All-fiber space-time mode-locked laser based on shunt regulation and control

Abstract

The invention discloses an all-fiber space-time mode-locked laser based on shunt regulation and control, which belongs to the technical field of communication and comprises a pumping source, a wavelength division multiplexer, a few-mode erbium-ytterbium co-doped fiber, a few-mode polarization controller, a first photon lantern, a second photon lantern, a saturable absorber, a few-mode fiber beam splitter and a few-mode isolator. According to the invention, mode branching is realized by adopting photon lantern characteristics, accurate regulation and control of parameters such as dispersion in different transverse modes are realized by adding a compensation optical fiber, stable space-time mode locking output can be realized in a 1550nm wave band, single pulse energy reaches 6nJ, the number of modes participating in mode locking can be flexibly adjusted, and pulse output with different space-time characteristics can be generated; the design possibility of the space-time mode-locked laser is expanded, the range of adjustable parameters of the space-time mode-locked laser is remarkably expanded, and higher flexibility and wider potential are provided for application of the space-time mode-locked laser in multiple fields such as nonlinear optical research and high-energy pulse lasers.

Description

Technical Field

[0001] The present invention belongs to the field of communication technologies, and particularly relates to an all-fiber spatio-temporal mode-locked laser based on splitting regulation. Background Art

[0002] Fiber mode-locked lasers have triggered revolutionary changes in many fields, including microscopy imaging, microfabrication, high-resolution spectroscopy, and time-frequency measurement. Among them, single-mode fiber provides a simple method for realizing fiber mode-locked lasers due to its good cost-effectiveness, high reliability, and low loss. However, with the continuous growth of the demand for high-energy lasers and the pursuit of larger data capacities in the field of optical fiber communication, single-mode fiber is facing unprecedented challenges. Therefore, in order to break through the bottleneck that the pulse energy is limited by transverse mode constraints and strong nonlinearity, the spatio-temporal mode-locking technology of multi-mode fiber lasers has emerged. This technology realizes further expansion of the energy scale by utilizing more modal content and a larger modal area. In addition, with the increase in the number of transverse modes, multi-mode fiber has more complex and unique spatio-temporal characteristics than single-mode fiber. These characteristics have inspired extensive research on spatio-temporal mode-locked lasers.

[0003] Currently, spatio-temporal mode-locked lasers mainly rely on spatial filtering technology to balance inter-modal dispersion. Since multi-mode fiber usually contains dozens or even hundreds of transverse modes, spatial filtering technology faces many technical challenges in precisely regulating each transverse mode and achieving stable spatio-temporal mode-locking. This method is difficult to precisely control the parameters of a single transverse mode, limiting the in-depth regulation and optimization of modal characteristics and their evolution processes, making it difficult to increase the output power by increasing the number of modes. In addition, spatial filtering technology is often accompanied by large energy losses, which not only increases the demand for pump power but also limits the possibility of further increasing energy. At the same time, in fibers with large modal dispersion, it is also difficult for spatial filtering technology to effectively balance inter-modal dispersion. Summary of the Invention

[0004] Aiming at the deficiencies existing in the prior art, the present invention provides an all-fiber spatio-temporal mode-locked laser based on shunt regulation. A photon lantern is used to achieve mode splitting, and compensation fibers are added to precisely regulate parameters such as different transverse mode dispersions, effectively utilizing the energy of each transverse mode, and achieving the balance of dispersion between different transverse modes in a few-mode step-index fiber with large mode dispersion. At the same time, the laser interacts with nanomaterials through evanescent field coupling in a self-made few-mode tapered saturable absorber to achieve the output of high-energy spatio-temporal mode-locked pulses. In addition, the laser can also adjust the number of transverse modes participating in spatio-temporal mode locking and output spatio-temporal mode-locked pulses with different spatio-temporal distribution characteristics. The all-fiber spatio-temporal mode-locked laser based on shunt regulation of the present invention broadens the design possibilities of spatio-temporal mode-locked lasers and has good application prospects in multiple fields such as nonlinear optical research and high-energy pulsed lasers.

[0005] The present invention is realized through the following technical solutions:

[0006] An all-fiber spatio-temporal mode-locked laser based on shunt regulation, comprising: a pump source 1, a wavelength division multiplexer 2, a few-mode erbium-ytterbium co-doped fiber 3, a few-mode polarization controller 4, a first photon lantern 5, a second photon lantern 6, a saturable absorber 7, a few-mode fiber splitter 8, and a few-mode isolator 9; the wavelength division multiplexer 2 includes a port A and a port B; the few-mode fiber splitter 8 includes a port A and a port B; wherein, the pump light output by the pump source 1 is sequentially input into the few-mode erbium-ytterbium co-doped fiber 3, the few-mode polarization controller 4, the first photon lantern 5, the second photon lantern 6, the saturable absorber 7, and the few-mode fiber splitter 8 through the port A of the wavelength division multiplexer 2, and is output from the port A of the few-mode fiber splitter 8 to the few-mode isolator 9, and finally re-enters the loop through the port B of the wavelength division multiplexer 2, and then oscillates in the resonant cavity; the generated light beam is output from the port B of the few-mode fiber splitter 8.

[0007] Further, the pump source 1 outputs 915 nm pump light.

[0008] Further, the few-mode erbium-ytterbium co-doped fiber 3 adopts a double-clad fiber structure, the gain range is 1530 - 1570 nm, and the fiber length is 2 m.

[0009] Further, both the first photon lantern 5 and the second photon lantern 6 are mode-selective photon lanterns, supporting LP 01 , LP 11 , LP 21 and LP 02 four modes.

[0010] Further, the saturable absorber 7 adopts a tapered fiber structure, the length of the tapered part is 2 cm, and nanogold material is drop-cast at the waist, with a diameter of 15 μm, and can pass through LP01 , LP 11 , LP 21 and LP 02 mode.

[0011] Furthermore, the few-mode fiber splitter 8 has a 1×2 structure, and the splitting ratio is 1:9.

[0012] Furthermore, the few-mode polarization controller 4, the few-mode fiber splitter 8, and the few-mode isolator 9 are all made of few-mode fibers, and the core and cladding diameters of the few-mode fibers are 18.5 μm and 125 μm respectively.

[0013] The working principle of an all-fiber spatio-temporal mode-locked laser based on splitting regulation according to the present invention is as follows:

[0014] The present invention uses a photon lantern to realize the splitting regulation of a spatio-temporal mode-locked laser. The schematic diagram is as Figure 2 shown. After calculating the cavity length, according to the cavity length requirement, the modes are demultiplexed into single-mode fiber outputs through the first photon lantern, and appropriate lengths of single-mode fibers are added to the single-mode fiber ends respectively to achieve precise balancing of the dispersions of different modes. Subsequently, the different modes are multiplexed back into the few-mode fiber for transmission through the second photon lantern. This method makes full use of all the modes transmitted in the few-mode fiber, and while achieving the dispersion balance of different modes, it avoids the energy loss caused by mode rejection.

[0015] The present invention uses a tapered fiber structure to prepare a real saturable absorber, as Figure 3 shown. When the fiber is used as a saturable absorber, the transmitted light interacts with the nanomaterials attached to its surface through the evanescent field at the waist of the fiber, thereby realizing the saturable absorption effect. Among them, the intensity of the evanescent field decays exponentially with the distance from the fiber interface, and its distribution can be expressed by Equation (1)

[0016] E(x) = E 0 * exp(-x / d p )(1)

[0017] where d p is the penetration depth of the evanescent field.

[0018] Since the distance that the evanescent field penetrates through the core is at the μm level, therefore, the fiber needs to be tapered so that the diameter of the waist of the tapered fiber is thin enough to ensure that the evanescent field excited in the core can be coupled with the nanomaterials dropped on the fiber surface, thereby effectively realizing the saturable absorption effect.

[0019] In addition, in a few-mode fiber spatio-temporal mode-locked laser, it is necessary to ensure that each transverse mode can pass through the thinnest part of the waist of the tapered fiber, so as to avoid the inability to transmit due to mode cut-off. In the tapered fiber, as shown in Equation (2), the number of mode fields propagating in the fiber can be obtained by calculating the given wavelength, fiber radius, and effective refractive indices of the core and cladding.

[0020]

[0021] Among them, a is the core radius, n core and n clad are the effective refractive indices of the core and cladding respectively.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] A fully fiberized spatio-temporal mode-locked laser based on splitting regulation according to the present invention uses a photon lantern to realize mode splitting, and realizes precise regulation of parameters such as dispersion of different transverse modes by adding compensating fibers, thereby realizing stable spatio-temporal mode-locking operation in all-step-index fibers. Compared with the traditional scheme relying on spatial filtering technology, this splitting regulation scheme can precisely control different transverse modes, and can realize dispersion compensation without filtering out the transverse modes with large walk-off, effectively utilizing all transverse modes. It not only avoids energy loss caused by spatial filtering, enables the output power to increase proportionally with the increase in the number of modes participating in spatio-temporal mode-locking, but also provides a novel and feasible technical path for realizing high-energy mode-locked lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. In all the drawings, similar elements or parts are generally denoted by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to actual scale.

[0025] Figure 1 is a schematic structural diagram of a fully fiberized spatio-temporal mode-locked laser based on splitting regulation according to the present invention;

[0026] Figure 2 is a schematic diagram of the principle of the splitting regulation technology according to the present invention;

[0027] Figure 3 is a schematic structural diagram of a real saturable absorber structure of a tapered fiber according to the present invention;

[0028] Figure 4 is a schematic structural diagram of a test system for the saturable absorption effect according to the present invention;

[0029] Figure 5 is a test result diagram of the absorption effect of the saturable absorber according to the present invention;

[0030] Figure 6 For LP 01 With LP 11 Mode-locked spectroscopy, beam profile, and time-domain pulse test result diagrams;

[0031] Among them, (a) is for LP 01 With LP 11 Mode-locked spectroscopy and beam profile test result diagrams;

[0032] (b) is for LP 01 With LP 11 Mode time-domain pulse test result diagram;

[0033] Figure 7 For LP 01 , LP 11 And LP 21 Mode-locked spectroscopy, beam profile, and time-domain pulse test result diagrams;

[0034] Among them, (a) is for LP 01 , LP 11 And LP 21 Mode-locked spectroscopy and beam profile test result diagrams;

[0035] (b) is for LP 01 , LP 11 And LP 21 Mode-locked time-domain pulse test result diagram;

[0036] Figure 8 For LP 01 , LP 11 , LP 21 And LP 02 Mode-locked spectroscopy, beam profile, and time-domain pulse test result diagrams;

[0037] Among them, (a) is for LP 01 , LP 11 , LP 21 And LP 02 Mode-locked spectroscopy and beam profile test result diagrams

[0038] (b) is for LP 01 , LP 11 , LP 21 And LP 02 Mode-locked time-domain pulse test result diagrams

[0039] Figure 9 Are the pulse energy test result diagrams of different spatio-temporal mode-locked outputs under different pump power conditions;

[0040] In the figure: pump source 1, wavelength division multiplexer 2, few-mode erbium-ytterbium co-doped fiber 3, few-mode polarization controller 4, first photon lantern 5, second photon lantern 6, saturable absorber 7, few-mode fiber splitter 8, few-mode isolator 9, pulsed laser 11, first photon lantern 12, second photon lantern 13, tunable attenuator 14, 3dB coupler 15, saturable absorber 17, first optical power meter 18, second optical power meter 19. Detailed implementation manners

[0041] To clearly and completely describe the technical solution of the present invention and its specific working process, in combination with the accompanying drawings of the specification, the specific implementation manners of the present invention are as follows:

[0042] Embodiment 1

[0043] As Figure 1 shown, this embodiment provides a fully few-mode fiber spatio-temporal mode-locked laser based on shunt regulation, which is composed of a pump source 1, a wavelength division multiplexer 2, a few-mode erbium-ytterbium co-doped fiber 3, a few-mode polarization controller 4, a first photon lantern 5, a second photon lantern 6, a saturable absorber 7, a few-mode fiber splitter 8, and a few-mode isolator 9. The connection manner is as described below:

[0044] The output end of the pump source 1 is connected to the A port of the wavelength division multiplexer 2. The output port of the wavelength division multiplexer 2 is sequentially connected to the few-mode erbium-ytterbium co-doped fiber 3 and the few-mode polarization controller 4. The output end of the few-mode polarization controller 4 is connected to the few-mode end of the first photon lantern 5. The four single-mode ends of its demultiplexing output are respectively connected to the four single-mode ends of the second photon lantern 6. Its few-mode end is sequentially connected to the saturable absorber 7 and the few-mode fiber splitter 8. The A port of the few-mode fiber splitter 8 is connected to the few-mode isolator 9 and finally connected to the B port of the wavelength division multiplexer, thus forming a ring cavity; the generated light beam is output from the B port of the few-mode fiber splitter 8.

[0045] The pump source 1 in this embodiment uses a multimode pump source from SkyEra Laser, with a central wavelength of 915 nm and a maximum output power of 5 W;

[0046] The few-mode erbium-ytterbium co-doped fiber 3 has a double-clad fiber structure, with a fiber length of 2 m, a core diameter of 12.0 μm, and a numerical aperture of 0.2;

[0047] The first photon lantern 5 and the second photon lantern 6 adopt mode-selective photon lanterns, which can support LP 01 、LP 11 、LP 21 and LP 02 four modes for mode multiplexing and demultiplexing;

[0048] The few-mode fiber splitter 8 has a 1×2 structure with a splitting ratio of 1:9 and is used to maintain the unidirectional operation of the laser and extract 10% of the optical output.

[0049] The saturable absorber 7 adopts a tapered fiber structure and is fabricated by the flame-brush drawing method. Its waist diameter is 15 μm, and the length of the tapered part is 2 cm. The nano-gold material in solution form is deposited on the tapered fiber by the drop-casting method and dried at room temperature. To verify that the fabricated tapered fiber can achieve the saturable absorption effect, the experimental structure shown in Figure 6 is used for testing. This system consists of a pulsed laser 11, a first photon lantern 12, a second photon lantern 13, an adjustable attenuator 14, a 3dB coupler 15, a saturable absorber 17, a first optical power meter 18, and a second optical power meter 19. To obtain the transmittance of the saturable absorber at different power densities, after the pulsed light passes through the tunable attenuator 14, it enters the 3db coupler 15 and is split into two laser beams with equal power. One beam of light is used as the reference light to directly measure the power, and the other beam of light is measured for power after passing through the tapered fiber. The input optical power is changed by the tunable attenuator 14. Among them, the pulsed light output by the pulsed laser 11 is 500 ps with a repetition frequency of 20 MHz. The test results are as shown in Figure 5 As the pulsed peak power density gradually increases, the transmittance of the tapered fiber saturable absorber for 1550-nm lasers in different transverse modes gradually increases. The modulation depths of each transverse mode are 7.74%, 8.05%, 8.43%, and 8.79% respectively. The test results show that the fabricated tapered fiber saturable absorber exhibits good saturable absorption characteristics at 1550 nm.

[0050] In this embodiment, the total fiber length in the cavity is 13.5 m, and the group delays of each mode in the cavity are 0, 14.33 ps / m, 25.6 ps / m, and 32 ps / m respectively. According to the fiber length and parameters, each meter of single-mode fiber can compensate for a 4.829-ns delay. Therefore, at the demultiplexing single-mode end of the first photon lantern 5, 8.9 cm of fiber is added in the LP 01 branch, 7.2 cm of fiber is added in the LP 11 branch, and 4.0 cm of fiber is added in the LP 21 branch to achieve the balance of inter-mode dispersion.

[0051] In this embodiment, for the output optical pulse, a spectral analyzer with a resolution of 0.1 nm is used to measure the mode-locked spectrum. After the optical pulse is photoelectrically converted by a multimode photodetector, the time-domain sequence of the mode-locked optical pulse is measured by an oscilloscope. In addition, a CCD camera is used to record the beam profile of the mode-locked optical pulse.

[0052] In this embodiment, the 01 branch and the 11 branch of the two-photon lantern are interconnected. When the pump power reaches 550 mw, LP 01 and LP 11 spatiotemporal mode-locked pulse outputs of two modes can be achieved. The spatiotemporal mode-locked time-domain pulse waveform, spectral pattern, and beam profile are as shown in Figure 6 . At this pump level, the pulse power of the spatiotemporal mode-locked output is 0.93 mw, and the corresponding pulse energy is 60.5 pj. Connect the LP 01 branch, LP 11 branch, and LP 21 branch of the two-photon lantern to each other. When the pump power reaches 500 mw, LP 01 , LP 11 , and LP 21 spatiotemporal mode-locked pulse outputs of three modes can be achieved. The spatiotemporal mode-locked time-domain pulse waveform, spectral pattern, and beam profile are as shown in Figure 7 . At this pump level, the pulse power of the spatiotemporal mode-locked output is 1.02 mw, and the corresponding pulse energy is 66.3 pj. Connect the LP 01 branch, LP 11 branch, LP 21 , and LP 02 branch of the two-photon lantern to each other. When the pump power reaches 450 mw, LP 01 , LP 11 , LP 21 , and LP 02 spatiotemporal mode-locked pulse outputs of four modes can be achieved. The spatiotemporal mode-locked time-domain pulse waveform, spectral pattern, and beam profile are as shown in Figure 8 . At this pump level, the pulse power of the spatiotemporal mode-locked output is 1.1 mw, and the corresponding pulse energy is 72.2 pj.

[0053] In this embodiment, in order to confirm the relationship between the number of transverse modes participating in spatiotemporal mode locking and the output pulse energy, the pulse energy of the spatiotemporal mode-locked output in each case was measured under different pump power conditions. The experimental results are as shown in Figure 9 . At a pump power of 5 w, the output power of the four-mode can reach 93 mw, and the single-pulse energy is 6 nJ. Moreover, as the number of modes participating in spatiotemporal mode locking increases, the pulse energy shows an obvious upward trend. This indicates that increasing the participation of transverse modes can effectively improve the output energy of the laser.

[0054] In summary, a fully fiberized spatio-temporal mode-locked laser based on split-path regulation established in this embodiment uses a photon lantern to achieve mode splitting, and precisely regulates parameters such as different transverse mode dispersions by adding compensation fibers, realizing stable spatio-temporal mode-locked operation in all-step fibers. The single-pulse energy can reach 6 nj, and the number of modes participating in mode locking can be flexibly adjusted. Through specific mode combination methods, mode-locked pulse outputs with different spatio-temporal distribution characteristics are achieved. It expands the parameter space that can be explored in spatio-temporal mode-locked lasers and breaks through the energy limitation of traditional spatial filtering techniques, providing a novel and feasible technical path for the research of high-energy mode-locked lasers.

[0055] The preferred embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0056] In addition, it should be noted that, in the case of no contradiction, the various specific technical features described in the above specific embodiments can be combined in any appropriate manner. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0057] Furthermore, any combination can be made between different embodiments of the present invention as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. An all-fiber spatiotemporal mode-locked laser based on branching control, characterized in that: include: The invention comprises a pump source (1), a wavelength division multiplexer (2), a few-mode erbium co-doped optical fiber (3), a few-mode polarization controller (4), a first photon lantern (5), a second photon lantern (6), a saturable absorber (7), a few-mode optical fiber beam splitter (8), and a few-mode isolator (9); the wavelength division multiplexer (2) comprises an A port and a B port; the few-mode optical fiber beam splitter (8) comprises an A port and a B port; wherein the pump light output by the pump source (1) is transmitted through the wavelength division multiplexer (2) The A port of the optical fiber inputs the least-mode erbium-ytterbium co-doped optical fiber (3), the few-mode polarization controller (4), the first photon lantern (5), the second photon lantern (6), the saturable absorber (7) and the few-mode optical fiber beam splitter (8) in sequence, and the A port of the few-mode optical fiber beam splitter (8) outputs the least-mode isolator (9), and finally re-enters the loop through the B port of the wavelength division multiplexer (2), and then oscillates in the resonant cavity; the generated light beam is output from the B port of the few-mode optical fiber beam splitter (8).

2. The all-fiber spatiotemporal mode-locked laser based on branching control as claimed in claim 1, characterized in that: The pump source (1) outputs 915nm pump light.

3. The all-fiber spatiotemporal mode-locked laser based on branching control as claimed in claim 1, characterized in that: The minority-mode erbium-ytterbium co-doped optical fiber (3) adopts a double-cladding optical fiber structure, has a gain range of 1530-1570 nm, and an optical fiber length of 2 m.

4. The all-fiber spatiotemporal mode-locked laser based on branching control as claimed in claim 1, characterized in that: The first photon lantern (5) and the second photon lantern (6) are both mode-selective photon lanterns that support LP 01 LP 11 LP 21 and LP 02 Four modes.

5. The all-fiber spatiotemporal mode-locked laser based on branching control as claimed in claim 1, characterized in that: The saturable absorber (7) adopts a tapered optical fiber structure, the length of the tapered part is 2 cm, the waist is drop-cast with nano-gold material, the diameter is 15 μm, and can be passed through LP 01 LP 11 LP 21 and LP 02 model.

6. The all-fiber spatiotemporal mode-locked laser based on branching control as claimed in claim 1, characterized in that: The few-mode optical fiber beam splitter (8) has a 1×2 structure and a splitting ratio of 1:

9.

7. The all-fiber spatiotemporal mode-locked laser based on branching control as claimed in claim 1, characterized in that: The few-mode polarization controller (4), the few-mode optical fiber beam splitter (8) and the few-mode isolator (9) are all made of few-mode optical fiber, and the diameters of the few-mode optical fiber core and cladding are 18.5 μm and 125 μm respectively.

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