Multi-output low-frequency picosecond mode-locked fiber laser based on cascaded differential beam splitter

Abstract

This invention discloses a multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter, comprising a fiber grating, a gain fiber, a cascaded beam splitter network, a saturable absorber, and a pump coupler for injecting pump light into the gain fiber. The fiber grating, gain fiber, cascaded beam splitter network, and saturable absorber are sequentially connected to form a linear resonant cavity with a target set length. The fiber grating in the linear resonant cavity is connected to the pump coupler to construct the linear cavity fiber laser. This invention can control the length of the entire linear cavity fiber laser by controlling the target set length of the linear resonant cavity composed of the fiber grating, gain fiber, cascaded beam splitter network, and saturable absorber, thereby making the linear cavity fiber laser smaller. At the same time, once the target set length of the linear resonant cavity is well controlled, a low repetition frequency can be directly achieved in the fundamental frequency mode without the need for complex harmonic mode-locking control.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more specifically to a multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter. Background Technology

[0002] Picosecond and femtosecond fiber lasers, with their core advantages such as compact structure, high conversion efficiency, and excellent beam quality, have become the core pulse source in cutting-edge fields such as precision machining, biomedicine, and scientific measurement, playing an irreplaceable role in many application scenarios with stringent requirements for laser pulse width and beam quality.

[0003] In applications such as pump-probe, optical sampling, and some nonlinear frequency conversion, low repetition frequency pulses of 10–20 MHz are often required to avoid heat accumulation and match the time window. According to the relationship between repetition frequency and cavity length, a single-pass cavity length of about 5.2 meters is required to achieve 20 MHz and about 10.4 meters is required to achieve 10 MHz.

[0004] Traditional solutions achieve low repetition rates by directly lengthening the gain fiber or adding a delay fiber, but this results in bulky lasers, sensitivity to environmental vibrations, severe accumulation of nonlinear effects, and difficulty in miniaturization. Furthermore, if multiple fixed-delay pulse sequences need to be obtained from a single laser to meet parallel processing or time-division multiplexing requirements, traditional solutions often involve building a tree-shaped beam-splitting network or a Mach-Zehnder interferometer array outside the laser output. This not only increases system size and cost but also makes the transmission of each light path via different external paths susceptible to environmental disturbances, leading to increased inter-path timing jitter and a significant decrease in synchronization accuracy.

[0005] Existing technologies have attempted to directly insert multiple fiber beam splitters into the laser resonator cavity to achieve built-in multiple outputs. However, after cascading fiber beam splitters with conventional splitting ratios (such as 50:50 and 70:30), the total transmittance within the cavity will decrease sharply. For example, the total transmittance of cascading five 50:50 beam splitters is only about 3.1%. Excessive cavity loss will significantly increase the mode-locking threshold, and may even prevent mode-locking from starting, or may only be able to operate within an extremely narrow pump power range. The stability is extremely poor, and it is very easy to degrade to continuous light or Q-switching state, which cannot meet the practical application requirements of stable mode-locking under a wide pump range.

[0006] Therefore, how to solve the problems of large size, complex multi-output system and poor synchronization of low repetition rate fiber lasers in traditional design schemes, and large loss and difficulty in stable mode locking caused by multi-stage beam splitting in the cavity has become the research topic to be solved in this invention. Summary of the Invention

[0007] The purpose of this invention is to provide a multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] A multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter includes a fiber grating, a gain fiber, a cascaded beam splitter network, a saturable absorber, and a pump coupler for injecting pump light into the gain fiber.

[0010] The fiber grating, the gain fiber, the cascaded beam splitter network, and the saturable absorber are sequentially connected to form a linear resonant cavity with a target set length. The fiber grating in the linear resonant cavity is connected to a pump coupler to construct a linear cavity fiber laser for achieving a low repetition frequency.

[0011] In use, the pump source injects pump light into the beginning of the linear resonant cavity through the pump coupler, and the pump light passes through the fiber grating and enters the gain fiber where it is absorbed. The fiber grating acts as a spectral filter to achieve simultaneous output of multiple wavelengths.

[0012] The cascaded optical splitting network includes N fiber beam splitters, where N is a positive integer, and each fiber beam splitter has a first input terminal, a second input terminal, a first output terminal, and a second output terminal.

[0013] N fiber optic beam splitters are cascaded in sequence, wherein the gain fiber is connected to the first input terminal of the first fiber optic beam splitter; the first output terminal of the first fiber optic beam splitter is connected to the first input terminal of the next fiber optic beam splitter, and the first output terminal of the next fiber optic beam splitter is connected to the first input terminal of the next fiber optic beam splitter, until the first output terminal of the Nth fiber optic beam splitter is connected to the saturable absorber, which is located at the end of the linear resonant cavity and is used to stabilize mode locking at low repetition frequencies.

[0014] In the above scheme, the length of the entire linear cavity fiber laser can be controlled by controlling the target set length of the linear resonant cavity composed of fiber grating, gain fiber, cascaded beam splitter network and saturable absorber, so as to make the linear cavity fiber laser smaller. At the same time, after the target set length of the linear resonant cavity is well controlled, a low repetition frequency can be directly achieved in the fundamental frequency mode without the need for complex harmonic mode-locking control.

[0015] Unlike existing technologies, this invention, through the coordinated design of a linear resonant cavity and a cascaded beam splitter network within the linear resonant cavity, can achieve low-repetition-frequency multi-path output of the laser beam while maintaining the compact size of the linear cavity fiber laser, and the output is stable.

[0016] In a further technical solution, the target set length is proportional to the low repetition frequency of the linear cavity fiber laser, and this set proportional relationship is determined by the following formula:

[0017] f rep = c / (2nL); where c is the speed of light, L is the target length, n is the effective refractive index of the fiber, and f rep For low repetition frequency of linear cavity fiber lasers;

[0018] The low repetition frequency of the linear cavity fiber laser is between 10 and 20 MHz.

[0019] With the above design, by controlling the low repetition frequency of the linear cavity fiber laser within the range of 10-20MHz, the target length can be limited to a certain range. In this way, the length of the linear cavity fiber laser can be constrained within a certain range, and the phenomenon of large volume will not occur.

[0020] In a further technical solution, the fiber optic beam splitter is a 2×2 fiber optic beam splitter;

[0021] In use, the cascaded optical splitter network is configured with 5-7 fiber beam splitters.

[0022] With the above design, the low repetition rate of a linear cavity fiber laser can be controlled by limiting the number of fiber beam splitters in the cascaded beam splitter network. That is, with the low repetition rate of the linear cavity fiber laser already controlled, the number of fiber beam splitters can be further controlled, thereby allowing both the target set length and the low repetition rate to be controlled.

[0023] In a further technical solution, the 2×2 fiber optic beam splitter is a fused biconical or planar waveguide coupler with a splitting ratio of 9:1.

[0024] In the above scheme, the 9:1 splitting ratio design ensures that the total intracavity loss after cascading 5 or 7 stages remains within a reasonable range, far lower than the traditional 50:50 beam splitting scheme. This results in a laser with an extremely low mode-locking threshold and an extremely wide stable operating range, making it insensitive to pump power fluctuations.

[0025] In a further technical solution, the first input end of the fiber optic beam splitter is configured to receive and amplify the signal laser beam generated after the pump light is absorbed by the gain fiber.

[0026] Furthermore, the fiber optic beam splitter is configured to split the signal laser beam input at the first input end into two beams, wherein the first signal laser beam, with an optical power ratio of 90%, is transmitted to the first output end, and the second signal laser beam, with an optical power ratio of 10%, is transmitted to the second output end.

[0027] With the above design, the optical power loss of the first output terminal can be reduced.

[0028] In a further technical solution, at least one of the second output end and the second input end of each fiber beam splitter serves as the signal output port of the linear cavity fiber laser, thereby enabling the linear cavity fiber laser to form multiple outputs.

[0029] The above design is used to achieve multiple outputs of a linear cavity fiber laser.

[0030] In a further technical solution, the splitting ratio of the fiber optic beam splitter and the number N of the fiber optic beam splitters satisfy the following formula:

[0031] T total = [9 / (9+1)] N ;

[0032] Among them, T total The total transmittance of the linear resonant cavity is between 40% and 60%.

[0033] With the above design, the number of fiber optic splitters can be controlled by controlling the total transmittance of the linear resonant cavity, so that the length of the linear resonant cavity will not be too large, that is, the volume will not be too large.

[0034] In a further technical solution, adjacent fiber optic bundlers in the cascaded optical splitting network are connected by connecting optical fibers, and the time delay between the output pulses of any two fiber optic bundlers and the length difference of the connecting optical fiber between the corresponding two fiber optic bundlers satisfy: Δτ=ΔL×n eff / c;

[0035] Where Δτ is the time delay between the output pulses of any two fiber optic bundle splitters, ΔL is the length difference of the connecting fiber between any two fiber optic bundle splitters, c is the speed of light, and n eff The refractive index of the optical fiber is given.

[0036] With the above design, the length of the connecting fiber between any two fiber optic bundle splitters and the time delay between the output pulses of any two fiber optic bundle splitters can be controlled according to the formula, thereby ensuring that the output pulses of each channel are strictly synchronized in the time domain.

[0037] In a further technical solution, in the linear cavity fiber laser, the maximum pump power Pmax and the minimum pump power Pmin of the pump source satisfy: Pmax / Pmin>3;

[0038] Where Pmin is the minimum pump power required to achieve stable mode locking, and Pmax is the maximum pump power that can be withstood to achieve stable mode locking.

[0039] With the above design, the 9:1 splitting ratio and the number N of fiber optic beam splitters are matched to satisfy the total intracavity transmittance T.total =0.9 N Then, the transmittance range is controlled between 0.4 and 0.6 to match the modulation depth of the saturable absorber and the saturation power of the gain fiber. This suppresses multi-pulse instability while maintaining a low threshold, and achieves stable mode-locking of the pump power in the range of Pmin to Pmax, where Pmax / Pmin>3.

[0040] In a further technical solution, when the fiber optic beam splitter in the cascaded optical splitting network is configured to be 5, the target setting length is 5.0 meters to 5.5 meters;

[0041] When the number of fiber beam splitters in the cascaded optical splitting network is 7, the target set length is 10.0 meters to 11.0 meters.

[0042] Specifically, when N=5, the linear cavity fiber laser operates at a repetition frequency of 20MHz, with a physical cavity length of less than 6 meters, achieving compact low-frequency multi-channel pulse output.

[0043] When N=7, the linear cavity fiber laser operates at a repetition frequency of 10MHz, with a physical cavity length between 10 and 11 meters, providing no fewer than 7 synchronous pulse outputs.

[0044] Due to the application of the above-mentioned solution, the technical solution of this application has the following advantages and effects compared with the prior art:

[0045] In this invention, the length of the entire linear cavity fiber laser can be controlled by controlling the target set length of the linear resonant cavity composed of fiber grating, gain fiber, cascaded beam splitter network and saturable absorber, so as to make the linear cavity fiber laser smaller in size. At the same time, after the target set length of the linear resonant cavity is well controlled, a low repetition frequency can be directly achieved in the fundamental frequency mode without the need for complex harmonic mode-locking control.

[0046] In this invention, multi-channel synchronous output is achieved by utilizing the four-port characteristics of the fiber optic beam splitter. Specifically, since each fiber optic beam splitter provides 1-2 outputs, 5 fiber optic beam splitters can provide 5-10 outputs, and 7 fiber optic beam splitters can provide 7-14 outputs. At the same time, the pulses output by all fiber optic beam splitters are generated from the same source, resulting in much higher synchronization accuracy and lower inter-channel time jitter.

[0047] Unlike existing technologies, this invention, through the coordinated design of a linear resonant cavity and a cascaded beam splitter network within the linear resonant cavity, can achieve low-repetition-frequency multi-path output of the laser beam while maintaining the compact size of the linear cavity fiber laser, and the output is stable. Attached Figure Description

[0048] Figure 1This is a schematic diagram of the first embodiment of the linear cavity fiber laser in this invention.

[0049] Figure 2 This is a schematic diagram of the second embodiment of the linear cavity fiber laser in this invention.

[0050] Figure 3 This is a pulse diagram of the linear cavity fiber laser in the second embodiment of the present invention (pulse diagram of a 10MHz seven / fourteen-channel output picosecond laser with a pump power of 150mW to 450mW).

[0051] Figure 4 This is a pulse diagram of the linear cavity fiber laser in the first embodiment of the present invention (pulse diagram of a 20MHz five / ten-channel output picosecond laser with a pump power of 150mW to 450mW).

[0052] Figure 5 This is a pulse diagram (pump power 1.5W) of the linear cavity fiber laser in the present invention as the first comparative example.

[0053] Figure 6 This is a pulse diagram of the linear cavity fiber laser in the present invention when it is the first comparative example (pump power 450mW).

[0054] Figure 7 This is a pulse diagram of the linear cavity fiber laser in the present invention when it is the first comparative example (pump power 1.85W).

[0055] Figure 8 This is a pulse diagram (pump power 2W) of the linear cavity fiber laser in the present invention as the first comparative example.

[0056] In the above diagram: 1. Fiber Bragg grating;

[0057] 2. Gain fiber;

[0058] 3. Cascaded optical splitter network; 31. Fiber optic beam splitter;

[0059] 311. First input terminal; 312. Second input terminal; 313. First output terminal; 314. Second output terminal;

[0060] 4. Saturable absorber;

[0061] 5. Pump coupler;

[0062] 6. Linear resonant cavity. Detailed Implementation

[0063] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0064] The terms "first," "second," etc., used in this article do not specifically refer to order or sequence, nor are they intended to limit this case; they are merely used to distinguish components or operations described using the same technical terms.

[0065] The terms "connection" or "positioning" as used in this article can refer to two or more components or devices making direct physical contact with each other, or making indirect physical contact with each other, or to two or more components or devices operating or moving with each other.

[0066] The terms “include,” “including,” and “have” used in this article are all open-ended, meaning they include but are not limited to.

[0067] Unless otherwise specified, the terms used herein generally have their ordinary meaning in the context of the art, the subject matter, and the specific context. Certain terms used to describe this case will be discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing this case.

[0068] The terms “front,” “back,” “up,” “down,” “left,” and “right” used in this article are directional terms. In this case, they are only used to describe the positional relationship between the structures and are not intended to limit the specific direction of the protection scheme or its actual implementation.

[0069] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the scope of this work. Singular forms such as “a,” “this,” “this,” “the,” and “the” as used herein also include plural forms.

[0070] This application aims to design a laser that can achieve a low repetition rate output of a laser beam while maintaining a compact size, and is stable during output.

[0071] See Figures 1-8 As shown, a multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter includes a fiber grating 1, a gain fiber 2, a cascaded beam splitter network 3, a saturable absorber 4, and a pump coupler 5 for injecting pump light into the gain fiber 2.

[0072] The fiber grating 1, the gain fiber 2, the cascaded beam splitter network 3, and the saturable absorber 4 are sequentially connected to form a linear resonant cavity 6 with a target set length. The fiber grating 1 in the linear resonant cavity 6 is connected to the pump coupler 5 to construct a linear cavity fiber laser for achieving a low repetition frequency.

[0073] In use, the pump source injects pump light into the beginning of the linear resonant cavity 6 through the pump coupler 5, and the pump light passes through the fiber grating 1 and enters the gain fiber 2 to be absorbed. The fiber grating 1 acts as a spectral filter to achieve simultaneous output of multiple wavelengths.

[0074] The cascaded optical splitting network 3 includes N optical fiber beam splitters 31, where N is a positive integer. Each optical fiber beam splitter 31 has a first input terminal 311, a second input terminal 312, a first output terminal 313, and a second output terminal 314.

[0075] N fiber optic beam splitters 31 are cascaded in sequence, wherein the gain fiber 2 is connected to the first input terminal 311 of the first fiber optic beam splitter 31; the first output terminal 313 of the first fiber optic beam splitter 31 is connected to the first input terminal 311 of the next fiber optic beam splitter 31, and the first output terminal 313 of the next fiber optic beam splitter 31 is connected to the first input terminal 311 of the next fiber optic beam splitter 31, until the first output terminal 313 of the Nth fiber optic beam splitter 31 is connected to the saturable absorber 4, which is located at the end of the linear resonant cavity 6 and is used to stabilize mode locking at low repetition frequency.

[0076] In this invention, the length of the entire linear cavity fiber laser can be controlled by controlling the target set length of the linear resonant cavity 6, which is composed of fiber grating 1, gain fiber 2, cascaded beam splitter network 3 and saturable absorber 4, so that the volume of the linear cavity fiber laser is smaller. At the same time, after the target set length of the linear resonant cavity 6 is well controlled, a low repetition frequency can be directly achieved in the fundamental frequency mode without the need for complex harmonic mode-locking control.

[0077] In this invention, by utilizing the four-port characteristic of the fiber optic beam splitter 31, multi-channel synchronous output is achieved. Specifically, since each fiber optic beam splitter 31 provides 1-2 outputs, 5 fiber optic beam splitters 31 can provide 5-10 outputs, and 7 fiber optic beam splitters 31 can provide 7-14 outputs. At the same time, the pulses output by all fiber optic beam splitters 31 are generated from the same source, resulting in much higher synchronization accuracy and lower inter-channel time jitter.

[0078] Unlike existing technologies, this invention, through the coordinated design of the linear resonant cavity 6 and the cascaded beam splitting network 3 within the linear resonant cavity 6, can achieve low repetition frequency multi-path output of the laser beam while maintaining the compact size of the linear cavity fiber laser, and the output is stable.

[0079] Inter-channel time jitter refers to the deviation in the arrival time of signal edges between output channels in a multi-output system. Unlike traditional single-channel jitter (the deviation of the signal edge from the ideal time position), inter-channel time jitter focuses on the relative time consistency between channels.

[0080] Preferably, the target set length is proportional to the low repetition frequency of the linear cavity fiber laser, and this set proportional relationship is determined by the following formula:

[0081] f rep = c / (2nL); where c is the speed of light, L is the target length, n is the effective refractive index of the fiber, and f rep For low repetition frequency of linear cavity fiber lasers;

[0082] The low repetition frequency of the linear cavity fiber laser is between 10 and 20 MHz.

[0083] With the above design, by controlling the low repetition frequency of the linear cavity fiber laser within the range of 10-20MHz, the target length can be limited to a certain range. In this way, the length of the linear cavity fiber laser can be constrained within a certain range, and the phenomenon of large volume will not occur.

[0084] Preferably, the fiber optic beam splitter 31 is a 2×2 fiber optic beam splitter 31;

[0085] In use, the fiber optic beam splitters 31 in the cascaded optical splitting network 3 are configured to be 5-7.

[0086] With the above design, the low repetition frequency of the linear cavity fiber laser can be controlled by limiting the number of fiber beam splitters 31 in the cascaded beam splitting network 3. That is, when the low repetition frequency of the linear cavity fiber laser is already controlled, the number of fiber beam splitters 31 can be further controlled, so that both the target set length and the low repetition frequency can be controlled.

[0087] Preferably, the 2×2 fiber optic splitter 31 is a fused biconical or planar waveguide coupler with a splitting ratio of 9:1.

[0088] In this invention, the 9:1 splitting ratio design ensures that the total intracavity loss after cascading 5 or 7 stages remains within a reasonable range, far lower than that of the traditional 50:50 beam splitting scheme. This results in a laser with an extremely low mode-locking threshold and an extremely wide stable operating range, making it insensitive to pump power fluctuations.

[0089] The 2×2 fiber optic splitter 31 is a fused biconical or planar waveguide coupler, and its splitting ratio tolerance is controlled within ±1%, which means that in practice, the actual splitting ratio is allowed to fluctuate within the range of 8.91:1.09 to 9.09:0.91.

[0090] Preferably, the first input end 311 of the fiber beam splitter 31 is configured to receive and amplify the signal laser beam generated after the pump light is absorbed by the gain fiber 2.

[0091] Furthermore, the fiber optic beam splitter 31 is configured to split the signal laser beam input at the first input terminal 311 into two beams, wherein the first signal laser beam with an optical power ratio of 90% is transmitted to the first output terminal 313, and the second signal laser beam with an optical power ratio of 10% is transmitted to the second output terminal 314.

[0092] With the above design, the optical power loss of the first output terminal 313 can be reduced.

[0093] Preferably, at least one of the second output terminal 314 and the second input terminal 312 of each of the fiber beam splitters 31 serves as the signal output port of the linear cavity fiber laser, thereby enabling the linear cavity fiber laser to form multiple outputs.

[0094] The above design is used to achieve multiple outputs of a linear cavity fiber laser.

[0095] In practice, the second input terminal 312 of each 2×2 fiber beam splitter 31 is connected to an angular physical contact (APC) connector to eliminate parasitic reflections, or as an additional signal output port, so that the entire laser system provides 2N output channels.

[0096] In other words, in practice, of the first input terminal 311, the second input terminal 312, the first output terminal 313, and the second output terminal 314 of the fiber optic beam splitter 31, only the first input terminal 311 is used for input. The other three ports (the second input terminal 312, the first output terminal 313, and the second output terminal 314) can all be used for output, and are used according to actual needs.

[0097] Preferably, the splitting ratio of the fiber optic beam splitter 31 and the number N of the fiber optic beam splitters 31 satisfy the following formula:

[0098] T total = [9 / (9+1)] N ;

[0099] Among them, T total The total transmittance of the linear resonant cavity is between 40% and 60%.

[0100] With the above design, the number of fiber optic splitters 31 can be controlled by controlling the total transmittance of the linear resonant cavity 6, so that the length of the linear resonant cavity 6 will not be too large, that is, the volume will not be too large.

[0101] It should be noted that [9 / (9+1)] N The meaning of 9÷(9+1) is that the splitting ratio of 9:1 indicates that the input optical power is divided into 10 parts, of which 9 parts are allocated to the first output end 313 of the fiber optic beam splitter 31 and 1 part is allocated to the second output end 314 of the fiber optic beam splitter 31. Since multiple fiber optic beam splitters 31 are connected by the first output end 313 of the previous fiber optic beam splitter 31 to the first input end 311 of the next fiber optic beam splitter 31, 9÷(9+1) is the transmittance of a single fiber optic beam splitter 31, that is, the proportion of the input optical power at the first output end 313 is 0.9.

[0102] Preferably, adjacent fiber optic bundle splitters 31 in the cascaded optical splitter network 3 are connected by connecting optical fibers, and the time delay between the output pulses of any two fiber optic bundle splitters 31 and the length difference of the connecting optical fiber between the corresponding two fiber optic bundle splitters 31 satisfy: Δτ=ΔL×n eff / c;

[0103] Where Δτ is the time delay between the output pulses of any two fiber optic bundle splitters, ΔL is the length difference of the connecting fiber between any two fiber optic bundle splitters, c is the speed of light, and n eff The refractive index of the optical fiber is given.

[0104] With the above design, the length of the connecting fiber between any two fiber optic bundle splitters 31 and the time delay between the output pulses of any two fiber optic bundle splitters 31 can be controlled according to the formula, thereby ensuring that the output pulses of each channel are strictly synchronized in the time domain.

[0105] Preferably, in the linear cavity fiber laser, the maximum pump power Pmax and the minimum pump power Pmin of the pump source satisfy: Pmax / Pmin>3;

[0106] Where Pmin is the minimum pump power required to achieve stable mode locking, and Pmax is the maximum pump power that can be withstood to achieve stable mode locking.

[0107] With the above design, the 9:1 splitting ratio and the number N of fiber optic beam splitters 31 are matched to satisfy the total intracavity transmittance T. total =0.9 NThen, the transmittance range is controlled between 0.4 and 0.6 to match the modulation depth of the saturable absorber 4 and the saturation power of the gain fiber 2, thereby suppressing multi-pulse instability while maintaining a low threshold, and achieving stable mode-locking of the pump power in the range of Pmin to Pmax, where Pmax / Pmin>3.

[0108] Preferably, when the fiber optic beam splitter 31 in the cascaded optical splitting network 3 is configured to be 5, the target set length is 5.0 meters to 5.5 meters;

[0109] When the fiber optic beam splitter 31 in the cascaded optical splitting network 3 is configured to be 7, the target set length is 10.0 meters to 11.0 meters.

[0110] Specifically, when N=5, the linear cavity fiber laser operates at a repetition frequency of 20MHz, with a physical cavity length of less than 6 meters, achieving compact low-frequency multi-channel pulse output.

[0111] When N=7, the linear cavity fiber laser operates at a repetition frequency of 10MHz, with a physical cavity length between 10 and 11 meters, providing no fewer than 7 synchronous pulse outputs.

[0112] Working principle: Please refer to Figure 1 and Figure 2 ;

[0113] In use, the pump source injects pump light into the beginning of the linear resonant cavity 6 through the pump coupler 5, and after the pump light passes through the fiber grating 1, it enters the gain fiber 2 and is absorbed. Subsequently, the signal laser beam generated and amplified after the pump light is absorbed by the gain fiber 2 will pass through multiple fiber beam splitters 31 in sequence. Finally, the signal laser beam at a low repetition frequency is stably mode-locked by the saturable absorber 4.

[0114] Specifically, such as Figure 1 and Figure 2 As shown, the present invention provides two laser embodiments, namely Embodiment 1 and Embodiment 2.

[0115] Example 1, as Figure 1 Example 1 of a linear cavity fiber laser: a 20MHz five / ten-output picosecond laser. The design is as follows:

[0116] Gain medium (gain fiber 2): 0.5-meter-long ytterbium-doped fiber (YDF, which has high gain in the 1000-1100nm band).

[0117] The pump source is a 980nm semiconductor laser, which is connected to the linear resonant cavity 6 through a WDM wavelength division multiplexer (the wavelength division multiplexer is the core component of the pump coupler 5).

[0118] For the reflective end, select fiber grating 1 with a center wavelength of 1030nm (reflectivity >90%).

[0119] The mode-locking element is a saturable absorber 4 with a pigtail (modulation depth 1.5%, saturation flux 150μJ / cm², recovery time 5ps).

[0120] The cascaded optical splitting network 3 uses five commercial 2×2 fused tapered fiber couplers, and the splitting ratio of each 2×2 fused tapered fiber coupler is strictly calibrated to 90:10 = (9:1).

[0121] The connection method of Embodiment 1 is as follows: the gain fiber 2 is connected to the first input end 311 of the first fiber beam splitter 31; the first output end 313 of the first fiber beam splitter 31 is connected to the first input end 311 of the next fiber beam splitter 31, the first output end 313 of the next fiber beam splitter 31 is connected to the first input end 311 of the next fiber beam splitter 31, until the first output end 313 of the Nth fiber beam splitter 31 is connected to the saturable absorber 4.

[0122] The fiber length in Example 1 is the sum of the lengths of all connecting fiber segments plus the equivalent optical path of devices such as gain fiber 2. Based on the 20MHz design target, the target length L of the linear resonant cavity 6 is designed to be L≈5.2 meters. Calculation verification: f rep =c / (2nL)≈3×10⁸ / (2×1.46×5.2)≈19.8MHz, which meets the design target of 20MHz.

[0123] Experimental results of Example 1:

[0124] When the pump power is adjusted within the range of 150mW to 450mW, the linear cavity fiber laser can self-start and maintain a stable single-pulse mode-locked state. The emission spectrum shows that the fundamental frequency peak is located at 19.8MHz, the signal-to-noise ratio (SNR) is greater than 70dB, there is no Q-switching sideband, and the pulse width of all 5 outputs is 10ps (hyperbolic secant fitting). The energy decreases slightly in sequence (due to cascaded beam splitting), but it can be further balanced by optimizing the beam splitting ratio gradient. The time interval between each pulse is determined by the length of the connecting fiber. In this embodiment 1, it is set to a delay of 2ns per stage, with a total time window of 8ns.

[0125] Example 2, as Figure 2 As shown, Example 2 of a linear cavity fiber laser is a 10MHz seven- or fourteen-channel output picosecond laser.

[0126] Example 2 has a similar structure to Example 1, except that:

[0127] The number of cascades in the cascaded optical splitter network 3 is 7 x 9:1 2×2 fiber optic beam splitters 31.

[0128] Based on the design target of 10MHz, the target length L of the linear resonant cavity 6 is designed as: target length L≈10.4 meters. Calculation verification: f rep ≈3×10⁸ / (2×1.46×10.4)≈9.9MHz, which meets the design target of 10MHz; at this time, the gain fiber 2 is selected to be 0.8 meters.

[0129] Experimental results of Example 2: Stable mode-locking was achieved over a wide pump range of 150mW to 480mW; 7 (or 14) synchronous pulse outputs were provided with a repetition frequency of 9.9MHz; due to the total transmittance of 7-level 9:1 splitting of 0.97≈47.8%, the cavity loss was moderate, and excellent stability was still maintained, proving the scalability of the design.

[0130] Comparative Example 1 is the same as Example 1 in all other respects, except that the 9:1 fiber beam splitter 31 in Example 1 is replaced with a 50:50 fiber beam splitter 31.

[0131] The total transmittance of the 5-stage cascade after the experiment in Comparative Example 1 was measured to be only 0.55 = 3.125%.

[0132] Furthermore, experiments revealed that mode-locking was difficult to initiate in Comparative Example 1 when the pump power was in the range of 150mW to 450mW. Even with the pump power increased to 2W, the laser in Comparative Example 1 still struggled to initiate mode-locking, or only exhibited unstable mode-locking at an extremely narrow power point (e.g., 1.85W-1.90W), degenerating into continuous light or noise with slight fluctuations. This fully demonstrates the importance of using a 9:1 fiber beam splitter 31 for multi-stage cascading in Embodiments 1 and 2 of this invention.

[0133] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter, characterized in that: It includes a fiber grating (1), a gain fiber (2), a cascaded optical splitter network (3), a saturable absorber (4), and a pump coupler (5) for injecting pump light into the gain fiber (2). The fiber grating (1), the gain fiber (2), the cascaded beam splitter (3) and the saturable absorber (4) are connected in sequence to form a linear resonant cavity (6) with a target set length. The fiber grating (1) in the linear resonant cavity (6) is connected to the pump coupler (5) to construct a linear cavity fiber laser for realizing a low repetition frequency. In use, the pump source injects pump light into the head end of the linear resonant cavity (6) through the pump coupler (5), and the pump light passes through the fiber grating (1) and enters the gain fiber (2) to be absorbed. The fiber grating (1) serves as a spectral filter to achieve simultaneous output of multiple wavelengths. The cascaded optical splitting network (3) includes N fiber beam splitters (31), where N is a positive integer. Each fiber beam splitter (31) has a first input terminal (311), a second input terminal (312), a first output terminal (313), and a second output terminal (314). N fiber optic bundle splitters (31) are cascaded in sequence, wherein the gain fiber (2) is connected to the first input end (311) of the first fiber optic bundle splitter (31); the first output end (313) of the first fiber optic bundle splitter (31) is connected to the first input end (311) of the next fiber optic bundle splitter (31), and the first output end (313) of the next fiber optic bundle splitter (31) is connected to the first input end (311) of the next fiber optic bundle splitter (31), until the first output end (313) of the Nth fiber optic bundle splitter (31) is connected to the saturable absorber (4), which is located at the end of the linear resonant cavity (6) and is used to stabilize mode locking at low repetition frequency; The target set length is proportional to the low repetition frequency of the linear cavity fiber laser, and this proportional relationship is determined by the following formula: f rep = c / (2nL); where c is the speed of light, L is the target length, n is the effective refractive index of the fiber, and f rep For low repetition frequency of linear cavity fiber lasers; The low repetition frequency of the linear cavity fiber laser is between 10-20 MHz; The fiber optic beam splitter (31) is a 2×2 fiber optic beam splitter (31). When in use, the fiber optic beam splitters (31) in the cascaded optical splitting network (3) are configured to be 5-7; The 2×2 fiber optic beam splitter (31) is a fused biconical or planar waveguide coupler with a splitting ratio of 9:

1. In the cascaded optical splitting network (3), adjacent fiber optic bundlers (31) are connected by connecting optical fibers. The time delay between the output pulses of any two fiber optic bundlers (31) and the length of the connecting optical fiber between the corresponding two fiber optic bundlers (31) satisfy the following: Δτ=ΔL×n eff / c; Where Δτ is the time delay between the output pulses of any two fiber optic bundle splitters (31), ΔL is the length of the connecting fiber between any two fiber optic bundle splitters (31), c is the speed of light, and n eff The refractive index of the optical fiber; The second output terminal (314) of each of the fiber optic beam splitters (31) is configured to output an optical signal, and the second input terminal (312) is configured to selectively output an optical signal; When there are 5 fiber optic splitters (31), the 5 fiber optic splitters (31) are configured to provide 5-10 outputs; when there are 7 fiber optic splitters (31), the 7 fiber optic splitters (31) are configured to provide 7-14 outputs.

2. The multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter according to claim 1, characterized in that: The first input end (311) of the fiber beam splitter (31) is configured to receive and amplify the signal laser beam generated after the pump light is absorbed by the gain fiber (2); Furthermore, the fiber beam splitter (31) is configured to split the signal laser beam input at the first input end (311) into two beams, wherein the first signal laser beam with an optical power of 90% is transmitted to the first output end (313), and the second signal laser beam with an optical power of 10% is transmitted to the second output end (314).

3. The multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter according to claim 1, characterized in that: At least one of the second output terminal (314) and the second input terminal (312) of each of the fiber beam splitters (31) serves as the signal output port of the linear cavity fiber laser, thereby enabling the linear cavity fiber laser to form multiple outputs.

4. The multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter according to claim 1, characterized in that: The splitting ratio of the fiber optic beam splitter (31) and the number N of the fiber optic beam splitters (31) satisfy the following formula: T total ＝[9 / （9+1）] N ； Among them, T total The total transmittance of the linear resonant cavity is between 40% and 60%.

5. The multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter according to claim 1, characterized in that: In the linear cavity fiber laser, the maximum pump power Pmax and the minimum pump power Pmin of the pump source satisfy: Pmax / Pmin>3; Where Pmin is the minimum pump power required to achieve stable mode locking, and Pmax is the maximum pump power that can be withstood to achieve stable mode locking.

6. The multi-output low-frequency picosecond mode-locked fiber laser based on a cascaded differential beam splitter according to claim 1, characterized in that: When the fiber optic bundle splitter (31) in the cascaded optical splitting network (3) is configured to be 5, the target set length is 5.0 meters to 5.5 meters; When the fiber optic bundle splitter (31) in the cascaded optical splitting network (3) is configured to be 7, the target set length is 10.0 m to 11.0 m.

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