Slow saturable absorber, hybrid mode locking structure preparation method and pulse laser

Abstract

The invention provides a slow saturable absorber, a preparation method of a mixed mode locking structure and a pulse laser, and the preparation method of the slow saturable absorber comprises the steps: adding selenium powder, sodium hydroxide and copper nitrate trihydrate into deionized water according to a first preset proportion, and obtaining a colloidal suspension; transferring the colloidal turbid liquid into a stainless steel high-pressure kettle for reaction to obtain copper selenide nanosheets; washing the copper selenide nanosheet by using alcohol; and depositing the washed copper selenide nanosheet on a D-type optical fiber, and coupling a laser to the D-type optical fiber in the deposition process to obtain the slow saturable absorber. According to the scheme, the problems that an existing real saturable absorber is low in anti-damage threshold value, mode-locked laser achieved based on the existing real saturable absorber is low in output power and poor in stability and long-term operation stability are solved, and the problems that an existing artificial saturable absorber is difficult to self-start, low in modulation depth and sensitive to polarization are solved.

Description

Technical Field

[0001] The invention relates to the technical field of ultrafast fiber lasers, and in particular to a slow saturable absorber, a hybrid mode-locking structure preparation method and a pulse laser. Background Art

[0002] Due to the advantages of good heat dissipation, higher peak power, narrower pulse width, more ideal conversion efficiency, lower production cost and easy operation, ultrafast mode-locked pulse lasers have wide and in-depth applications in optical communications, laser sensing, micromachining and biomedicine.

[0003] There are currently two ways to achieve ultrafast mode locking in pulsed lasers, namely active debugging and passive modulation. Among them, active debugging requires an external modulator, which is not only inconvenient to operate but also expensive. The passive modulation method is to integrate a device with a nonlinear response to light, namely a saturable absorber, in the resonant cavity of the pulsed laser. This method is simple to operate, low-cost and can be fully optically realized, and has become the current mainstream method.

[0004] Among them, saturable absorbers are divided into real saturable absorbers and artificial saturable absorbers. Real saturable absorbers are some two-dimensional materials with saturable absorption characteristics, and artificial saturable absorbers are mainly fiber structures such as nonlinear polarization rotation and nonlinear multimode interference. Some mainstream two-dimensional materials include graphene, transition metal disulfide, topological insulators and black scales. These materials generally have advantages such as wide working bandwidth, large modulation depth and high carrier concentration, but these materials also face problems such as poor damage threshold, poor stability of ultrafast mode-locked lasers based on them, low output power and poor long-term operation stability. Artificial saturable absorbers have higher damage thresholds and higher output powers due to their all-fiber structures, but they also face problems such as difficulty in self-starting, low debugging depth, poor stability and polarization sensitivity. Summary of the invention

[0005] The present application provides a slow saturable absorber, a method for preparing a hybrid mode-locked structure, and a pulsed laser, in order to solve the problems of low damage threshold of existing real saturable absorbers, low output power of mode-locked lasers based on them, poor stability and long-term operation stability, as well as the problems of difficulty in self-starting, low modulation depth, and polarization sensitivity of existing artificial saturable absorbers.

[0006] In a first aspect, the present application provides a method for preparing a slow saturable absorber, the preparation method comprising:

[0007] Adding selenium powder, sodium hydroxide and copper nitrate trihydrate into deionized water according to a first preset ratio to obtain a colloidal suspension;

[0008] The colloidal suspension is transferred to a stainless steel autoclave for reaction to obtain copper selenide nanosheets;

[0009] Washing the copper selenide nanosheets with alcohol;

[0010] The washed copper selenide nanosheets are deposited on a D-type optical fiber, and a laser is coupled to the D-type optical fiber during the deposition process to obtain a slow saturable absorber.

[0011] Preferably, the preparation method further comprises:

[0012] Adding selenium powder, sodium hydroxide and copper nitrate trihydrate to deionized water in a first preset ratio to obtain a target mixture;

[0013] The target mixture is magnetically stirred for a first preset time to obtain the colloidal suspension.

[0014] Preferably, the preparation method further comprises:

[0015] The colloidal suspension is transferred to a stainless steel autoclave lined with tetrafluoroethylene to react for a second preset time to obtain copper selenide nanosheets.

[0016] Preferably, the preparation method further comprises:

[0017] During the process of depositing the copper selenide nanosheets on the D-type optical fiber, the output power of the D-type optical fiber on which the copper selenide nanosheets are being deposited is monitored, and when the output power of the D-type optical fiber reaches a stable state, the deposition of the copper selenide nanosheets is stopped.

[0018] Preferably, the preparation method further comprises:

[0019] adding a first mole of the selenium powder and a second mole of the copper nitrate trihydrate to a first volume of the deionized water to obtain a mixture;

[0020] A third mole of sodium hydroxide is added to the mixture to obtain the target mixture.

[0021] Preferably, the preparation method further comprises:

[0022] Take a D-type optical fiber with a preset polishing length and a preset polishing depth;

[0023] coupling the laser to the D-type optical fiber to obtain a coupled optical fiber;

[0024] The copper selenide nanosheets suspended in alcohol are dropped onto the surface of the D-type optical fiber to obtain the slow saturable absorber.

[0025] Preferably, the wavelength of the laser is 976 nm and the power is 100 mW.

[0026] In a second aspect, the present application also provides a method for preparing a hybrid locking structure, the preparation method comprising:

[0027] Take a multimode graded-index optical fiber of a first preset length;

[0028] The two ends of the multimode graded-index optical fiber are respectively fused with a common single-mode optical fiber of a second preset length to obtain a target optical fiber;

[0029] The two ends of the target optical fiber are respectively fused with two polarization controllers to obtain a fast saturable absorber;

[0030] The fast saturable absorber and the slow saturable absorber are welded to obtain a hybrid mode-locking structure.

[0031] In a third aspect, the present application further provides a pulsed laser, the pulsed laser comprising a hybrid mode-locked structure, a polarization-independent optical isolator, an erbium-doped gain fiber, a wavelength division multiplexer, a pump source and an output coupler; the hybrid mode-locked structure comprises a slow saturable absorber and a fast saturable absorber, the fast saturable absorber comprises a multimode gradient-index fiber and two polarization controllers;

[0032] The hybrid mode-locked structure, the polarization-independent optical isolator, the erbium-doped gain fiber, the wavelength division multiplexer and the output coupler are sequentially fused in series end to end to form a laser circuit; the polarization-independent optical isolator is fused to the pump source, and the output coupler is fused to the external output port;

[0033] The pump source is configured to provide a basic light source for the laser circuit;

[0034] The wavelength division multiplexer is configured to integrate two wavelengths of light into one optical fiber;

[0035] The erbium-doped gain fiber is configured to generate a population inversion required for lasing;

[0036] The polarization-independent optical isolator is configured to control the optical single-phase output;

[0037] The output coupler is configured to observe laser output quality.

[0038] Preferably, the wavelength division multiplexer is further configured to integrate the basic light source and the excitation laser emitted by the pump source into the same optical fiber;

[0039] The erbium-doped gain fiber is further configured to excite the gain material in the erbium-doped gain fiber to generate a population inversion required for laser.

[0040] As can be seen from the above content, the present application provides a slow saturable absorber, a method for preparing a hybrid mode-locked structure, and a pulsed laser, wherein the method for preparing the slow saturable absorber comprises adding selenium powder, sodium hydroxide, and copper nitrate trihydrate to deionized water according to a first preset ratio to obtain a colloidal suspension; transferring the colloidal suspension to a stainless steel autoclave for reaction to obtain copper selenide nanosheets; washing the copper selenide nanosheets with alcohol; depositing the washed copper selenide nanosheets on a D-type optical fiber, and coupling a laser to the D-type optical fiber during the deposition process to obtain a slow saturable absorber. The present application solves the problems of low damage threshold of existing real saturable absorbers, low output power of mode-locked lasers based on them, poor stability and long-term operation stability, and the problems of difficulty in self-starting, low modulation depth, and polarization sensitivity of existing artificial saturable absorbers through the above scheme. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solution of the present application, the drawings required for use in the embodiments are briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0042] Figure 1 A flowchart of a method for preparing a slow saturable absorber for this application;

[0043] Figure 2 This is a flow chart of a method for preparing a hybrid locking structure of the present application;

[0044] Figure 3 A schematic diagram of a pulsed laser for this application;

[0045] Figure 4 is the spectrum of pulsed laser;

[0046] Figure 5 is the output waveform of the pulse laser;

[0047] Figure 6 is the RF diagram of the pulsed laser;

[0048] Figure 7 is the pulse width diagram of the pulsed laser;

[0049] Figure 8 is the input and output power diagram of the pulsed laser;

[0050] Fig. 9 is the spectrum stability diagram of the pulsed laser;

[0051] Fig.10 The output spectra of pulsed lasers based on slow saturable absorbers, fast saturable absorbers and hybrid mode-locked structures are compared.

[0052] Fig.11 Comparison of the output pulse widths of pulse lasers based on slow saturable absorbers, fast saturable absorbers, and hybrid mode-locked structures. DETAILED DESCRIPTION

[0053] 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.

[0054] It should be noted that the brief description of terms in this application is only for the convenience of understanding the embodiments described below, and is not intended to limit the embodiments of this application. Unless otherwise specified, these terms should be understood according to their common and usual meanings.

[0055] The terms "first", "second", "third", etc. in the specification and claims of this application and the above drawings are used to distinguish similar or similar objects or entities, and do not necessarily mean to limit a specific order or sequence, unless otherwise noted. It should be understood that the terms used in this way can be interchangeable under appropriate circumstances.

[0056] Figure 1 The present invention provides a flow chart of a method for preparing a slow saturable absorber.

[0057] See also Figure 1 It can be seen that this embodiment provides a method for preparing a slow saturable absorber, and the preparation method includes:

[0058] S100, adding selenium powder, sodium hydroxide and copper nitrate trihydrate to deionized water according to a first preset ratio to obtain a colloidal suspension. Specifically, in this embodiment, it is necessary to add selenium powder, sodium hydroxide and copper nitrate trihydrate to deionized water according to a first preset ratio to obtain a target mixture; and magnetically stirring the target mixture for a first preset time to obtain the colloidal suspension.

[0059] Exemplarily, 10 mmol of selenium (Se) powder and 10 mmol of copper nitrate trihydrate [Cu(No3)2·3H2O] are added to 60 mL of deionized water, 130 mmol of sodium hydroxide (NaOH) is added to the mixture, and the mixture is magnetically stirred for about 2 hours to obtain a colloidal suspension.

[0060] S200, transferring the colloidal suspension to a stainless steel autoclave for reaction to obtain copper selenide nanosheets. Specifically, in this embodiment, the colloidal suspension is transferred to a stainless steel autoclave lined with tetrafluoroethylene for reaction for a second preset time to obtain copper selenide nanosheets.

[0061] Exemplarily, 60 mL of the suspension was transferred into a 100 mL stainless steel autoclave lined with tetrafluoroethylene and reacted at 160° C. for 12 hours to obtain a black precipitate (copper selenide nanosheets).

[0062] S300, washing the copper selenide nanosheets with alcohol. Specifically, in this embodiment, the black precipitate (copper selenide nanosheets) is formed at the end of the reaction and washed twice with alcohol. Finally, the prepared copper selenide nanosheets are stored in an alcohol solution for further use.

[0063] S400, depositing the washed copper selenide nanosheets on a D-type optical fiber, and coupling a laser to the D-type optical fiber during the deposition process to obtain a slow saturable absorber. Specifically, in this embodiment, a D-type optical fiber with a preset polishing length and a preset polishing depth is taken; the laser is coupled to the D-type optical fiber to obtain a coupled optical fiber; and the copper selenide nanosheets suspended in alcohol are dropped onto the surface of the D-type optical fiber to obtain the slow saturable absorber.

[0064] It should be noted that, in the process of depositing the copper selenide nanosheets on the D-type optical fiber, it is necessary to monitor the output power of the D-type optical fiber on which the copper selenide nanosheets are being deposited, and stop depositing the copper selenide nanosheets when the output power of the D-type optical fiber reaches a stable state.

[0065] Exemplarily, a D-type optical fiber with a side polishing length of 17 mm and a side polishing depth of about 53 μm is used to deposit CuSe nanosheets. During the deposition process, a laser with a wavelength of 976 nm and a power of 100 mW is coupled to the D-type optical fiber. Then the copper selenide nanosheets suspended in an alcohol solution are slowly dropped onto the surface of the D-type optical fiber. As the copper selenide nanosheets are gradually deposited on the D-type optical fiber, changes in the output power can be observed. During the deposition process, the output power gradually decreases. Once the output power reaches a stable state, it is confirmed that the copper selenide nanosheets are fully deposited on the D-type optical fiber.

[0066] Figure 2 This is a flow chart of a method for preparing a hybrid locking structure of the present application.

[0067] See also Figure 2 It can be seen that this embodiment also provides a method for preparing a hybrid locking structure, the preparation method comprising:

[0068] S500, taking a multimode graded-index optical fiber of a first preset length;

[0069] S600, respectively fusing two ends of the multimode graded-index optical fiber with common single-mode optical fibers of a second preset length to obtain a target optical fiber;

[0070] S700, fusing two polarization controllers at two ends of the target optical fiber to obtain a fast saturable absorber;

[0071] S800, welding the fast saturable absorber and the slow saturable absorber to obtain a hybrid mode-locking structure.

[0072] Specifically, in this embodiment, steps S500 to S800 all need to be prepared on the basis of the slow saturable absorber prepared in steps S100 to S400.

[0073] For example, a 10-cm-long multimode gradient-index optical fiber is taken, and a certain length of ordinary single-mode optical fiber is fused to its two ends, and then two polarization controllers are installed on the single-mode optical fibers at both ends of the multimode optical fiber. Finally, one end of the single-mode optical fiber is fused with the D-type optical fiber with copper selenide nanosheets deposited to form a hybrid mode-locked structure.

[0074] The prepared slow saturable absorber and fast saturable absorber are integrated as a whole into the resonant cavity of a pulsed laser to generate high-performance ultrafast mode-locked laser.

[0075] Figure 3 This is a schematic diagram of a pulsed laser according to the present application.

[0076] See also Figure 3 It can be seen that this embodiment also provides a pulse laser. The core improvement of the pulse laser provided by this embodiment lies in the hybrid mode-locked structure obtained by step S100 to step S800. The pulse laser includes a hybrid mode-locked structure 1, a polarization-independent optical isolator 2, an erbium-doped gain fiber 3, a wavelength division multiplexer 4, a pump source 5 and an output coupler 6; the hybrid mode-locked structure 1 includes a slow saturable absorber 11 and a fast saturable absorber 12, and the fast saturable absorber 12 includes a multimode gradient-index fiber 121 and two polarization controllers 122;

[0077] The hybrid mode-locked structure 1, the polarization-independent optical isolator 2, the erbium-doped gain fiber 3, the wavelength division multiplexer 4 and the output coupler 6 are sequentially fused end to end in series to form a laser circuit; the polarization-independent optical isolator 2 is fused to the pump source 5, and the output coupler 6 is fused to the external output port;

[0078] The pump source 5 is configured to provide a basic light source for the laser circuit; the wavelength division multiplexer 4 is configured to integrate two wavelengths of light into one optical fiber; the erbium-doped gain fiber 3 is configured to generate the population inversion required for laser; the polarization-independent optical isolator 2 is configured to control the single-phase output of light; and the output coupler 6 is configured to observe the laser output quality.

[0079] Specifically, in this embodiment, the pump source 5 provides a basic light source for exciting the laser for the pulse laser, and the wavelength division multiplexer 4 is used to integrate the light of two wavelengths into one optical fiber, that is, to integrate the light source of the pump source 5 and the excited laser into the same optical fiber; the function of the erbium-doped gain fiber 3 is to excite the material in the gain fiber through the light emitted by the pump source 5 to reverse the number of particles required to generate the laser in the gain fiber; the polarization-independent optical isolator 2 is used to control the single-phase output of light; the output coupler 6 is used to observe the laser output quality, 90% of the light continues to circulate in the ring cavity, and 10% of the laser light is output from the ring cavity.

[0080] The polarization-independent optical isolator 2 is a device that allows light to pass through from only one direction and blocks light from the opposite direction. It is manufactured based on the non-reciprocity principle of Faraday rotation and uses the Faraday effect of magneto-optical crystals to achieve directional control of light.

[0081] The erbium-doped gain fiber 3 refers to an optical fiber doped with a small amount of the rare earth element erbium. This optical fiber can amplify optical signals within the range of 1550nm. When pumped by an external light source, the gain of the optical signal can be significantly improved. The main function of the erbium-doped gain fiber 3 in the pulse laser is to amplify the laser pulse. In the pulse laser system, the erbium-doped gain fiber 3 is usually used as a part of the power amplifier, and the input seed laser pulse is amplified to the required energy level through the excitation of the pump light. This amplification process not only improves the power of the laser pulse, but also maintains a good beam quality, so that the pulse laser has higher accuracy and reliability in long-distance measurement and remote sensing applications. Since the erbium-doped fiber laser operates in the 1.55μm band, the laser in this band is safe for the human eye, and the atmosphere has high laser transparency to this band. Therefore, the erbium-doped gain fiber laser is particularly suitable for long-range laser radar applications in the atmosphere, such as remote sensing measurement of atmospheric wind speed, observation of atmospheric chemical small molecules, and ranging and identification of remote targets.

[0082] The wavelength division multiplexer 4 mainly plays two roles in the pulse laser. First, it can merge multiple laser signals of different wavelengths into a wavelength multiplexed signal output, thereby increasing the output power of the laser. This is undoubtedly a huge advantage for application scenarios that require high-power output. Through wavelength division multiplexing technology, the outputs of multiple lasers can be combined to form a laser beam with more powerful power and richer wavelengths, thereby meeting various complex application requirements. Secondly, the wavelength division multiplexer 4 can also realize the simultaneous input and output of multiple signals, which is also of great significance in pulse lasers. In some application scenarios that need to process multiple signals at the same time, the wavelength division multiplexer 4 can enable multiple signals to be transmitted in the same optical fiber, greatly improving the capacity and efficiency of optical fiber communication. This not only reduces the complexity and cost of the system, but also improves the stability and reliability of the system.

[0083] The application of wavelength division multiplexer 4 in pulsed lasers also involves some specific technical implementations. For example, in order to realize the merging and separation of laser signals of different wavelengths, devices such as grating wavelength division multiplexer or fiber wavelength division multiplexer are needed. These devices can realize efficient processing of laser signals of different wavelengths through precise optical design and manufacturing processes. At the same time, it is also necessary to cooperate with spectrometers, real-time OSC and PD and other equipment for spectral analysis and time domain analysis to ensure the performance and stability of the system.

[0084] Exemplarily, the prepared slow saturable absorber and fast saturable absorber are integrated into the resonant cavity by fiber fusion splicing, and the performance of the pulsed laser is tested. The test results are displayed by an oscilloscope, a spectrometer, an optical power meter and an autocorrelator.

[0085] The test results are as follows Figure 4-8 When the power of pump source 5 is 87 mW, the establishment of mode-locked operation can be observed, and self-starting can be achieved, and its polarization sensitivity is very low. Figure 4 This is the output spectrum of the pulsed laser. It can be seen that its central wavelength is 1568.2nm and the 3dB bandwidth is 8.1nm. Figure 5 This is the output time domain diagram of the pulse laser. It can be seen that the pulse interval is 56.1ns, corresponding to the fundamental frequency of 17.8MHz. Figure 6 This is the RF diagram of the hybrid mode-locked pulse laser. It can be seen that the signal-to-noise ratio is as high as 82dB, indicating that the pulse laser has very high stability. Figure 7 The autocorrelation curve is shown, and the pulse width is only 762fs. Figure 8The relationship between the output power and input power of the pulsed laser is shown. The conversion efficiency is 3.9%. The maximum power of pump source 5 is 902mW, the output power is as high as 32.5mW, and the corresponding peak power is as high as 2396W. This proves that the pulsed laser has a high damage threshold. In order to test the long-term working stability of the pulsed laser, Fig. 9 As shown in the figure, the output spectrum is recorded every 4 hours at the highest power of pump source 5. It can be seen that within 24 hours, the output spectrum has no jitter or deformation, indicating the super long-term working stability of the pulsed laser. In addition, in order to further highlight the advantages of hybrid mode locking, we also use slow saturable absorbers and fast saturable absorbers to achieve mode locking operation, and compare them with the performance of hybrid mode locking, as shown in the figure. Figure 10-11 shown. Fig.10 The output spectrum comparison of the three mode-locking methods is shown. It can be seen that the spectrum width of the hybrid mode-locked output is the widest, and the 3dB bandwidth is nearly doubled. Fig.11 The output pulse width comparison of the three mode-locking methods is shown. It can be seen that the pulse width of the hybrid mode-locking output is the narrowest, only 762fs. We compared the signal-to-noise ratio of the three mode-locking methods and found that the signal-to-noise ratio of the hybrid mode-locking is as high as 82dB, which is much higher than other methods, proving the great advantages and application potential of the hybrid mode-locking.

[0086] in, Figure 4-Figure 11 In the formula, Wavelength is wavelength, Intensity is intensity, Time is time, Frequency is frequency, Pump power is pump power, Output power is output power, FWHM is bandwidth, λ c is the center wavelength, τ is the pulse width, Span is the span, RBW is the resolution, Experimental data is the experimental data, and Fitting data is the fitting data.

[0087] This embodiment has the following advantages:

[0088] The mode-locked laser of the present invention has the advantages of good heat dissipation, small size, low cost, etc., and is very suitable for commercial use; and has good stability, can achieve long-term operation, and has the advantages of high repetition frequency, narrow pulse width and large output power, so it can be used as an ultrafast pulse laser source in the field of interaction between femtosecond laser and micro-nano materials, medical processing, laser communication, ablation cooling material removal and optical frequency combing and other fields.

[0089] This embodiment uses CuSe nanosheets and nonlinear multimode interference composite structures for the first time to realize a self-starting ultra-high stability hybrid mode-locked fiber laser. The hybrid mode-locked laser has the advantages of high repetition frequency, narrow pulse width, high output power, good working stability, etc., filling the gap that passive mode-locked fiber lasers are difficult to obtain good output characteristics and highly stable pulsed lasers.

[0090] The technical solution of this embodiment optimizes the preparation method of the saturable absorber. The composite structure of the fast saturable absorber and the slow saturable absorber overcomes the limitations of the saturable absorber and is an attractive way because this method can not only exhibit the characteristics of the two saturable absorbers, but also has better effects than both saturable absorbers, thus solving the technical problem that people have always been eager to solve but have never succeeded in solving.

[0091] The technical solution of this embodiment overcomes technical prejudice: it overcomes the problems of low damage threshold, low modulation depth, wide pulse width, and low stability based on two-dimensional materials as saturable absorption devices. It also overcomes the problems of poor stability, difficulty in achieving self-starting, and polarization sensitivity based on nonlinear multi-mode interference locking.

[0092] Similar parts between the embodiments provided in this application can be referenced to each other. The specific implementation methods provided above are only a few examples under the general concept of this application and do not constitute a limitation on the protection scope of this application. For those skilled in the art, any other implementation methods expanded based on the scheme of this application without creative work belong to the protection scope of this application.

Claims

1. A method for preparing a slow saturable absorber, characterized in that: The preparation method comprises: Adding selenium powder, sodium hydroxide and copper nitrate trihydrate into deionized water according to a first preset ratio to obtain a colloidal suspension; The colloidal suspension is transferred to a stainless steel autoclave for reaction to obtain copper selenide nanosheets; Washing the copper selenide nanosheets with alcohol; The washed copper selenide nanosheets are deposited on a D-type optical fiber, and a laser is coupled to the D-type optical fiber during the deposition process to obtain a slow saturable absorber.

2. The method for preparing a slow saturable absorber according to claim 1, characterized in that: The preparation method further comprises: Adding selenium powder, sodium hydroxide and copper nitrate trihydrate to deionized water in a first preset ratio to obtain a target mixture; The target mixture is magnetically stirred for a first preset time to obtain the colloidal suspension.

3. The method for preparing a slow saturable absorber according to claim 1, characterized in that: The preparation method further comprises: The colloidal suspension is transferred to a stainless steel autoclave lined with tetrafluoroethylene to react for a second preset time to obtain copper selenide nanosheets.

4. The method for preparing a slow saturable absorber according to claim 1, characterized in that: The preparation method further comprises: During the process of depositing the copper selenide nanosheets on the D-type optical fiber, the output power of the D-type optical fiber on which the copper selenide nanosheets are being deposited is monitored, and when the output power of the D-type optical fiber reaches a stable state, the deposition of the copper selenide nanosheets is stopped.

5. The method for preparing a slow saturable absorber according to claim 3, characterized in that: The preparation method further comprises: adding a first mole of the selenium powder and a second mole of the copper nitrate trihydrate to a first volume of the deionized water to obtain a mixture; A third mole of sodium hydroxide is added to the mixture to obtain the target mixture.

6. The method for preparing a slow saturable absorber according to claim 1, characterized in that: The preparation method further comprises: Take a D-type optical fiber with a preset polishing length and a preset polishing depth; coupling the laser to the D-type optical fiber to obtain a coupled optical fiber; The copper selenide nanosheets suspended in alcohol are dropped onto the surface of the D-type optical fiber to obtain the slow saturable absorber.

7. The method for preparing a slow saturable absorber according to claim 1, characterized in that: The wavelength of the laser is 976nm and the power is 100mW.

8. A method for preparing a hybrid locking structure, characterized in that: The method for preparing the closed-mode structure is prepared by using the slow saturable absorber obtained in any one of claims 1 to 7, and the preparation method comprises: Take a multimode graded-index optical fiber of a first preset length; The two ends of the multimode graded-index optical fiber are respectively fused with a common single-mode optical fiber of a second preset length to obtain a target optical fiber; The two ends of the target optical fiber are respectively fused with two polarization controllers to obtain a fast saturable absorber; The fast saturable absorber and the slow saturable absorber are welded to obtain a hybrid mode-locking structure.

9. A pulse laser, characterized in that: The mode-locked pulse laser comprises the hybrid mode-locked structure according to claim 8, the pulse laser comprises a hybrid mode-locked structure (1), a polarization-independent optical isolator (2), an erbium-doped gain fiber (3), a wavelength division multiplexer (4), a pump source (5) and an output coupler (6); the hybrid mode-locked structure (1) comprises a slow saturable absorber (11) and a fast saturable absorber (12), the fast saturable absorber (12) comprises a multimode gradient-index fiber (121) and two polarization controllers (122); The hybrid mode-locked structure (1), the polarization-independent optical isolator (2), the erbium-doped gain optical fiber (3), the wavelength division multiplexer (4) and the output coupler (6) are sequentially fused end to end in series to form a laser circuit; the polarization-independent optical isolator (2) is fused to the pump source (5), and the output coupler (6) is fused to an external output port; The pump source (5) is configured to provide a basic light source for the laser circuit; The wavelength division multiplexer (4) is configured to integrate two wavelengths of light into one optical fiber; The erbium-doped gain fiber (3) is configured to generate a population inversion required for lasing; The polarization-independent optical isolator (2) is configured to control the single-phase output of light; The output coupler (6) is configured to observe the laser output quality.

10. A pulse laser according to claim 9, characterized in that: The wavelength division multiplexer (4) is also configured to integrate the basic light source and the excitation laser emitted by the pump source (5) into the same optical fiber; The erbium-doped gain fiber (3) is also configured to excite the gain material in the erbium-doped gain fiber (3) to generate a population inversion required for laser light.

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