Ultra-short pulse generation method and device based on single-mode and dual-mode fiber laser

Abstract

The invention discloses an ultrashort pulse generation method and device based on a single-mode and dual-mode fiber laser. The method comprises the steps that pump light enters an erbium-doped fiber through a dual-mode wavelength division multiplexer, erbium ions are excited to an upper energy level to form population inversion, stimulated radiation is generated, and a laser signal is amplified; through the dual-mode optical fiber coupler, a small amount of laser is output to the outside of the cavity for measurement, and the remaining laser continues to oscillate in the cavity; the polarization state of the remaining laser in the resonant cavity is adjusted through a polarization controller so as to ensure that the laser is stably transmitted in the resonant cavity, and mode locking of the laser is realized by using a carbon nanotube saturable absorber so as to ensure that stable ultrashort pulses are generated; the laser passes through the dual-mode isolator to ensure that the light pulse runs unidirectionally in the resonant cavity and enters the dual-mode wavelength division multiplexer again to form circulation; and through periodic mode coupling, a stable mode-locked pulse is finally formed. The device is suitable for a few-mode optical fiber mode-locked laser, is simple in structure, is easy to construct and maintain, and is extremely high in stability and reliability.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber lasers, and more specifically, relates to an ultrashort pulse generation method and device based on single-mode and double-mode optical fiber lasers. Background Art

[0002] In recent years, people have conducted a lot of research on single-mode fiber lasers and found that the output pulse energy of the laser is close to the limit due to the core diameter and nonlinear effects of single-mode optical fibers. Therefore, multi-mode / few-mode fiber lasers have begun to enter the public's field of vision. Multi-mode / few-mode optical fibers have a larger core diameter and can support multiple spatial modes, which can greatly increase the data capacity of optical communications and the pulse energy of fiber lasers. Therefore, multi-mode / few-mode fiber lasers have become one of the research hotspots in recent years.

[0003] Few-mode fiber lasers are a good platform for studying the dynamic characteristics of spatiotemporal solitons. From an application perspective, few-mode fiber lasers can be used to generate ultrafast column vector light and vortex light. However, current research on few-mode fiber lasers is mainly focused on experimental exploration, and theoretical research on intracavity pulse formation and evolution is still in its infancy.

[0004] In 2017, Wright et al. first realized spatiotemporal mode locking in a multimode fiber laser. The gain fiber plays a spatial filtering role and causes the total energy of the multimode pulse to reach a gain saturation state, which largely eliminates the interaction between different transverse modes caused by gain. The graded-index multimode fiber ensures that the mode dispersion is small and reduces the time domain walk-off of different modes. Experimental results show that strong spectral filtering and spatial filtering can offset the mode dispersion and chromatic dispersion in multimode fiber lasers. During spatiotemporal mode locking, multiple transverse modes completely overlap in the time domain, greatly improving the output pulse energy of the fiber laser.

[0005] At present, all reports on multi-mode / few-mode fiber mode-locked lasers are achieved through the method of spatiotemporal mode locking, but whether the stable mode locking of multi-mode / few-mode fiber lasers must achieve spatial mode locking still needs further exploration. Summary of the invention

[0006] In view of the above defects or improvement needs of the prior art, the present invention provides an ultrashort pulse generation method and device based on single-mode and dual-mode fiber lasers, wherein a dual-mode wavelength division multiplexer, an erbium-doped fiber, the dual-mode fiber coupler, the polarization controller, the carbon nanotube saturable absorber and the dual-mode isolator are sequentially arranged and welded through a step-refractive index dual-mode fiber to form a ring-shaped laser resonant cavity, which has a simple structure and is easy to construct and maintain, simplifies the operation steps, and improves the feasibility and popularity of the system. The precise coordination and efficient collaboration between the components ensure that the laser resonant cavity exhibits extremely high stability and reliability, and ensures the continuity and consistency of the generation of ultrashort pulses. Carbon nanotubes are used as saturable absorbers, and their unique physical properties make them very suitable for light absorption, It performs well in saturation effect and nonlinear optical response, providing strong support for the generation of ultrashort pulses. Combined with the characteristics of dual-mode optical fiber, this method also shows significant advantages in optical signal transmission, mode control and noise suppression, laying a solid foundation for the precise regulation and output of ultrashort pulses. Compared with traditional multimode optical fiber, few-mode optical fiber can better control the transmission mode and dispersion of optical signals, thereby providing higher bandwidth and lower loss. When the mode-locked laser is combined with the few-mode optical fiber, more efficient and stable optical signal generation and transmission can be achieved. By utilizing the nonlinear effects and interactions between modes in the optical fiber, and by precisely controlling the distribution and dynamic behavior of the light field in the laser cavity, stable locking of multiple optical fiber modes is achieved.

[0007] In order to achieve the above object, according to one aspect of the present invention, a method for generating ultrashort pulses based on single-mode and double-mode fiber lasers is provided, comprising the following steps:

[0008] S100: The pump light enters the erbium-doped fiber through a dual-mode wavelength division multiplexer, excites erbium ions to the upper energy level to form a population inversion, generates stimulated radiation, and amplifies the laser signal;

[0009] S200: The laser passes through the dual-mode fiber coupler, a small amount of laser is output to the outside of the cavity for measurement, and the remaining laser continues to oscillate in the cavity;

[0010] S300: The remaining laser light is used to adjust the polarization state in the resonant cavity through the polarization controller to ensure stable transmission of the laser light in the cavity. The carbon nanotube saturable absorber is used to achieve mode locking of the laser light to ensure the generation of stable ultrashort pulses.

[0011] S400: The laser passes through a dual-mode isolator to ensure that the optical pulse runs unidirectionally in the resonant cavity, and re-enters the dual-mode wavelength division multiplexer to form a cycle;

[0012] S500: After periodic mode coupling, a stable mode-locked pulse is finally formed.

[0013] Furthermore, the laser enters the step-index dual-mode fiber through the erbium-doped fiber to produce a mode coupling effect, and part of the fundamental mode energy is converted into a higher-order mode.

[0014] Furthermore, the laser enters the erbium-doped fiber through the step-index dual-mode fiber to produce a mode coupling effect, and the high-order mode is converted into the fundamental mode.

[0015] Furthermore, during the conversion between the fundamental mode and the higher-order mode, the mode coupling coefficient between the erbium-doped fiber and the step-index dual-mode fiber is calculated:

[0016]

[0017] Among them, C 11 LP in Erbium-doped fiber 01 Mode and LP in step-index dual-mode fiber 01 Coupling coefficient between modes;

[0018] C 12 LP in Erbium-doped fiber 01 Mode and LP in step-index dual-mode fiber 11 Coupling coefficient between modes;

[0019] F(x,y) is the LP in the erbium-doped fiber 01 Mode field distribution of the mode;

[0020] F1(x,y) and F2(x,y) are the LP 01 and LP 11 Mode field distribution of the mode;

[0021] LP 01 is the fundamental mode in the optical fiber;

[0022] LP 11 are the higher-order modes in the optical fiber.

[0023] Furthermore, a displacement distance r0 is introduced into the erbium-doped fiber and the step-index dual-mode fiber, and the mode field distribution after the fiber is lateral shifted is obtained as F(x-x0,y-y0), where r0 = (x0 2 +y0 2 ) 1 / 2 , the LP in the erbium-doped fiber 01 The mode field distribution F(x,y) of the mode is shifted by a spatial distance r0 relative to the mode field distribution of the two modes in the step-index dual-mode fiber, which is equivalent to realizing the dislocation fusion operation, and the mode coupling coefficient after the dislocation is obtained:

[0024]

[0025] Among them, r0 is the dislocation distance;

[0026] F(x-x0,y-y0) is the LP in the erbium-doped fiber 01 Mode field distribution after lateral translation of the mode.

[0027] Furthermore, when the pulse enters the step-index dual-mode fiber from the erbium-doped fiber, the light fields of the two modes in the step-index dual-mode fiber are obtained as follows:

[0028] u1=C 11 u,

[0029] u2=C 12 u;

[0030] Where u is the LP in the erbium-doped fiber 01 the amplitude of the mode;

[0031] u1 is the LP in the step-index dual-mode fiber 01 the amplitude of the mode;

[0032] u2 is the LP in the step-index dual-mode fiber 11 The amplitude of the mode.

[0033] Furthermore, when the pulse enters the erbium-doped fiber from the step-index dual-mode fiber, the LP in the erbium-doped fiber is obtained. 01 The light field of the mode is:

[0034] u=C 11 u1+C 12 u2.

[0035] Where u is the LP in the erbium-doped fiber 01 the amplitude of the mode;

[0036] u1 is the LP in the step-index dual-mode fiber 01 the amplitude of the mode;

[0037] u2 is the LP in the step-index dual-mode fiber 11 The amplitude of the mode.

[0038] According to another aspect of the present invention, there is provided an ultrashort pulse generating device based on a single-mode and dual-mode fiber laser, comprising: a pump source, a dual-mode wavelength division multiplexer, an erbium-doped fiber, a dual-mode fiber coupler, a polarization controller, a carbon nanotube saturable absorber, a dual-mode isolator and a step-index dual-mode fiber;

[0039] The dual-mode wavelength division multiplexer, the erbium-doped optical fiber, the dual-mode optical fiber coupler, the polarization controller, the carbon nanotube saturable absorber and the dual-mode isolator are sequentially fused through a step-refractive-index dual-mode optical fiber to form a ring-shaped laser resonant cavity.

[0040] Furthermore, the wavelength of the pump light emitted by the pump source is 976 / 980 nm.

[0041] Furthermore, the core diameter of the step-refractive-index dual-mode optical fiber is 14 μm, and the refractive index difference between the core and the cladding is 0.005.

[0042] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0043] 1. The present invention discloses an ultrashort pulse generation method based on single-mode and double-mode fiber lasers. Compared with traditional multimode fiber, few-mode fiber can better control the transmission mode and dispersion of optical signals, thereby providing higher bandwidth and lower loss. When the mode-locked laser is combined with the few-mode fiber, more efficient and stable optical signal generation and transmission can be achieved.

[0044] 2. The present invention provides an ultrashort pulse generation method based on single-mode and double-mode fiber lasers, which utilizes the nonlinear effects in the optical fiber and the interaction between modes, and achieves stable locking of multiple optical fiber modes by precisely controlling the distribution and dynamic behavior of the light field in the laser cavity.

[0045] 3. The present invention provides an ultrashort pulse generation method based on single-mode and dual-mode fiber lasers. The dual-mode wavelength division multiplexer, erbium-doped fiber, the dual-mode fiber coupler, the polarization controller, the carbon nanotube saturable absorber and the dual-mode isolator are sequentially arranged and welded through a step-refractive index dual-mode fiber to form a ring-shaped laser resonant cavity. The structure is simple, easy to build and maintain, and the operation steps are simplified, and the feasibility and popularity of the system are improved. With the precise coordination and efficient collaboration among the components, the laser resonant cavity exhibits extremely high stability and reliability, ensuring the continuity and consistency of ultrashort pulse generation. Carbon nanotubes are used as saturable absorbers. Their unique physical properties excel in light absorption, saturation effect and nonlinear optical response, providing strong support for the generation of ultrashort pulses. Combined with the characteristics of dual-mode optical fibers, the method also exhibits significant advantages in optical signal transmission, mode control and noise suppression, laying a solid foundation for the precise regulation and output of ultrashort pulses. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A schematic diagram of a process for generating ultrashort pulses based on single-mode and double-mode fiber lasers according to an embodiment of the present invention;

[0047] Figure 2 This is a schematic structural diagram of an ultrashort pulse generating device based on single-mode and double-mode fiber lasers according to an embodiment of the present invention;

[0048] Figure 3The pulse shape at the output coupler evolves with the number of turns when the misalignment distance is 0.8 μm in an ultrashort pulse generation method based on single-mode and double-mode fiber lasers according to an embodiment of the present invention;

[0049] Figure 4 This is a spectrum diagram of two modes after reaching a steady state in an ultrashort pulse generation method based on a single-mode and dual-mode fiber laser according to an embodiment of the present invention;

[0050] Figure 5 A pulse shape diagram of two modes in a method for generating ultrashort pulses based on single-mode and double-mode fiber lasers in an embodiment of the present invention in logarithmic coordinates;

[0051] Figure 6 This is a mode-locked pulse spectrum diagram of an ultrashort pulse generating device based on a single-mode and dual-mode fiber laser according to an embodiment of the present invention when the pump power is 64mW;

[0052] Figure 7 This is a mode-locked pulse spectrum diagram of an ultrashort pulse generating device based on a single-mode and dual-mode fiber laser according to an embodiment of the present invention when the pump power is 66mW;

[0053] Figure 8 This is a mode-locked pulse spectrum diagram of an ultrashort pulse generating device based on a single-mode and double-mode fiber laser in an embodiment of the present invention when the misalignment distance is small;

[0054] Fig. 9 This is a mode-locked pulse spectrum diagram of an ultrashort pulse generating device based on single-mode and double-mode fiber lasers at a large misalignment distance according to an embodiment of the present invention.

[0055] In all the drawings, the same reference numerals represent the same technical features, specifically: 1-pump source, 2-dual-mode wavelength division multiplexer, 3-erbium-doped fiber, 4-dual-mode fiber coupler, 5-polarization controller, 6-carbon nanotube saturable absorber, 7-dual-mode isolator, 8-step refractive index dual-mode fiber. DETAILED DESCRIPTION

[0056] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0057] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0058] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0059] In this patent, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of more restrictions, the elements defined by the sentence "includes..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0060] like Figure 1 As shown, an embodiment of the present invention provides a method for generating ultrashort pulses based on single-mode and dual-mode fiber lasers, which specifically includes the following steps:

[0061] S100: The pump light enters the erbium-doped optical fiber 3 through the dual-mode wavelength division multiplexer 2, excites the erbium ions to the upper energy level to form a population inversion, generates stimulated radiation, and amplifies the laser signal;

[0062] A pump source 1 with a specific wavelength is used to emit high-intensity pump light. The pump light passes through a precisely designed dual-mode wavelength division multiplexer 2. The dual-mode wavelength division multiplexer 2 can couple optical signals of different wavelengths into the same optical fiber while avoiding interference from light of other wavelengths. In the erbium-doped optical fiber 3, the pump light interacts with the erbium ions to excite the erbium ions from the ground state to the upper energy level. With the continuous action of the pump light, the number of erbium ions in the upper energy level gradually increases, forming a population inversion state, that is, the number of particles in the upper energy level is greater than that in the lower energy level. When the population inversion reaches a certain level, stimulated radiation will occur between the erbium ions, releasing light of the same wavelength as the signal light, thereby amplifying the signal light.

[0063] S200: The laser passes through the dual-mode fiber coupler 4, a small amount of laser is output to the outside of the cavity for measurement, and the remaining laser continues to oscillate in the cavity;

[0064] A fiber coupler is a device that can transmit one or more optical signals from one optical fiber to another. The generated laser is distributed through a dual-mode fiber coupler 4. A small part of the laser is output outside the cavity. The output pulse optical performance is tested by a spectrometer, oscilloscope and other equipment to monitor the operating status of the laser. The remaining laser continues to oscillate in the resonant cavity. Through multiple reflections and amplification, the intensity and stability of the laser are further increased.

[0065] S300: The remaining laser light is adjusted in polarization state in the resonant cavity by the polarization controller 5 to ensure stable transmission of the laser light in the cavity, and the carbon nanotube saturable absorber 6 is used to realize mode locking operation of the laser light to ensure the generation of stable ultrashort pulses;

[0066] The polarization controller 5 is used to finely adjust the polarization state of the laser in the resonant cavity. The adjustment of the polarization controller 5 can ensure the stable transmission of the laser in the resonant cavity. Since the laser may be affected by various factors during the transmission process (such as the bending of the optical fiber, temperature changes, etc.), the polarization state changes, thereby affecting the stability and performance of the laser. Through the polarization controller 5, the polarization state of the laser can be adjusted to keep it in the optimal state, reducing the loss caused by polarization mismatch. The carbon nanotube saturable absorber 6 is introduced into the resonant cavity. The absorber has the characteristic of being sensitive to the laser intensity. When the laser intensity reaches a certain level, the carbon nanotube saturable absorber 6 will be saturated, and the absorption of the laser will be reduced, thereby realizing the mode-locking operation of the laser. The mode-locking operation can ensure the generation of stable ultra-short pulses and improve the performance of the laser.

[0067] S400: The laser passes through the dual-mode isolator 7 to ensure that the optical pulse runs unidirectionally in the resonant cavity, and re-enters the dual-mode wavelength division multiplexer 2 to form a cycle;

[0068] The laser passes through the dual-mode isolator 7, which is a device that only allows unidirectional transmission of optical signals. It is used to ensure that the optical pulse runs unidirectionally in the resonant cavity. The unidirectional operation can avoid unnecessary reflection and interference of the optical pulse in the resonant cavity, and improve the stability and efficiency of the laser. The laser passes through the dual-mode isolator 7 to ensure that the optical pulse is always transmitted in one direction, and re-enters the dual-mode wavelength division multiplexer 2, and enters the erbium-doped optical fiber 3 together with the pump light for amplification. This process is continuously circulated in the resonant cavity to form a stable laser oscillation until a stable mode-locked pulse is formed.

[0069] S500: After periodic mode coupling, a stable mode-locked pulse is finally formed.

[0070] In the resonant cavity, after multiple cycles and amplifications, the intensity and stability of the laser gradually increase. At the same time, the laser undergoes a periodic mode coupling process in the resonant cavity, that is, the energy exchange and redistribution between different modes. This process helps to optimize the output characteristics of the laser, such as wavelength, pulse width, etc. Finally, under the action of the carbon nanotube saturable absorber 6, the laser forms a stable mode-locked pulse output. The mode-locked pulse has an ultra-short pulse width and stable frequency characteristics, and is suitable for various high-precision and high-speed laser applications.

[0071] Furthermore, the laser enters the step-index dual-mode fiber 8 through the erbium-doped fiber 3 to produce a mode coupling effect, and part of the fundamental mode energy is converted into a higher-order mode; the laser enters the erbium-doped fiber 3 through the step-index dual-mode fiber 8 to produce a mode coupling effect, and the higher-order mode is converted into the fundamental mode.

[0072] When the laser passes through the erbium-doped fiber 3 and enters the step-index dual-mode fiber 8, a mode coupling effect occurs due to the difference in the mode field diameter and refractive index distribution of the two optical fibers. This effect causes part of the fundamental mode energy to be converted into a higher-order mode, and the remaining energy is still converted between fundamental modes. That is, the LP in the erbium-doped fiber 3 01 Mode and step-index dual-mode fiber 8 LP 01 Mode conversion and erbium-doped fiber 3 in LP 01 Mode and step-index dual-mode fiber 8 LP 11 When the laser enters the erbium-doped fiber 3 through the step-index dual-mode fiber 8, the high-order mode is converted into the fundamental mode, that is, the step-index dual-mode fiber 8 in the LP 11 Mode LP with Erbium-doped Fiber 3 01 Switch between modes.

[0073] Furthermore, in the process of mutual conversion between the fundamental mode and the high-order mode, the more the mode field distributions of the modes in the two optical fibers overlap, the higher the mode coupling efficiency is. It is necessary to consider the mode coupling effect between the two optical fibers. According to the overlap integral, the mode coupling coefficient between the erbium-doped optical fiber 3 and the step-index dual-mode optical fiber 8 is calculated:

[0074]

[0075] Among them, C 11 Erbium-doped fiber 3 LP 01 Mode and step-index dual-mode fiber 8 LP 01 Coupling coefficient between modes;

[0076] C 12 Erbium-doped fiber 3 LP 01 Mode and step-index dual-mode fiber 8 LP 11 Coupling coefficient between modes;

[0077] F(x,y) is the LP in erbium-doped fiber 3 01 Mode field distribution of the mode;

[0078] F1(x,y) and F2(x,y) are the LP 01 and LP 11 Mode field distribution of the mode;

[0079] LP 01 is the fundamental mode in the optical fiber;

[0080] LP 11 are the higher-order modes in the optical fiber.

[0081] Furthermore, when optical fibers are butt-jointed or coupled, the central axes of the two optical fibers should be completely aligned under ideal conditions. The existence of the misalignment distance will reduce the accuracy of the butt-joint connection, and the misalignment distance will affect the coupling efficiency of light from one optical fiber to another. The misalignment distance r0 is introduced into the erbium-doped optical fiber 3 and the step-index dual-mode optical fiber 8, and the mode field distribution after the optical fiber is lateral shifted is F(x-x0,y-y0), where r0=(x0 2 +y0 2 ) 1 / 2 , the LP in the Erbium-doped fiber 3 01 The mode field distribution F(x, y) of the mode is translated by a spatial distance r0 relative to the mode field distribution of the two modes in the step-index dual-mode optical fiber 8, which is equivalent to realizing a dislocation fusion operation, and the mode coupling coefficient after the dislocation is obtained:

[0082]

[0083] Among them, r0 is the dislocation distance;

[0084] F(x-x0,y-y0) is the LP in erbium-doped fiber 3 01 Mode field distribution after lateral translation of the mode.

[0085] When the pulse enters the step-index dual-mode fiber 8 from the erbium-doped fiber 3, as the offset distance increases, more LP in the step-index dual-mode fiber 8 11 The mode is excited, and LP 01 The mode coupling efficiency hardly changes, and the LP 01 The coupling efficiency of the mode is always much higher than that of LP 01 mode, due to the erbium-doped fiber 3 LP 01 Mode field distribution of the mode and LP in step-index dual-mode fiber 8 01 The mode field distributions of the modes always have a high degree of overlap.

[0086] Stable mode locking can only be achieved when the misalignment distance does not exceed 1 μm, so C 11Always greater than C 12 , resulting in step-index dual-mode fiber 8 LP 01 The energy of the mode is always higher than that of LP 11 model.

[0087] Furthermore, when the pulse enters the step-refractive-index dual-mode optical fiber 8 from the erbium-doped optical fiber 3, the light fields of the two modes in the step-refractive-index dual-mode optical fiber 8 are obtained respectively:

[0088] u1=C 11 u,

[0089] u2=C 12 u;

[0090] Where u is the LP in the erbium-doped fiber 01 the amplitude of the mode;

[0091] u1 is the LP in the step-index dual-mode fiber 01 the amplitude of the mode;

[0092] u2 is the LP in the step-index dual-mode fiber 11 The amplitude of the mode.

[0093] Furthermore, when the pulse enters the erbium-doped optical fiber 3 from the step-refractive-index dual-mode optical fiber 8, the optical field of the LP01 mode in the erbium-doped optical fiber 3 is obtained as follows:

[0094] u=C 11 u1+C 12 u2.

[0095] Where u is the LP in the erbium-doped fiber 01 the amplitude of the mode;

[0096] u1 is the LP in the step-index dual-mode fiber 01 the amplitude of the mode;

[0097] u2 is the LP in the step-index dual-mode fiber 11 The amplitude of the mode.

[0098] like Figure 2 As shown, the present invention provides an ultrashort pulse generating device based on single-mode and dual-mode fiber lasers, comprising a pump source 1, a dual-mode wavelength division multiplexer 2, an erbium-doped fiber 3, a dual-mode fiber coupler 4, a polarization controller 5, a carbon nanotube saturable absorber 6, a dual-mode isolator 7 and a step-refractive-index dual-mode fiber 8;

[0099] The dual-mode wavelength division multiplexer 2, the erbium-doped optical fiber 3, the dual-mode optical fiber coupler 4, the polarization controller 5, the carbon nanotube saturable absorber 6 and the dual-mode isolator 7 are sequentially fused through the step-refractive index dual-mode optical fiber 8 to form a ring-shaped laser resonant cavity.

[0100] The pump source 1 emits pump light through the dual-mode wavelength division multiplexer 2 and enters the erbium-doped optical fiber 3; the erbium-doped optical fiber 3 excites erbium ions to the upper energy level, realizes population inversion, generates stimulated radiation, and amplifies the signal light; the dual-mode optical fiber coupler 4 is used to output a small part of the laser to the outside of the cavity for measurement; the polarization controller 5 is used to adjust the polarization state in the cavity; the carbon nanotube saturable absorber 6 is used to realize laser mode locking; the dual-mode isolator 7 ensures that the light pulse runs unidirectionally in the cavity.

[0101] Furthermore, the wavelength of the pump light emitted by the pump source 1 is 976 / 980 nm.

[0102] Furthermore, the core diameter of the step-refractive-index dual-mode optical fiber 8 is 14 μm, and the refractive index difference between the core and the cladding is 0.005.

[0103] like Figure 3 As shown in the figure, the pulse shape evolves with the number of turns at the output coupler when the misalignment distance r0 is 0.8μm; from the first turn to the eightieth turn is the locking mode establishment stage, the trailing edge of the pulse base undergoes continuous broadening, and after eighty turns, a small pulse is generated from the asymmetric base and gradually evolves into a stable AP, always maintaining a time domain interval of 6.98ps with the DP.

[0104] like Figure 4-5 As shown in the figure, the spectra of the two modes after reaching the steady state have a modulation period of 1.14nm and a time domain interval of 0.81ps, which corresponds to the time walk-off of the two modes at the output coupler. It can be seen that the locking of the transverse mode is not a necessary condition for achieving stable mode locking in the few-mode fiber laser. The time domain interval between DP and AP on each mode is 6.98ps, which just corresponds to the spectral modulation period of 1.14nm on each mode. This shows that the spectral modulation is formed by the interference between DP and AP on each mode, rather than by LP. 01 With LP 11 The modes interfere with each other. The intensity of AP is about 30 dB smaller than that of DP, indicating that AP is not caused by traditional soliton splitting, because traditional soliton splitting usually produces two solitons with equal intensity. The intensity of AP does not meet the soliton condition and has a large time domain interval with DP, indicating that AP does not maintain its own stability by exchanging energy with DP.

[0105] An embodiment 1 of the present invention:

[0106] like Figure 6 As shown, this embodiment of the present invention proposes an ultrashort pulse generation method based on a single-mode and dual-mode fiber laser. When the misalignment distance is large, the pump power is set to 64mW and the polarization controller is adjusted. The specific steps include:

[0107] S100: When the pump power is 64mW, the pump light is emitted through the dual-mode wavelength division multiplexer into the erbium-doped fiber, which excites the erbium ions to the upper energy level to form a population inversion, generates stimulated radiation, and amplifies the laser signal;

[0108] S200: The laser passes through the dual-mode fiber coupler, a small amount of laser is output to the outside of the cavity for measurement, and the remaining laser continues to oscillate in the cavity;

[0109] S300: The remaining laser light is used to adjust the polarization state in the resonant cavity through the polarization controller to ensure stable transmission of the laser light in the cavity. The carbon nanotube saturable absorber is used to achieve mode locking of the laser light to ensure the generation of stable ultrashort pulses.

[0110] S400: The laser passes through a dual-mode isolator to ensure that the optical pulse runs unidirectionally in the resonant cavity, and re-enters the dual-mode wavelength division multiplexer to form a cycle;

[0111] S500: After periodic mode coupling, a stable mode-locked pulse is finally formed.

[0112] When the misalignment distance is large, by setting the pump power to 64mW and adjusting the polarization controller, a stable mode-locked pulse can be obtained in the laser. The central wavelength of the spectrum is 1557.59nm, Kelly sidebands appear in the spectrum, and there is obvious modulation with a modulation period of 1.17nm.

[0113] An embodiment 2 of the present invention:

[0114] like Figure 7 As shown, the embodiment of the present invention provides another method for generating ultrashort pulses based on single-mode and double-mode fiber lasers. In this embodiment, other contents are the same as those in embodiment 2, except that: the pump power of the pump source is 66 mW.

[0115] When the misalignment distance is large, the pump power is increased to 66 mW and the polarization controller is adjusted to obtain a stable mode-locked pulse in the laser. The central wavelength of the spectrum is 1557.60 nm, and the spectrum also shows obvious modulation. At this time, the intensity of the Kelly sideband is significantly increased.

[0116] An embodiment 3 of the present invention:

[0117] like Figure 8 As shown, this embodiment of the present invention proposes a method for generating ultrashort pulses based on single-mode and dual-mode fiber lasers. When the displacement distance is small, the pump power is set to 62mW and the polarization controller is adjusted. The specific steps include:

[0118] S100: When the pump power is 62mW, the pump light is emitted through the dual-mode wavelength division multiplexer into the erbium-doped fiber, which excites the erbium ions to the upper energy level to form a population inversion, generate stimulated radiation, and amplify the laser signal;

[0119] S200: The laser passes through the dual-mode fiber coupler, a small amount of laser is output to the outside of the cavity for measurement, and the remaining laser continues to oscillate in the cavity;

[0120] S300: The remaining laser light is used to adjust the polarization state in the resonant cavity through the polarization controller to ensure stable transmission of the laser light in the cavity. The carbon nanotube saturable absorber is used to achieve mode locking of the laser light to ensure the generation of stable ultrashort pulses.

[0121] S400: The laser passes through a dual-mode isolator to ensure that the optical pulse runs unidirectionally in the resonant cavity, and re-enters the dual-mode wavelength division multiplexer to form a cycle;

[0122] S500: After periodic mode coupling, a stable mode-locked pulse is finally formed.

[0123] When the pump power is 62mW and the misalignment distance is small, by adjusting the polarization controller, a stable mode-locked pulse can be obtained in the laser. The central wavelength of the spectrum is 1557.84nm, and modulation appears in the spectrum, but the depth of the spectrum modulation is low at this time.

[0124] An embodiment 4 of the present invention:

[0125] like Fig. 9 As shown, the embodiment of the present invention provides another method for generating ultrashort pulses based on single-mode and double-mode fiber lasers. In this embodiment, other contents are the same as those in embodiment 2, except that: the misalignment distance is larger.

[0126] When the pump power is 62mW and the offset distance is large, the polarization controller can be adjusted to obtain stable mode-locked pulses in the laser. The central wavelength of the spectrum is 1557.66nm, and the spectral modulation depth of the output pulse is significantly increased. It is found that the spectral modulation is formed by the interference between DP and AP in each mode, and increasing the offset distance will cause more energy to be coupled to LP. 11 mode, resulting in an increase in the intensity of DP.

[0127] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is 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 in the protection scope of the present invention.

Claims

1. A method for generating ultrashort pulses based on single-mode and dual-mode fiber lasers, characterized in that: The steps include: S100: The pump light enters the erbium-doped fiber through a dual-mode wavelength division multiplexer, excites erbium ions to the upper energy level to form a population inversion, generates stimulated radiation, and amplifies the laser signal; S200: The laser passes through the dual-mode fiber coupler, a small amount of laser is output to the outside of the cavity for measurement, and the remaining laser continues to oscillate in the cavity; S300: The remaining laser light is used to adjust the polarization state in the resonant cavity through the polarization controller to ensure stable transmission of the laser light in the cavity. The carbon nanotube saturable absorber is used to achieve mode locking of the laser light to ensure the generation of stable ultrashort pulses. S400: The laser passes through a dual-mode isolator to ensure that the optical pulse runs unidirectionally in the resonant cavity, and re-enters the dual-mode wavelength division multiplexer to form a cycle; S500: After periodic mode coupling, a stable mode-locked pulse is finally formed.

2. The method for generating ultrashort pulses based on single-mode and double-mode fiber lasers according to claim 1, characterized in that: The laser enters the step-index dual-mode fiber through the erbium-doped fiber to produce a mode coupling effect, and part of the fundamental mode energy is converted into a higher-order mode.

3. The method for generating ultrashort pulses based on single-mode and double-mode fiber lasers according to claim 1, characterized in that: The laser enters the erbium-doped fiber through the step-index dual-mode fiber to produce a mode coupling effect, and the high-order mode is converted into the fundamental mode.

4. A method for generating ultrashort pulses based on single-mode and double-mode fiber lasers according to any one of claims 1 to 3, characterized in that: In the process of mutual conversion between the fundamental mode and the higher-order mode, the mode coupling coefficient between the erbium-doped fiber and the step-index dual-mode fiber is calculated: Among them, C 11 LP in Erbium-doped fiber 01 Mode and LP in step-index dual-mode fiber 01 Coupling coefficient between modes; C 12 LP in Erbium-doped fiber 01 Mode and LP in step-index dual-mode fiber 11 Coupling coefficient between modes; F(x,y) is the LP in the erbium-doped fiber 01 Mode field distribution of the mode; F1(x,y) and F2(x,y) are the LP 01 and LP 11 Mode field distribution of the mode; LP 01 is the fundamental mode in the optical fiber; LP 11 are the higher-order modes in the optical fiber.

5. The method for generating ultrashort pulses based on single-mode and double-mode fiber lasers according to claim 4, characterized in that: Introducing the offset distance r0 in the erbium-doped fiber and the step-index dual-mode fiber, the mode field distribution after the fiber is lateral shifted is F(x-x0,y-y0), where r0 = (x0 2 +y0 2 ) 1 / 2 , the LP in the erbium-doped fiber 01 The mode field distribution F(x,y) of the mode is shifted by a spatial distance r0 relative to the mode field distribution of the two modes in the step-index dual-mode fiber, which is equivalent to realizing the dislocation fusion operation, and the mode coupling coefficient after the dislocation is obtained: Among them, r0 is the dislocation distance; F(x-x0,y-y0) is the LP in the erbium-doped fiber 01 Mode field distribution after lateral translation of the mode.

6. A method for generating ultrashort pulses based on single-mode and double-mode fiber lasers according to any one of claims 1 to 3, characterized in that: When the pulse enters the step-index dual-mode fiber from the erbium-doped fiber, the light fields of the two modes in the step-index dual-mode fiber are: u1=C 11 u, u2=C 12 u; Where u is the LP in the erbium-doped fiber 01 the amplitude of the mode; u1 is the LP in the step-index dual-mode fiber 01 the amplitude of the mode; u2 is the LP in the step-index dual-mode fiber 11 The amplitude of the mode.

7. A method for generating ultrashort pulses based on single-mode or double-mode fiber lasers according to any one of claims 1 to 3, characterized in that: When the pulse enters the erbium-doped fiber from the step-index dual-mode fiber, the LP in the erbium-doped fiber is obtained. 01 The light field of the mode is: u=C 11 u1+C 12 u2。 Where u is the LP in the erbium-doped fiber 01 the amplitude of the mode; u1 is the LP in the step-index dual-mode fiber 01 the amplitude of the mode; u2 is the LP in the step-index dual-mode fiber 11 The amplitude of the mode.

8. A novel ultrashort pulse generating device based on a single-step refractive index dual-mode fiber laser, used to implement an ultrashort pulse generating method based on a single-mode and dual-mode fiber laser as described in any one of claims 1 to 7, characterized in that: It comprises a pump source (1), a dual-mode wavelength division multiplexer (2), an erbium-doped optical fiber (3), a dual-mode optical fiber coupler (4), a polarization controller (5), a carbon nanotube saturable absorber (6), a dual-mode isolator (7) and a step-refractive-index dual-mode optical fiber (8); The dual-mode wavelength division multiplexer (2), the erbium-doped optical fiber (3), the dual-mode optical fiber coupler (4), the polarization controller (5), the carbon nanotube saturable absorber (6) and the dual-mode isolator (7) are sequentially fused through a step-refractive-index dual-mode optical fiber (8) to form a ring-shaped laser resonant cavity.

9. The ultrashort pulse generating device based on single-mode and double-mode fiber lasers according to claim 8, characterized in that: The wavelength of the pump light emitted by the pump source (1) is 976 / 980 nm.

10. The ultrashort pulse generating device based on single-mode and double-mode fiber lasers according to claim 8, characterized in that: The core diameter of the step-refractive-index dual-mode optical fiber (8) is 14 μm, and the refractive index difference between the core and the cladding is 0.005.

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