecond pulse propagating and accumulating in the optical fiber ring cavity (11) is controlled to achieve the step-by-step generation of high-order chaotic pulses. Finally, the high-order chaotic pulses are output through the low-energy output port of the output coupler (7).

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