A method for improving the output power of a mode-locked fiber laser oscillator

Abstract

The application relates to the technical field of mode-locked fiber lasers, and particularly discloses a method for improving the output power of a mode-locked fiber laser oscillation stage, which constructs a resonant cavity containing at least two gain units, a mode-locked unit and the like; initial gain is established by injecting pump light into a first gain unit, and then light is injected into a second gain unit to realize mode locking and power improvement; and parameters are adjusted to maintain stable output. The first gain unit reduces the mode-locking threshold, and the second gain unit improves the pulse energy; the two units are functionally separated and cooperated in the same cavity, and the nonlinear effect in the cavity is relieved. The application does not need a cavity amplification structure, can synchronously complete mode locking establishment and power improvement in the oscillation stage, breaks through the power bottleneck, takes into account stability and reliability, has good expansibility, and is suitable for multiple power levels and application scenarios.

Description

Technical Field

[0001] This invention relates to the field of mode-locked fiber laser technology, specifically to a method for improving the output power of the oscillator stage of a mode-locked fiber laser. Background Technology

[0002] The performance indicators of mode-locked fiber lasers typically include average output power, single pulse energy, spectral bandwidth, pulse width, radio frequency signal-to-noise ratio, and long-term operational stability. Among these, the average output power and single pulse energy that the oscillator stage can stably output directly determine the upper limit of the system's subsequent applications (such as the seed power required for subsequent amplification, nonlinear frequency conversion efficiency, pulse compression potential, etc.).

[0003] Most existing mode-locked fiber lasers employ passive mode-locking, achieving pulse establishment and steady-state maintenance through saturable absorption mechanisms. Passive mode-locking can rely on a real saturable absorber or be achieved through artificial saturable absorption methods such as nonlinear polarization rotation. While these approaches can achieve stable mode-locked output, they generally face technical bottlenecks when increasing the output power of the oscillator stage: as the pump power increases, the intracavity nonlinearity significantly intensifies, easily leading to problems such as an increased mode-locking threshold, saturation or even a decline in output power growth, and decreased mode-locking stability. From a physical perspective, optical fibers have a long nonlinear action length. Under high power conditions, pulse propagation in the fiber is affected by multiple nonlinear processes, including cross-phase modulation, stimulated Raman scattering, four-wave mixing, and soliton correlation effects, resulting in significant changes in spectral and temporal characteristics. If these nonlinear effects lack effective structural design and operating point control, they often limit further increases in oscillator stage power and adversely affect mode-locking stability.

[0004] To achieve higher output power, a master oscillator power amplifier structure is typically used in engineering, which involves introducing an external amplifier after the oscillator stage to boost the pulse power. However, this approach does not fundamentally solve the problem of limited output power of the oscillator stage, and the system structure is complex, requiring high levels of isolation and nonlinear management, which is detrimental to system compactness and long-term stable operation.

[0005] Therefore, from the perspective of system simplification and stability improvement, it is necessary to explore a new approach to enhance the power of the oscillator stage: while ensuring mode-locking stability, the oscillation setup and power enhancement processes should be completed collaboratively within the same resonant cavity. This can be achieved by rationally allocating intracavity gain and controlling the pumping method to improve the stable power output of the oscillator stage. Furthermore, to further optimize the spectral and pulse characteristics of the high-power mode-locked output, controlled intracavity nonlinear adjustment techniques can be introduced. For example, by placing fiber optic devices with specific nonlinear and dispersion characteristics within the resonant cavity, the spectral evolution of the intracavity pulse can be modulated, thereby improving the output spectral width and energy carrying capacity without compromising mode-locking stability. However, these measures represent further optimization of the output characteristics, and their prerequisite remains the effective enhancement of the oscillator stage's output power.

[0006] In summary, there is an urgent need for a method that focuses on improving the power of the oscillator stage. This method involves a holistic design of the resonant cavity structure and operating mode to increase the output power of the oscillator stage of the mode-locked fiber laser while ensuring mode-locking stability, and to provide a foundation for further performance optimization. Summary of the Invention

[0007] The purpose of this invention is to provide a method for improving the output power of the oscillator stage in a mode-locked fiber laser, addressing the common problems of limited output power growth and decreased stability in existing mode-locked fiber lasers during oscillator stage power enhancement. Specifically, this invention involves a holistic design of the resonant cavity structure and operating mode of the mode-locked fiber laser, enabling the oscillation setup and power enhancement processes to be completed collaboratively within the same resonant cavity. This improves the average power and single-pulse energy that the oscillator stage can stably output while ensuring stable mode-locked operation. Compared to solutions relying on external amplification or a single gain segment, this invention can effectively enhance the output power of the oscillator stage without significantly increasing system complexity. Furthermore, this invention aims to maintain a stable mode-locked operating state even under high-pump conditions through reasonable pump coordination and in-cavity mode-locking condition control, avoiding problems such as power drop, multi-pulse operation, or output instability caused by gain saturation or enhanced nonlinear effects.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for improving the output power of the oscillator stage of a mode-locked fiber laser, comprising the following steps: A mode-locked fiber laser resonator containing at least two gain units is constructed. The resonator is further provided with a mode-locking unit, a unidirectional transmission component, and an output coupling structure. Injecting the first pump light into the first gain unit enables the resonant cavity to obtain the initial gain required for oscillation establishment; Based on the initial gain, a second pump light is injected into the second gain unit to put the laser into a mode-locked state and boost the power of the pulse in the resonant cavity; Adjusting the mode-locking conditions and pump coordination within the resonant cavity enables the laser to maintain stable mode-locked output under high pump conditions; The mode-locked pulse output with high average power and high single-pulse energy is obtained directly from the oscillator stage through the output coupling structure.

[0009] Furthermore, the first gain unit is located at a first position within the resonant cavity and is used to participate in the mode-locking process under relatively low pump power conditions, thereby reducing the system mode-locking threshold; the second gain unit is located at a different position within the resonant cavity than the first gain unit and is used to boost the energy of the cavity pulse after mode-locking is established.

[0010] Furthermore, the mode-locking unit employs a real saturable absorber or an artificial saturable absorber structure based on nonlinear effects to form a stable pulse modulation mechanism within the resonant cavity.

[0011] Furthermore, the unidirectional transmission component is used to ensure unidirectional laser oscillation and to directly obtain high-power mode-locked pulse output from the oscillation stage through the output coupling structure.

[0012] Furthermore, the power ratio and operating sequence of the first pump light and the second pump light can be adjusted.

[0013] Furthermore, the operating point of the first gain unit is set in a region conducive to stable mode locking, and the operating point of the second gain unit is set after mode locking is formed, thereby distributing and mitigating intracavity nonlinear effects through functional separation and co-cavity cooperation.

[0014] Furthermore, it also includes fiber optic devices, which have specific nonlinear and dispersive characteristics and are disposed within a resonant cavity, to control and adjust the spectral evolution of pulses within the resonant cavity in order to broaden the output spectrum or optimize pulse characteristics.

[0015] The advantages of this invention compared to the prior art are: 1) Achieve effective output power enhancement within the oscillator stage. This invention introduces multiple cooperative gain units within the same resonant cavity and, in conjunction with appropriate pumping methods, enables the mode-locking process and power enhancement process to be completed synchronously within the oscillator stage. This overcomes the technical bottleneck of limited output power in traditional oscillator stages and effectively improves the average output power and single-pulse energy of the oscillator stage.

[0016] 2) It can maintain stable mold-locking operation even under high pumping conditions. By designing the intracavity gain distribution and operating mode as a whole, different gain units can work together in a functional division, which is conducive to maintaining a stable mode-locking state under high pump power conditions and avoiding problems such as increased mode-locking threshold, mode-locking instability, multi-pulse operation or output power drop caused by increased pump power.

[0017] 3) Reduce the mode-locking threshold to improve oscillation start-up characteristics. This invention enables the laser to achieve stable mode-locking under low pump power conditions by setting up a gain unit for participating in mode-locking establishment, thereby reducing the mode-locking threshold and providing a stable working basis for subsequent power increases.

[0018] 4) Mitigating the limitation of power enhancement by intracavity nonlinear effects This invention effectively reduces the adverse effects of nonlinear effects concentrated on a single fiber segment under high power conditions by rationally distributing the gain and power enhancement processes within the cavity, thus helping to improve the stability and controllability of the oscillator stage under high power operating conditions.

[0019] 5) The system is more compact and reliable because it does not rely on an external amplification structure. This invention achieves power enhancement within the oscillator stage, eliminating the need for additional external amplifiers and complex isolation and nonlinear management structures. This simplifies system configuration, improves system integration, reduces system complexity, and facilitates long-term stable operation and engineering applications.

[0020] 6) It has good scalability and application adaptability. The method of the present invention can achieve flexible control of different output power and pulse characteristics by adjusting the pump power of each gain unit, the intracavity mode-locking condition and related parameters. It is suitable for different power levels and application scenarios and has good scalability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a high-power common-cavity fiber laser system; where WDM is wavelength division multiplexer, Combiner is beam combiner, λ / 2 is half-wave plate, λ / 4 is quarter-wave plate, BF is birefringent filter, COL is collimator, and Output is output; except for the dashed lines, all solid lines are optical fibers.

[0022] Figure 2 This is a graph showing the evolution of the system output power with the power of the second pump light when the first pump light has no power input.

[0023] Figure 3 This is the system output characteristic diagram when the second pump optical power is 7.792W; among which, Figure 3 (a) is the spectrum. Figure 3 (b) is the autocorrelation trace plot.

[0024] Figure 4 This is a graph showing the evolution of the system output power as pump 2 power increases when the first pump power is 350mW.

[0025] Figure 5 This is the system output characteristic diagram when the second pump optical power is 8W; among them, Figure 5 (a) is the spectrum. Figure 5 (b) is the autocorrelation trace plot.

[0026] Figure 6 This is a diagram of a cocavity fiber laser system with PCF-assisted spectral broadening.

[0027] Figure 7 This is a graph showing the evolution of the system output power as the second pump light power increases, with the first pump light power being 350mW.

[0028] Figure 8 (a) is the widest spectrum output by the system; Figure 8 (b) is the corresponding single pulse shape diagram.

[0029] Figure 9 It is a radio frequency spectrum diagram, with the inset being a long program sequence diagram of 0-1GHz.

[0030] As shown in the figure: 1. First gain unit, 2. Second gain unit, 3. Resonant cavity, 4. Unidirectional transmission component, 5. Output coupling structure, 6. First pump light, 7. Second pump light. Detailed Implementation

[0031] Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the present invention.

[0032] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use.

[0033] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0034] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0035] The following detailed description, in conjunction with the accompanying drawings, illustrates a method for improving the output power of an oscillator stage in a mode-locked fiber laser according to the present invention.

[0036] Combined with appendix Figure 1-9 This invention will be described in detail below.

[0037] This invention provides a method for improving the output power of the oscillator stage of a mode-locked fiber laser. At least two functionally synergistic gain units are introduced within the same resonant cavity 3, and a multi-pump collaborative driving method is employed to ensure that the mode-locking process and the power enhancement process are completed synchronously within the same resonant cavity 3. Through the overall design of the cavity gain distribution, pump injection method, and mode-locking operating point, the average power and single-pulse energy of the oscillator stage can be stably output while ensuring stable mode-locked operation.

[0038] Compared with traditional single-gain oscillators or schemes that rely on external amplification, this invention does not change the basic form of the mode-locking mechanism, but achieves an effective increase in the output power of the oscillator stage through the coordinated design of the internal structure and the operating mode.

[0039] According to a preferred embodiment of the present invention, the resonant cavity 3 of the mode-locked fiber laser includes: 1) First gain unit 1 The first gain unit 1, located at a first position within the resonant cavity 3, provides the initial gain required for oscillation establishment. This first gain unit 1 can effectively participate in the mode-locking process under relatively low pump power conditions, thereby reducing the system's mode-locking threshold.

[0040] 2) Second gain unit 2 It is located in the resonant cavity 3 at a different position than the first gain unit 1, and is used to further enhance the energy of the pulse in the cavity after mode-locking has been established, so as to increase the output power of the oscillation stage.

[0041] 3) Mold-locking unit A mode-locking unit is disposed in resonant cavity 3 to achieve passive mode-locking. This mode-locking unit can be a real saturable absorber or an artificial saturable absorber structure based on nonlinear effects, so as to form a stable pulse modulation mechanism within resonant cavity 3.

[0042] 4) Unidirectional transmission and output structure A unidirectional transmission component 4 is set in the resonant cavity 3 to ensure unidirectional laser oscillation, and a high-power mode-locked pulse output is directly obtained from the oscillation stage through the output coupling structure 5.

[0043] This invention employs a multi-channel pumping coordination method corresponding to the aforementioned multi-gain unit, specifically including: A first pump light 6 is injected into the first gain unit 1 to lower the mode-locking threshold and provide the initial gain required for oscillation establishment; a second pump light 7 is injected into the second gain unit 2 to establish mode-locking and increase the intracavity pulse energy, thereby increasing the output power of the oscillation stage. By reasonably matching the power ratio and operating sequence of the first pump light 6 and the second pump light 7, the laser can maintain a stable mode-locking state under high total pump conditions, avoiding problems such as mode-locking instability, multi-pulse operation, or output power drop caused by gain concentration or excessive nonlinearity.

[0044] In this invention, the first gain unit 1 mainly provides the initial gain required for oscillation establishment, and its operating point is preferably set in a region conducive to stable mode-locking; the second gain unit 2 mainly participates in power enhancement, and its operating point is set after mode-locking has been formed. Through this functional separation but co-cavity collaboration, the intracavity nonlinear effect is spatially distributed and mitigated, thereby suppressing the overall impact of unfavorable nonlinear processes on mode-locking stability.

[0045] Meanwhile, by adjusting the mode-locking unit and related intracavity parameters, the laser can maintain single-pulse mode-locked output even under high pump conditions, thereby improving the operational stability during the oscillator power boosting process.

[0046] In a further embodiment of the present invention, an optical fiber device with specific nonlinear and dispersion characteristics can be introduced into the resonant cavity 3 to control the spectral evolution of the pulse within the cavity. Through this nonlinear adjustment method, the output spectrum can be further broadened or the pulse characteristics optimized without compromising mode-locking stability, thereby improving the applicability of the oscillator-stage output pulse in subsequent compression or nonlinear applications.

[0047] The specific implementation process of the method for improving the output power of the oscillator stage of a mode-locked fiber laser according to the present invention is as follows: Example 1: A method for increasing the power of the oscillator stage of a common-cavity, multi-gain, synergistically pumped mode-locked fiber laser. Figure 1This is a high-power fiber laser system with common cavity, multiple gain, and coordinated pumping. In this system, the oscillator stage and the amplification stage coexist. The first pump beam 6 (Pump1) is a single-mode semiconductor laser with a maximum power output of 350mW and a wavelength of 980nm. The first pump beam 6 is coupled into the resonant cavity 3 through a wavelength division multiplexer (WDM). The first gain unit 1 (YDF1, LIEKKI, Yb1200-4 / 125) with a length of 25cm provides the initial gain. The second pump beam 7 (Pump2) is a multimode semiconductor laser with a maximum power output of 8W and a wavelength of 980nm. The second pump beam 7 is coupled into the resonant cavity 3 through a beam combiner. The second gain unit 2 (YDF2, LIEKKI, Yb1200-4 / 125) with a length of 25cm provides the initial gain. The MA-YDF-10 / 130VIII (Nufern) is a 5.3m double-clad large-mode-field ytterbium-doped fiber, providing gain and signal amplification. The unidirectional transmission component 4 uses a high-power collimating isolator (COL-ISO) and collimator to form the system's spatial optical path. This spatial optical path includes a half-wave plate, a quarter-wave plate, a birefringent filter (BF), and a polarizing beam splitter (PBS) to form the NPR mechanism. The isolator (ISO) and COL-ISO in the system constitute the unidirectional transmission component 4, ensuring unidirectional laser operation. The output is achieved at the reflecting end of the PBS. The overall cavity length is approximately 10.7m.

[0048] The second pump light 7 determines whether the laser can oscillate effectively. The system output can be divided into two cases: (1) No power input for the first pump light 6. When there is no power input for the first pump light 6, and the power input for the second pump light 7 is 4.15W, the laser achieves mode-locked output, and the system output power is as follows: Figure 2 As shown, the output power increases linearly with increasing pump power. When the pump power increases to a certain value, the output power tends to saturate. Further increasing the pump power causes the power to decrease. With a pump power of 7.792W, the maximum system output power is 2.426W, and the photoelectric conversion efficiency is 31.95%.

[0049] The pump power is 7.792W, and the system output spectrum is as follows: Figure 3 As shown in (a). The center wavelength is 1052 nm, and the 3dB bandwidth is 14.5 nm. The autocorrelation trace of the output pulse is as follows. Figure 3 As shown in (b), the corresponding pulse width is 394 ps (Gaussian linear fitting).

[0050] (2) The power of the first pump light 6 is 350mW When the first pump light 6 has a power input of 350mW and the second pump light 7 has a power input of 2.5W, the laser achieves mode-locked output, and the system's output power is as follows: Figure 4As shown, in this case, the system output power has no saturation point. When the pump power is 8W, the maximum system output power is 2.527W, and the optical-to-optical conversion efficiency is 30.26%.

[0051] The pump power is 8W, and the system output spectrum is as follows: Figure 5 As shown in (a). The center wavelength is 1061 nm, and the 3dB bandwidth is 17.24 nm. The autocorrelation trace of the output pulse is as follows. Figure 5 As shown in (b), the corresponding pulse width is 387 ps (Gaussian linear fitting).

[0052] Calculations show that the maximum single-pulse energy obtained by the system is 135nJ, and the corresponding peak power is 349W.

[0053] Example 2: A preferred embodiment of introducing an intracavity nonlinear adjustment unit Building upon the high-power fiber laser system with co-cavity multi-gain and synergistic pumping in Example 1, this example further expands upon the co-cavity fiber laser system based on photonic crystal fiber (PCF)-assisted spectral broadening. The experimental setup is as follows: Figure 6 As shown. With Figure 1 Compared to the experimental setup shown, a new 3-meter-long photonic crystal fiber (SC-5.0-1040, NKT) was added between YDF1 and ISO, while the rest remained unchanged. In this system, the PCF used is solid-core with a core diameter of 4.8μm, compatible with single-mode fiber, making splicing easy and with negligible loss.

[0054] As shown in Example 1, the power input of the first pump light 6 can significantly reduce the mode-locking threshold of the system. Therefore, in this example, the power of the first pump light 6 is set to 350mW. When the power of the second pump light 7 is 5W, the system achieves mode-locking, and its output power is as follows: Figure 7 As shown. Although the addition of the PCF increases the pump threshold of the system, it has no impact on the overall output efficiency of the system. Furthermore, no gain saturation was observed. When the power of the second pump light 7 is 8W, the system output power reaches a maximum of 2.594W, with an optical-to-optical conversion efficiency of 31.07%.

[0055] The widest spectrum obtained experimentally and the corresponding pulse are as follows: Figure 8 As shown, the center wavelength is 1061nm, the spectral width is 34.29nm, and the pulse width is 443ps.

[0056] The system operates in the widest radio frequency spectrum, such as Figure 9 As shown in the figure, the repetition frequency is 14.65MHz and the signal-to-noise ratio is 60dB. The inset shows a long sequence of signals from 0 to 1GHz, demonstrating very stable mode-locking. The maximum single-pulse energy obtained by the system is calculated to be 177nJ.

[0057] In this embodiment, the increase in oscillator output power is still achieved by the common cavity multi-gain structure and the cooperative pumping method. The in-cavity nonlinear adjustment unit is only used as a means of further optimizing the output characteristics and does not constitute a necessary condition for the increase in oscillator power.

[0058] As can be seen from the above embodiments, the present invention, through the overall design of the structure and operation mode of the mode-locked fiber laser resonator 3, enables the mode-locking process and the power enhancement process to be completed in the same resonator 3, thereby achieving an effective increase in the output power of the oscillator stage while ensuring stable mode-locking operation.

[0059] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for improving the output power of an oscillator stage in a mode-locked fiber laser, characterized in that, Includes the following steps: A mode-locked fiber laser resonator containing at least two gain units is constructed. The resonator is further provided with a mode-locking unit, a unidirectional transmission component, and an output coupling structure. Injecting the first pump light into the first gain unit enables the resonant cavity to obtain the initial gain required for oscillation establishment; Based on the initial gain, a second pump light is injected into the second gain unit to put the laser into a mode-locked state and boost the power of the pulse in the resonant cavity; Adjusting the mode-locking conditions and pump coordination within the resonant cavity enables the laser to maintain stable mode-locked output under high pump conditions; The mode-locked pulse output with high average power and high single-pulse energy is obtained directly from the oscillator stage through the output coupling structure.

2. The method for improving the output power of the oscillator stage of a mode-locked fiber laser according to claim 1, characterized in that: The first gain unit is located at a first position within the resonant cavity and is used to participate in the mode-locking process under relatively low pump power conditions, thereby reducing the system mode-locking threshold. The second gain unit is located at a different position within the resonant cavity than the first gain unit and is used to boost the energy of the pulse within the cavity after mode-locking is established.

3. The method for improving the output power of the oscillator stage of a mode-locked fiber laser according to claim 2, characterized in that: The mode-locking unit employs a real saturable absorber or an artificial saturable absorber structure based on nonlinear effects to form a stable pulse modulation mechanism within the resonant cavity.

4. A method for improving the output power of an oscillator stage in a mode-locked fiber laser according to claim 3, characterized in that: The unidirectional transmission component is used to ensure unidirectional laser oscillation and to obtain high-power mode-locked pulse output directly from the oscillation stage through the output coupling structure.

5. A method for improving the output power of an oscillator stage in a mode-locked fiber laser according to claim 4, characterized in that: The power ratio and operating sequence of the first pump light and the second pump light can be adjusted.

6. A method for improving the output power of an oscillator stage in a mode-locked fiber laser according to claim 5, characterized in that: The operating point of the first gain unit is set in a region that is conducive to stable mode locking, and the operating point of the second gain unit is set after mode locking is formed. The nonlinear effects in the cavity are distributed and mitigated by functional separation and co-cavity cooperation.

7. A method for improving the output power of an oscillator stage in a mode-locked fiber laser according to claim 6, characterized in that: It also includes fiber optic devices, which have specific nonlinear and dispersive characteristics and are disposed within a resonant cavity, to control and adjust the spectral evolution of pulses within the resonant cavity in order to broaden the output spectrum or optimize pulse characteristics.

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