Switchable pulsed polarization maintaining fiber laser and modulation method thereof

Abstract

The present application relates to the technical field of fiber laser, in particular to a polarization maintaining fiber laser with switchable pulse and a modulation method thereof. The polarization maintaining fiber laser with switchable pulse comprises a pump source for outputting pump light; the light emission direction of the pump source is sequentially connected with a polarization maintaining wavelength division multiplexer, a polarization maintaining fiber, a polarization maintaining fiber isolator, a directional carbon tube saturable absorber and a polarization maintaining coupler; the directional carbon tube saturable absorber is coupled to the end face of the polarization maintaining jumper, and the directional carbon tube saturable absorber has an included angle with the laser polarization direction in the polarization maintaining jumper, and the included angle is adjusted to generate multiple states of mode-locked pulses. The present application adjusts the included angle between the laser polarization direction and the directional carbon tube saturable absorber to achieve more convenient, fast and stable pulse switching. The present application does not need to frequently adjust the optical path structure or optical elements, and only needs to adjust the included angle between the polarization maintaining fiber and the directional carbon tube saturable absorber to achieve accurate pulse state switching.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and more specifically to a switchable pulse polarization-maintaining fiber laser and its modulation method. Background Technology

[0002] Pulsed lasers, with their high peak power and narrow pulse width, have shown great application potential in various fields such as materials processing, optical communication, biomedical imaging, and basic scientific research. Currently, the optical path of pulsed lasers is basically composed of single-mode fiber and related devices. Single-mode fiber, because it can support the propagation of a single mode of light, has been widely used in long-distance, high-bandwidth optical communication systems.

[0003] However, in pulsed laser systems, single-mode optical fibers are highly sensitive to environmental interference such as temperature changes and mechanical vibrations, resulting in weak environmental anti-interference capabilities of pulsed laser systems. This defect directly affects the signal-to-noise ratio of pulsed lasers, causing a significant decrease in the stability of pulsed lasers. Especially in complex and ever-changing practical application environments, the reliability of the system is difficult to guarantee, limiting its application in high-precision and high-requirement scenarios.

[0004] Furthermore, single-mode fiber offers limited control over polarization state, and the laser's polarization direction is easily affected by external environmental interference, resulting in random changes. To address these random variations in laser polarization state within single-mode fiber, a polarization controller is typically used in conjunction with the single-mode fiber to adjust the laser's polarization direction in real time, enabling pulse switching. However, the polarization controller's adjustment often fails to achieve precise control over the polarization direction, leading to significant pulse switching errors and impacting the accuracy and reliability of the pulsed laser system. Summary of the Invention

[0005] To address the problem in existing pulsed laser systems that single-mode fiber requires a polarization controller to switch pulses and cannot achieve precise control over the polarization direction, this invention provides a switchable pulse polarization-maintaining fiber laser and its modulation method.

[0006] This invention achieves more convenient, faster, and more stable pulse switching by using polarization-maintaining optical components and a directional carbon nanotube saturable absorber, and by directionally adjusting the angle between the laser polarization direction and the directional carbon nanotube saturable absorber. This invention eliminates the need for a polarization controller or changes to the optical path structure; precise pulse state switching can be achieved simply by adjusting the angle between the polarization-maintaining fiber and the directional carbon nanotube saturable absorber.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows.

[0008] This invention provides a switchable pulse polarization-maintaining fiber laser, comprising a pump source for outputting pump light; the light emission direction of the pump source is sequentially connected to a polarization-maintaining wavelength division multiplexer, a polarization-maintaining fiber, a polarization-maintaining fiber isolator, a directional carbon nanotube saturable absorber, and a polarization-maintaining coupler; the output end of the polarization-maintaining fiber isolator is connected to the directional carbon nanotube saturable absorber via a polarization-maintaining jumper; the directional carbon nanotube saturable absorber is coupled to the end face of the polarization-maintaining jumper, and the directional carbon nanotube saturable absorber and the laser polarization direction in the polarization-maintaining jumper have an angle, and by adjusting the angle, mode-locked pulses of various states can be generated.

[0009] Preferably, the end face of the polarization-maintaining jumper has a groove, the oriented carbon nanotube saturable absorber is coupled to the groove, and the oriented carbon nanotube saturable absorber remains covering the fiber core channel of the polarization-maintaining jumper.

[0010] Preferably, the end face of the polarization-maintaining jumper has two geometrically symmetrical stress structures to keep the laser polarization direction of the polarization-maintaining jumper along the stress structure direction.

[0011] Preferably, the two stress structures are symmetrically arranged on both sides of the fiber core channel of the polarization jumper.

[0012] Preferably, the direction of the groove is at an angle to the laser polarization direction in the polarization-maintaining jumper, and the mode-locked pulse in three states can be generated by changing the angle.

[0013] Preferably, the included angle is 30°, 45°, or 60°. Specifically, when the included angle is 30°, the mode-locking pulse is a soliton mode-locking pulse; or, when the included angle is 45°, the mode-locking pulse is a harmonic mode-locking pulse; or, when the included angle is 60°, the mode-locking pulse is a bound state mode-locking pulse.

[0014] Preferably, the polarization-maintaining fiber is an erbium-doped polarization-maintaining gain fiber; the core diameter of the polarization-maintaining fiber is 7 μm; and the length is 0.8 m.

[0015] Preferably, the polarization-maintaining coupler has a first output terminal and a second output terminal. The first output terminal is connected to the common terminal of the polarization-maintaining wavelength division multiplexer, and the second output terminal is connected to the input terminal of the pulse state detection device. The pulse state detection device is used to detect the pulse state of the optical path passing through the polarization-maintaining coupler.

[0016] A second aspect of the present invention provides a modulation method for a switchable pulse polarization-maintaining fiber laser as described in the first aspect, comprising the following steps: The directional carbon nanotube saturable absorber is coupled to the end face of the polarization-maintaining jumper. At the same time, the angle between the directional carbon nanotube saturable absorber and the laser polarization direction in the polarization-maintaining jumper is adjusted to generate mode-locked pulses in various states.

[0017] A preferred method for generating mode-locked pulses in multiple states is: The oriented carbon nanotube saturable absorber is coupled to the groove on the end face of the polarization-maintaining jumper, while ensuring that the oriented carbon nanotube saturable absorber covers the fiber core channel of the polarization-maintaining jumper. Adjust the direction of the groove so that it forms an angle with the laser polarization direction in the polarization-maintaining jumper. By changing the angle, three states of mode-locked pulses can be generated.

[0018] Preferably, the specific conditions for generating mode-locked pulses in three states are: When the included angle is 30°, the mode-locking pulse is a soliton mode-locking pulse; or, when the included angle is 45°, the mode-locking pulse is a harmonic mode-locking pulse; or, when the included angle is 60°, the mode-locking pulse is a bound-state mode-locking pulse. In this invention, the soliton mode-locking pulse is a conventional soliton mode-locking pulse.

[0019] The beneficial effects of this invention are: 1. This invention achieves more convenient, faster, and more stable pulse switching by using polarization-maintaining optical components and a directional carbon nanotube saturable absorber to directionally adjust the angle between the laser polarization direction and the directional carbon nanotube saturable absorber. This invention eliminates the need for frequent adjustments to the optical path structure or optical components; precise pulse state switching can be achieved simply by adjusting the angle between the polarization-maintaining fiber and the directional carbon nanotube saturable absorber.

[0020] 2. By directionally adjusting the angle between the laser polarization direction and the directional carbon nanotube saturable absorber, the present invention can actually achieve the adjustment of the modulation depth of the directional carbon nanotube saturable absorber, with a modulation depth of 6.2% to 10.9%.

[0021] 3. Compared with existing single-mode optical fibers and devices, the mode-locked pulses of the switchable pulse polarization-maintaining fiber laser of the present invention are all above 80dB, and the mode-locked pulse stability is stronger.

[0022] 4. The switchable pulse polarization-maintaining fiber laser of the present invention can meet the diverse pulse state requirements of different application scenarios by generating mode-locked pulses in multiple states. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a switchable pulse polarization-maintaining fiber laser according to an embodiment of the present invention.

[0024] Figure 2 This is a schematic diagram of the end face of a polarization-maintaining fiber according to one embodiment of the present invention.

[0025] Figure 3This is a schematic diagram of the end face of the bias-maintaining jumper in one embodiment of the present invention.

[0026] Figure 4 This is a schematic diagram showing the angle between the laser polarization direction of the polarization-maintaining jumper and the saturable absorber of the directional carbon nanotube, and a schematic diagram showing the position of the groove, according to an embodiment of the present invention. Specifically, (a) is a schematic diagram showing the angle between the laser polarization direction of the polarization-maintaining jumper and the saturable absorber of the directional carbon nanotube; and (b) is a schematic diagram showing the position of the groove.

[0027] Figure 5 This is a schematic diagram showing the growth direction of carbon nanotubes in an AFI zeolite crystal, representing a directional carbon nanotube saturable absorber according to an embodiment of the present invention.

[0028] Figure 6 Comparison of the tunable modulation depth of the oriented carbon nanotube saturable absorber at different angles. Among them, (a) is the tunable modulation depth of the oriented carbon nanotube saturable absorber when the included angle is 30°; (b) is the tunable modulation depth of the oriented carbon nanotube saturable absorber when the included angle is 60°.

[0029] Figure 7 These are optical microscope images of the AFI crystal. (a) and (b) are optical microscope images of the AFI crystal from different viewing angles, respectively.

[0030] Figure 8 These are scanning electron microscope (SEM) images of the AFI crystal. (a) and (b) are SEM images of the AFI crystal at different magnifications, respectively.

[0031] Figure 9 Optical microscope images of the CNT@AFI host and guest materials. Among them, (a) and (b) are optical microscope images of the CNT@AFI host and guest materials from different viewpoints.

[0032] Figure 10 The image shows the Raman spectrum of the CNT@AFI host-guest material.

[0033] Figure 11 The data plots are for traditional soliton mode-locking. Among them, (a) is the emission spectrum of traditional soliton mode-locking; (b) is the pulse sequence of mode-locking; (c) is the pulse autocorrelation curve of traditional soliton mode-locking; and (d) is the signal-to-noise ratio of the pulse.

[0034] Figure 12 The data diagrams are for harmonic mode-locking. Among them, (a) to (e) are the pulse sequences of second- to sixth-order harmonic mode-locking, respectively; (f) to (j) are the signal-to-noise ratios of second- to sixth-order harmonic mode-locking, respectively.

[0035] Figure 13The data for bound-state mode-locking are shown in the figure. Among them, (a) is the emission spectrum of bound-state mode-locking; and (b) is the pulse autocorrelation curve of bound-state mode-locking.

[0036] Figure label: 1. Pump source; 2. Polarization-maintaining wavelength division multiplexer; 3. Polarization-maintaining optical fiber; 4. Polarization-maintaining optical fiber isolator; 5. Directional carbon nanotube saturable absorber; 6. Polarization-maintaining coupler; 7. Polarization-maintaining jumper; 71. Groove; 72. Stress structure; 8. Pulse state detection equipment. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0038] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] In existing technologies, to address the random variations in laser polarization state within single-mode fibers, a single-mode fiber combined with a polarization controller is typically used to switch pulses. Specifically, while single-mode fibers can support single-mode light propagation, their control over polarization state is relatively limited, and the laser polarization direction is easily affected by external environmental interference, resulting in random changes.

[0040] To address this issue, existing technologies require a polarization controller to adjust the laser's polarization direction in real time to achieve pulse switching. This method has significant drawbacks. For example, it often involves a complex adjustment process. The polarization controller needs to adjust the laser's polarization direction mechanically or manually, a process that is not only complex but also requires specialized technicians, increasing system maintenance and operating costs. Furthermore, this method is difficult to achieve precise control and is inefficient. Due to the limited polarization state support of single-mode fiber, the polarization controller's adjustment often fails to achieve precise control of the polarization direction, resulting in significant pulse switching errors and impacting the system's accuracy and reliability. Frequent adjustments not only reduce system efficiency but may also lead to decreased system stability, further affecting the pulsed laser's performance.

[0041] Furthermore, existing technologies are limited in pulse generation and mode-locking techniques, only implementing Q-switching mode-locking and basic soliton mode-locking, and cannot achieve richer pulse state switching and diversified mode-locking techniques. This technological limitation makes it difficult for the system to meet the diverse pulse state requirements of different application scenarios.

[0042] This invention achieves more convenient, faster, and more stable pulse switching by using polarization-maintaining optical components and a directional carbon nanotube saturable absorber, and by directionally adjusting the angle between the laser polarization direction and the directional carbon nanotube saturable absorber. This invention eliminates the need for a polarization controller or changes to the optical path structure; precise pulse state switching can be achieved simply by adjusting the angle between the polarization-maintaining fiber and the directional carbon nanotube saturable absorber.

[0043] Specifically, this invention employs a fully polarization-maintaining optical path structure, ensuring that the laser direction inside the polarization-maintaining fiber laser is always along the stress direction, which is the slow axis direction of the polarization-maintaining fiber. This allows for the quantitative adjustment of the angle between the laser polarization direction and the carbon nanotubes, essentially adjusting the modulation depth of the carbon nanotubes from 6.2% to 10.9%. This makes pulse switching more convenient, faster, and more stable, and also enables precise correspondence between the angle and the pulse state; for example, a 30° angle corresponds to a traditional soliton mode-locked pulse, a 45° angle to a harmonic mode-locked pulse, and a 60° angle to a bound-state mode-locked pulse.

[0044] like Figure 11 As shown, traditional soliton mode-locking is a frequency-locking technique implemented during soliton propagation. A soliton is an isolated wave that propagates stably with a constant shape in a nonlinear medium. Mode-locking here refers to synchronizing the phase of multiple frequency components of a laser or oscillator by utilizing the characteristics of solitons. Traditional soliton mode-locking is commonly used in fiber optic communication to reduce signal attenuation, improve transmission quality, and ensure signal stability during long-distance transmission. Figure 11 Figure (a) shows a center wavelength of 1556.48 nm and a 3-dB bandwidth of 2.93 nm. There are obvious Kelly sidebands on both sides of the center wavelength, indicating that it is a conventional soliton mode-locked signal. Figure 11 In Figure (b), the repetition frequency is 27.95 MHz. Figure 11 In Figure (c), the pulse width is 976.2 fs after fitting with hyperbolic secant; Figure 11 In Figure (d), the pulse signal-to-noise ratio is 83dB, proving that the pulse stability is excellent.

[0045] like Figure 12 As shown, harmonic mode-locking is a technique that uses harmonic frequencies to achieve multi-frequency synchronization. By locking these harmonic frequencies, the system can generate a stable, broadband spectral output. Harmonic mode-locking is widely used in supercontinuum light sources, frequency combs, and multi-wavelength communications to provide broadband spectral coverage and meet the needs of multi-band transmission. Figure 12As shown in (a) to (e), the second to sixth order harmonic mode-locking, with periods of 17.86 ns, 11.71 ns, 8.89 ns, 7.16 ns, and 5.96 ns, respectively, correspond to repetition frequencies of 55.91 MHz, 84.27 MHz, 111.81 MHz, 139.76 MHz, and 167.71 MHz, respectively, and signal-to-noise ratios of 86 dB, 80 dB, 82 dB, 73 dB, and 62 dB, respectively. This demonstrates excellent mode-locking pulse stability, and the signal-to-noise ratio is significantly improved compared to other harmonic mode-locking methods. Figure 12 (f) ~ (j).

[0046] like Figure 13 As shown, bound-state mode-locking involves using the properties of bound states to achieve frequency locking in semiconductors or other quantum systems. A bound state is a state in which electrons and holes are combined, possessing a specific energy level structure. Bound-state mode-locking is commonly found in optoelectronic devices, such as quantum dot lasers. By controlling the dynamics of bound states, better frequency control and coherence can be achieved, improving device efficiency and stability. Figure 13 Figure (a) shows that the center wavelength is 1556.8 nm, the spectral modulation wavelength is 2.5 nm, and the time interval is 3.2 ps. Figure 13 Figure (b) contains three equally spaced peaks with a peak intensity ratio of 1:2:1, indicating that the binding force between sub-pulses is strong and that the two solito pulses have the same intensity and pulse width.

[0047] Each mode-locking technology is designed for different physical systems and application scenarios, and achieves frequency stability through a unique mechanism, thereby improving system performance and output quality.

[0048] This application uses fully polarization-maintaining optical components, such as polarization-maintaining fiber, polarization-maintaining fiber isolator, polarization-maintaining coupler, and polarization-maintaining wavelength division multiplexer. All of these are polarization-maintaining devices, eliminating the need for a polarization controller. Furthermore, due to the unique dual-stress pillar structure of polarization-maintaining fiber, its resistance to environmental interference is significantly enhanced. In polarization-maintaining fiber lasers, this translates to a significantly improved signal-to-noise ratio, with soliton mode-locking and harmonic mode-locking generally exceeding 80 dB, and stronger pulse stability.

[0049] This invention not only realizes soliton mode-locked pulses, but also successfully realizes various mode-locked pulse states such as harmonic mode-locked pulses and bound state mode-locked pulses, providing richer functional options for different application scenarios.

[0050] Specifically, soliton mode-locked pulses are known for their frequency and phase stability, making them suitable for applications requiring high-precision frequency control. For example, they are used in communication systems for frequency locking to ensure the stability of signal transmission.

[0051] Harmonic mode-locked pulses are suitable for applications that require the simultaneous use of multiple frequencies. For example, in ultrashort pulse generation, a wider spectrum can be achieved by superimposing multiple harmonics. In miniaturized devices, harmonic mode-locked pulses help optimize device size and efficiency, such as in miniaturized laser sources.

[0052] Bound-state mode-locked pulses are suitable for applications requiring the simultaneous processing of multiple signals, such as high-capacity communication. In this mode, each soliton can carry an independent signal and also serve as a stable information carrier for quantum communication and computation, thus providing greater security.

[0053] The technical solution of the present invention will be further described below through specific embodiments.

[0054] In the following embodiments, unless otherwise specified, the methods described are conventional methods; and unless otherwise specified, the reagents and materials described are commercially available.

[0055] like Figure 1 A switchable pulse polarization-maintaining fiber laser includes a pump source 1 for outputting pump light; the light output direction of the pump source 1 is sequentially connected to a polarization-maintaining wavelength division multiplexer 2, a polarization-maintaining fiber 3, a polarization-maintaining fiber isolator 4, a directional carbon nanotube saturable absorber 5, and a polarization-maintaining coupler 6; the output end of the polarization-maintaining fiber isolator 4 is connected to the directional carbon nanotube saturable absorber 5 through a polarization-maintaining jumper 7; the directional carbon nanotube saturable absorber 5 is coupled to the end face of the polarization-maintaining jumper 7, and the directional carbon nanotube saturable absorber 5 and the laser polarization direction in the polarization-maintaining jumper 7 have an angle, and the mode-locked pulses of various states can be generated by adjusting the angle.

[0056] In one embodiment of the present invention, a 976nm polarization-maintaining diode laser is used as pump source 1 to provide an excitation source for the optical path.

[0057] The polarization-maintaining wavelength division multiplexer 2 adopts a reflective structure. Its three ports are the reflector, output, and common ports. The reflector of the polarization-maintaining wavelength division multiplexer 2 is connected to the pump source 1, with an operating wavelength between 960nm and 990nm, and uses a PM980 fiber optic pigtail. The output port of the polarization-maintaining wavelength division multiplexer 2 is connected to the polarization-maintaining fiber 3, and the common port is connected to the polarization-maintaining coupler 6. Both the output and common ports of the polarization-maintaining wavelength division multiplexer 2 operate at wavelengths between 1520nm and 1580nm, using a PM1550 fiber optic pigtail.

[0058] The polarization-maintaining fiber 3 is a 0.8m long erbium-doped gain fiber with a core diameter of 7μm. The specific model is Nufern PM-ESF-7 / 125, and the typical absorption value for 976nm pump light is 24.0dB / m.

[0059] The polarization-maintaining fiber isolator 4 has a center wavelength of 1550nm, operates on the slow axis, and is cut off on the fast axis, ensuring unidirectional laser transmission. The output of the polarization-maintaining fiber isolator 4 is connected to the directional carbon nanotube saturable absorber 5 via a polarization-maintaining jumper 7. By connecting the polarization-maintaining jumpers 7 with different groove angles to the optical path, the angle between the directional carbon nanotube saturable absorber 5 and the laser polarization direction in the polarization-maintaining jumper 7 can be adjusted, generating mode-locked pulses in various states.

[0060] A polarization-maintaining coupler 6 with a coupling ratio of 20:80 splits the laser beam, with 80% of the energy circulating within the annular cavity and 20% output for measurement. A pulse state detection device 8, such as a spectrometer or oscilloscope, is used to record the pulse state.

[0061] Based on the above embodiment, the end face of the polarization-maintaining jumper 7 has a groove 71, and the oriented carbon nanotube saturable absorber 5 is coupled to the groove 71, and the oriented carbon nanotube saturable absorber 5 remains covering the fiber core channel of the polarization-maintaining jumper 7. As the direction of the groove 71 gradually deviates from the laser polarization direction in the polarization-maintaining jumper 7, the included angle gradually increases, generating mode-locked pulses in three states.

[0062] Based on the above implementation, the end face of the polarization-maintaining jumper 7 has two geometrically symmetrical stress structures 72, so that the laser polarization direction of the polarization-maintaining jumper 7 is kept along the stress structure direction.

[0063] Based on the above implementation method, two stress structures 72 are symmetrically arranged on both sides of the fiber core channel of the polarization jumper 7.

[0064] like Figure 3 and Figure 4 The polarization-maintaining jumper end face has the same stress column structure as the polarization-maintaining fiber, with a core channel in the middle. The laser propagates within this core channel, and its polarization direction remains along the stress direction, which is the slow axis direction of the polarization-maintaining fiber. The polarity bonds of the polarization-maintaining jumper are easily observed; they are typically metal sleeves, corresponding to the slow axis direction of the polarization-maintaining fiber, and can be used to determine the laser polarization direction. When coupling with a carbon nanotube saturable absorber, it is essential to ensure that the core channel is completely covered to prevent light leakage and to achieve modulation.

[0065] An angle θ is formed between the groove and the laser polarization direction in the polarization-maintaining jumper. A hexagonal prism-shaped oriented carbon nanotube saturable absorber is placed within the groove of the polarization-maintaining jumper. Since the carbon nanotubes are directionally grown, their orientation will be consistent with the direction of the groove. Simultaneously, the laser polarization direction in the polarization-maintaining jumper is consistent with the slow axis direction of the jumper, thus creating an angle θ between the oriented carbon nanotube and the laser polarization direction in the polarization-maintaining jumper. By changing the angle between the groove and the laser polarization direction in the polarization-maintaining jumper, the effective angle between the oriented carbon nanotube and the laser polarization direction in the polarization-maintaining jumper can be changed, effectively altering the modulation depth of the carbon nanotube. See [link to relevant documentation]. Figure 6The range is from 6.2% to 10.9%, which enables different nonlinear effects and generates mode-locked pulses in various states, including traditional soliton mode-locking, harmonic mode-locking, and bound state mode-locking.

[0066] Based on the above implementation, since the direction of the groove 71 forms an angle with the laser polarization direction in the polarization jumper 7, the mode-locked pulses in three states can be generated by changing the angle.

[0067] Based on the above implementation method, polarization-maintaining fiber 3 is a polarization-maintaining erbium-doped gain fiber; the core diameter of polarization-maintaining fiber 3 is 7.0 μm, the cladding diameter is 125.0 μm ± 1.5 μm, and the length is 0.8 m. For example, the model of polarization-maintaining fiber 3 is NufernPM-ESF-7 / 125.

[0068] In one embodiment of the present invention, such as Figure 2 The polarization-maintaining fiber 3 has two stress pillars, which are divided into a fast axis and a slow axis. The slow axis is along the direction of the stress pillars. The laser polarization direction of the polarization-maintaining fiber 3 is kept along the slow axis direction of the polarization-maintaining fiber.

[0069] In one embodiment of the present invention, the polarization-maintaining coupler 6 has a first output terminal and a second output terminal. The first output terminal is connected to the common terminal of the polarization-maintaining wavelength division multiplexer 2, and the second output terminal is connected to the input terminal of the pulse state detection device 8. The pulse state detection device 8 is used to detect the pulse state of the optical path passing through the polarization-maintaining coupler 6.

[0070] Based on the above embodiments, a modulation method for a switchable pulse polarization-maintaining fiber laser is provided, comprising the following steps: The directional carbon nanotube saturable absorber 5 is coupled to the end face of the polarization-maintaining jumper 7. At the same time, the angle between the directional carbon nanotube saturable absorber 5 and the laser polarization direction in the polarization-maintaining jumper 7 is adjusted to generate mode-locked pulses in various states.

[0071] Based on the above embodiments, a method for generating mode-locked pulses in multiple states is as follows: A directional carbon nanotube saturable absorber 5 is coupled to a groove 71 on the end face of a polarization-maintaining jumper 7, while ensuring that the directional carbon nanotube saturable absorber 5 covers the fiber core channel of the polarization-maintaining jumper 7; the direction of the groove 71 is adjusted so that it forms an angle with the laser polarization direction in the polarization-maintaining jumper 7; by changing this angle, three states of mode-locked pulses can be generated. In this embodiment of the invention, the angle is 30°, 45°, or 60°; by adjusting the direction of the groove 71 to the corresponding angle, three states of mode-locked pulses can be generated.

[0072] Based on the above implementation method, the specific conditions for generating mode-locked pulses in three states are as follows: When the included angle is 30°, the mode-locking pulse is a conventional soliton mode-locking pulse; or, when the included angle is 45°, the mode-locking pulse is a harmonic mode-locking pulse; or, when the included angle is 60°, the mode-locking pulse is a bound state mode-locking pulse.

[0073] The directional carbon nanotube saturable absorber 5 used in this invention is obtained through in-situ synthesis using a pyrolytic template agent, such as... Figure 5 As shown. The preparation method of the directional carbon nanotube saturable absorber 5 is as follows:

[0074] Step 1, Preparation of AFI crystals: First, AFI-type zeolite molecular sieve crystals were synthesized using a hydrothermal synthesis method. The synthesis steps are as follows: Weigh 1.6 mol of aluminum isopropoxide and 0.2 mol of zinc acetate dihydrate powder and place them in a polytetrafluoroethylene beaker. Add deionized water and stir for 12 hours to allow the aluminum isopropoxide and zinc acetate dihydrate to fully hydrolyze, thus obtaining the first reaction sol.

[0075] 1.23 mol of orthophosphoric acid was mixed with 10 mL of deionized water and added dropwise to the first reaction sol. The mixture was stirred for 2 hours to obtain the second reaction sol.

[0076] Using tri-n-propylamine as a template agent, 1.2 mol of the template agent was measured and mixed with 10 mL of deionized water. The mixture was then added dropwise to the second reaction sol, and stirring was continued for 2 hours to obtain the third reaction sol.

[0077] After stirring, the third reaction sol was allowed to stand for 1 hour, and then the polytetrafluoroethylene beaker was transferred to a stainless steel autoclave. The sealed autoclave was placed in an oven, and the crystallization temperature and time were set to allow the reaction to proceed under the autogenous pressure of the autoclave, resulting in the crystallized composite.

[0078] The crystallized composite was transferred to a glass beaker and deionized water was added. The sample was repeatedly cleaned in an ultrasonic cleaner. The remaining sample was dried in an electric heating drying oven at 80°C for 12 hours to obtain AFI type aluminum phosphate zeolite crystals, abbreviated as AFI crystals.

[0079] The gel molar ratio of aluminum isopropoxide, orthophosphoric acid, zinc acetate dihydrate, tri-n-propylamine, and deionized water was 1.6:1.23:0.2:1.2:600; the crystallization temperature was 165℃, and the crystallization time was 20 hours.

[0080] Figure 7 This is an optical microscope image of an AFI crystal. Figure 7 The results show that the AFI crystal has a smooth surface and good optical transparency. Figure 8 This is a scanning electron microscope image of an AFI crystal. Figure 8 The AFI crystals are uniform in size.

[0081] Step 2, Preparation of carbon nanotube@AFI type aluminum phosphate zeolite host-guest material: Using tri-n-propylamine as a template agent, oriented carbon nanotube@AFI type aluminum phosphate zeolite host-guest material was synthesized in situ by pyrolysis of the template agent. The specific method is as follows:

[0082] The AFI crystal was transferred into a muffle furnace, and the vacuum was slowly evacuated to 10°C. -3 After mbar heating, the temperature was increased from room temperature to 600℃ at a rate of 6.5℃ / min and held for 2 hours. After natural cooling to room temperature, the material was removed to obtain oriented carbon nanotubes@AFI type aluminum phosphate zeolite host-guest material, abbreviated as CNT@AFI host-guest material, which serves as an oriented carbon nanotube saturable absorber.

[0083] CNT@AFI host-guest materials are oriented carbon nanotubes grown inside the pores of AFI zeolite.

[0084] Figure 9 Optical microscope images of CNT@AFI host and guest materials. Figure 9 It can be observed that the carbonized AFI crystals still maintain their hexagonal prism shape and do not collapse. Moreover, due to the pyrolysis of the template agent into carbon nanotubes, they turn into a uniform black color.

[0085] To further confirm the successful synthesis of carbon nanotubes, Raman spectroscopy was used for characterization, with an excitation wavelength of 514 nm. The results are as follows: Figure 10 As shown. Figure 10 In the low-frequency region 510cm -1 It exhibits a typical radial breathing pattern, representing (4,2) chiral carbon nanotubes.

[0086] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A switchable pulse polarization-maintaining fiber laser, characterized in that, It includes a pump source (1) for outputting pump light; the light output direction of the pump source (1) is sequentially connected to a polarization-maintaining wavelength division multiplexer (2), a polarization-maintaining fiber (3), a polarization-maintaining fiber isolator (4), a directional carbon nanotube saturable absorber (5), and a polarization-maintaining coupler (6). The output end of the polarization-maintaining fiber isolator (4) is connected to the directional carbon nanotube saturable absorber (5) via a polarization-maintaining jumper (7); the directional carbon nanotube saturable absorber (5) is coupled to the end face of the polarization-maintaining jumper (7), and the directional carbon nanotube saturable absorber (5) has an angle with the laser polarization direction in the polarization-maintaining jumper (7), and the mode-locked pulses of various states are generated by adjusting the angle; the end face of the polarization-maintaining jumper (7) has a groove (71), the directional carbon nanotube saturable absorber (5) is coupled to the groove (71), and the directional carbon nanotube saturable absorber (5) remains covering the fiber core channel of the polarization-maintaining jumper (7); The direction of the groove (71) forms an angle with the laser polarization direction in the polarization-maintaining jumper (7). By changing the angle, three states of mode-locked pulses can be generated: when the angle is 30°, the mode-locked pulse is a soliton mode-locked pulse; or, when the angle is 45°, the mode-locked pulse is a harmonic mode-locked pulse; or, when the angle is 60°, the mode-locked pulse is a bound state mode-locked pulse.

2. The switchable pulse polarization-maintaining fiber laser according to claim 1, characterized in that, The polarization-maintaining jumper (7) has two geometrically symmetrical stress structures (72) on its end face so that the laser polarization direction of the polarization-maintaining jumper (7) is kept along the stress structure direction.

3. The switchable pulse polarization-maintaining fiber laser according to claim 2, characterized in that, The two stress structures (72) are symmetrically arranged on both sides of the core channel of the polarization jumper (7).

4. The switchable pulse polarization-maintaining fiber laser according to claim 1, characterized in that, The polarization-maintaining fiber (3) is a polarization-maintaining erbium-doped gain fiber; the core diameter of the polarization-maintaining fiber (3) is 7 μm and the length is 0.8 m.

5. The switchable pulse polarization-maintaining fiber laser according to claim 1, characterized in that, The polarization-maintaining coupler (6) has a first output terminal and a second output terminal. The first output terminal is connected to the common terminal of the polarization-maintaining wavelength division multiplexer (2), and the second output terminal is connected to the input terminal of the pulse state detection device (8). The pulse state detection device (8) is used to detect the pulse state of the optical path passing through the polarization-maintaining coupler (6).

6. A modulation method for a switchable pulse polarization-maintaining fiber laser according to any one of claims 1 to 5, characterized in that, Includes the following steps: The directional carbon nanotube saturable absorber (5) is coupled to the end face of the polarization-maintaining jumper (7). At the same time, the angle between the directional carbon nanotube saturable absorber (5) and the laser polarization direction in the polarization-maintaining jumper (7) is adjusted to generate mode-locked pulses in multiple states. The method for generating mode-locked pulses in multiple states is: The oriented carbon nanotube saturable absorber (5) is coupled to the groove (71) on the end face of the polarization jumper (7), while ensuring that the oriented carbon nanotube saturable absorber (5) covers the core channel of the polarization jumper (7). Adjust the direction of the groove (71) so that the direction of the groove (71) is at an angle to the laser polarization direction in the polarization jumper (7), and generate mode-locked pulses in three states by changing the angle; The specific conditions for generating mode-locked pulses in three states are: When the included angle is 30°, the mode-locking pulse is a soliton mode-locking pulse; Alternatively, when the included angle is 45°, the mode-locking pulse is a harmonic mode-locking pulse; Alternatively, when the included angle is 60°, the mode-locking pulse is a bound-state mode-locking pulse.

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