Nonlinear polarization evolution fiber laser for superposition self-similar dispersion management

Abstract

The invention relates to the field of fiber lasers, and particularly provides a nonlinear polarization evolution fiber laser for superposition self-similarity dispersion management, the fiber laser comprises an oscillator, an amplifier and a compressor, the oscillator is a full-polarization linear NPE mode-locked laser oscillator, the amplifier is a self-similarity amplifier, and the compressor comprises a transmission grating pair; pulse laser generated by the oscillator enters the amplifier, the amplifier carries out self-similarity amplification on input pulses, the amplified pulse laser enters the compressor, and the pulse laser is output by the compressor. According to the nonlinear polarization evolution fiber laser, self-similar dispersion management is superposed, the pulse in a parabola-like shape with the spectral width of 38.7 nm and the duration of 104 fs can be obtained, and generation of femtosecond laser is facilitated. According to the laser, the self-similar pulse can be realized more conveniently and efficiently, and meanwhile, the system instability caused by excessive additional laser coupling equipment is reduced.

Description

Technical Field

[0001] The present application relates to the field of fiber lasers, and in particular to a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management. Background Art

[0002] Nonlinear Polarization Evolution (NPE) fiber laser is an ultrashort pulse fiber laser based on nonlinear optical effects, which is widely used in ultrafast optics, optical communications, precision measurement, and bio-imaging. NPE achieves passive mode locking through the nonlinear polarization effect inside the fiber, which is an important mechanism for achieving ultrashort pulse lasers.

[0003] In high-power ultrafast fiber laser systems, self-similar amplification technology needs to be combined to increase the pulse energy of the laser. Self-similar amplification is an important laser amplification technology that ensures that the laser pulse maintains the self-similarity of its spectrum and pulse morphology during the amplification process by controlling gain and dispersion. It is widely used in ultrafast lasers, laser manufacturing, optical imaging and other fields, and is of great significance to scientific research, industrial processing, medical treatment, etc.

[0004] In the existing laser structure, the system for realizing self-similar amplification is usually more complicated, often requiring multiple gain stages and complex dispersion compensation devices. Nonlinear optical effects such as self-phase modulation and stimulated Raman scattering will appear between different optical elements and fiber connections, resulting in distortion of pulse morphology, which in turn affects the effect of self-similar amplification. The implementation process requires multiple optical elements and devices to achieve multiple processes such as gain, dispersion management, and pulse compression, such as couplers, wavelength division multiplexers, fiber connectors, and beam splitters. There are docking errors and coupling efficiency problems between these optical elements, which increases the complexity of the system. Couplers and optical elements usually need to be precisely docked and adjusted. Minor docking errors will cause light loss, affecting laser quality and system stability; they will also cause beam morphology distortion, increase the complexity of adjustment, and fail to efficiently produce self-similar amplification.

[0005] In summary, existing lasers cannot efficiently produce self-similar amplification, which is not conducive to the generation of ultrafast lasers. Summary of the invention

[0006] The purpose of the present invention is to provide a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management to address the deficiencies in the above-mentioned prior art, so as to solve the problem that the existing lasers cannot efficiently produce self-similar amplification, which is not conducive to the generation of ultrafast lasers.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows: The present application provides a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management, the fiber laser comprises an oscillator, an amplifier, and a compressor, the oscillator is a full polarization linear NPE mode-locked laser oscillator, the amplifier is a self-similar amplifier, and the compressor comprises a transmission grating pair; the pulse laser generated by the oscillator enters the amplifier, the amplifier performs self-similar amplification on the input pulse, the amplified pulse laser enters the compressor, and the pulse laser is output by the compressor.

[0008] Furthermore, the amplifier includes a third optical path segment, a fourth optical path segment, and a second optical fiber segment, and two ends of the second optical fiber segment are respectively connected to the third optical path segment and the fourth optical path segment.

[0009] Furthermore, the third optical path section includes a first optical fiber wavelength division multiplexer, a second optical fiber collimator, and a second laser diode; the output of the isolator enters the second optical fiber collimator.

[0010] Furthermore, the fourth optical path section includes a second optical fiber wavelength division multiplexer, a third optical fiber collimator, and a third laser diode.

[0011] Furthermore, the compressor includes a third reflector, a third transmission grating, a fourth transmission grating, and a fourth reflector.

[0012] Furthermore, the third transmission grating and the fourth transmission grating are arranged opposite to each other.

[0013] Furthermore, the oscillator includes a first optical path segment, a second optical path segment, and a first optical fiber segment, and two ends of the first optical fiber segment are respectively connected to the first optical path segment and the second optical path segment.

[0014] Furthermore, the first optical path section includes a first reflector, a λ / 8 wave plate, a Faraday rotator, a wavelength division multiplexing collimator, and a first laser diode.

[0015] Furthermore, the second optical path section includes a first optical fiber collimator, a polarization beam splitter, a first half-wave plate, a first transmission grating, a second transmission grating, a second reflector, a second half-wave plate, and an isolator.

[0016] Furthermore, the first optical fiber segment and the second optical fiber segment are Yb-doped optical fibers.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) The amplifier of the present application is a self-similar amplifier, and a transmission grating for dispersion management is provided in the compressor, that is, the nonlinear polarization evolution fiber laser of the present application is superimposed with self-similar dispersion management. Dispersion management compensates for the time delay difference caused by the dispersion effect of the optical fiber and other media during the propagation of the optical pulse, avoids the distortion of the pulse morphology, ensures the stability of the pulse in the entire gain process, avoids pulse broadening or distortion, and thus ensures the high efficiency of the self-similar amplification process.

[0018] (2) The duration of the pulse output by the oscillator of the present application is 585 fs. After passing through the self-similar amplifier, a parabolic pulse with a spectral width of 38.7 nm and a duration of 104 fs is obtained; this is shorter than the 200 fs of the conventional chirped pulse amplification technology. Parabolic pulses can achieve high-energy, high-peak-power femtosecond pulses.

[0019] (3) The solution of this application reduces the number of coupling devices between different laser components and amplifiers, making the optical path more direct and eliminating the need for additional laser coupling devices, thereby reducing potential optical losses and system complexity. In other words, the light source, gain medium, dispersion compensation, pulse compression and other components are directly placed in the optical path, avoiding the complex optical connections and fiber docking in traditional solutions, reducing the coupling requirements between multiple components, reducing docking errors between devices and coupling efficiency issues, and improving the overall stability of the laser system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic diagram of a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management provided by the present invention; Figure 2 The present invention provides a nonlinear polarization evolution fiber laser oscillator with superimposed self-similar dispersion management and output pulse spectrum and time characteristics; Figure 3 Comparison results of the self-similar amplification process in different directions of a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management provided by the present invention; Figure 4 The spectrum evolution of a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management provided by the present invention under different forward pump powers; Figure 5 The present invention provides a spectrum and time characteristics of the output pulse of a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management; Figure 6 The present invention provides a power stability test result of a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management.

[0021] Figure markings: 1-first reflector; 2-λ / 8 wave plate; 3-Faraday rotator; 4-wavelength division multiplexing collimator; 5-first laser diode; 6-first optical fiber segment; 7-first optical fiber collimator; 8-polarization beam splitter; 9-first half-wave plate; 10-first transmission grating; 11-second transmission grating; 12-second reflector; 13-second half-wave plate; 14-isolator; 15-first optical fiber wavelength division multiplexer; 16-second optical fiber collimator; 17-second laser diode; 18-second optical fiber segment; 19-second optical fiber wavelength division multiplexer; 20-third optical fiber collimator; 21-third laser diode; 22-third reflector; 23-third transmission grating; 24-fourth transmission grating; 25-fourth reflector. DETAILED DESCRIPTION

[0022] In order to make the implementation process of the present invention clearer, it will be described in detail below with reference to the accompanying drawings.

[0023] The present invention provides a nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management, the fiber laser comprising an oscillator, an amplifier, and a compressor; Figure 1 Schematic diagram of the fiber laser of this application, wherein Figure 1 (a) is an oscillator, Figure 1 (b) is the amplifier, Figure 1 (c) is a compressor. The output of the oscillator enters the amplifier, the amplifier amplifies the input, the output pulse enters the compressor, and finally, the laser is output by the compressor. Specifically, the pulse laser generated by the oscillator enters the amplifier, the amplifier performs self-similar amplification on the input pulse, the amplified pulse laser enters the compressor, and the pulse laser is output by the compressor.

[0024] The oscillator is a fully polarized linear NPE laser oscillator, which is the seed pulse source of the entire system and is used to generate the initial mode-locked laser pulse. The mode-locking is mainly achieved through the NPE effect. The oscillator includes a first optical path segment, a second optical path segment, and a first optical fiber segment 6. The two ends of the first optical fiber segment 6 are respectively connected to the first optical path segment and the second optical path segment. The first optical fiber segment 6 is a Yb-doped optical fiber with a length of 65 cm. The Yb-doped optical fiber of this length can play a role in reducing the positive dispersion in the resonant cavity; and the cavity length is short, and a higher repetition frequency pulse (121.7 MHz) can be obtained. The first optical fiber segment 6 is used as a gain medium. After pumping, stimulated radiation is generated to form an initial laser. The first optical path segment includes a first reflector 1, a λ / 8 wave plate 2, a Faraday rotator 3, a wavelength division multiplexing collimator 4, and a first laser diode 5; the first laser diode 5 is a single-mode 976 nm laser diode with a pump power of 1.4 W. Specifically, one end of the first optical fiber segment 6 is connected to the wavelength division multiplexing collimator 4, and the other end of the wavelength division multiplexing collimator 4 is provided with a Faraday rotator 3, and the side of the Faraday rotator 3 away from the wavelength division multiplexing collimator 4 is provided with a λ / 8 wave plate 2, and the side of the λ / 8 wave plate 2 away from the Faraday rotator 3 is provided with a first reflector 1; the end of the wavelength division multiplexing collimator 4 close to the first optical fiber segment 6 is also connected to the first laser diode 5, and the first laser diode 5 is used to provide pump light, and the pump light is coupled to the gain medium through the wavelength division multiplexing collimator 4. The light beam between the first reflector 1 and the wavelength division multiplexing collimator 4 is spatial light.

[0025] The second optical path segment includes a first fiber collimator 7, a polarization beam splitter 8, a first half-wave plate 9, a first transmission grating 10, a second transmission grating 11, a second reflector 12, a second half-wave plate 13, and an isolator 14. Specifically, the other end of the first fiber segment 6 is connected to the first fiber collimator 7, and the first fiber collimator 7 is arranged away from the first fiber segment 6. The polarization beam splitter 8 divides the light field into a transmission light path and a reflection light path. The first half-wave plate 9, the first transmission grating 10, the second transmission grating 11, and the second reflector 12 are arranged in sequence on the transmission light path. The first transmission grating 10 and the second transmission grating 11 are arranged opposite to each other. In order to compensate for the positive dispersion value brought by the optical fiber and make the net dispersion in the cavity close to zero and biased negative, the laser in the cavity can be controlled to pass through a pair of parallel transmission gratings (LightSmyth, T-1000-1040-3212-94) produces negative dispersion. Setting the spacing of the grating pair can control the negative dispersion value. According to the dispersion calculation formula of the grating pair, the spacing of the grating pair is 8mm; in addition, the line density of the grating pair is 1000 lines / mm. In order to ensure the diffraction efficiency, according to the principle that the diffraction efficiency of the transmission grating is the largest when the angle between the light incident angle and the grating normal is the Littrow angle, the tilt angle of the grating pair is adjusted to deviate from the vertical direction by 31°. The second half-wave plate 13 and the isolator 14 are arranged in sequence on the reflection light path, which play the role of polarization control and optical isolation in the laser oscillator. Specifically, the second half-wave plate 13 is used to adjust the polarization state of the light passing through the polarization beam splitter 8 to optimize the nonlinear polarization evolution locking process and ensure the stable formation and efficient transmission of the pulse. The isolator 14 is used to prevent the unexpected reflection of light outside the linear resonant cavity or return to the resonant cavity to prevent the return light from interfering with the laser oscillator and improve the stability of the system.

[0026] Among them, the λ / 8 wave plate 2 and the Faraday rotator 3 are combined to act as a non-reciprocal phase shifter to provide a non-reciprocal phase shift and reduce the mode locking threshold. The first reflector 1 and the second reflector 12 are used to reflect the laser, so that the light can go back and forth multiple times in the resonant cavity to enhance the mode locking effect. The polarization beam splitter 8 and the first half-wave plate 9 are used to adjust the polarization state to form a stable mode locking. The first transmission grating 10 and the second transmission grating 11 play the role of dispersion compensation and wavelength selection. By applying different path delays to light of different wavelengths, fine management of dispersion is achieved, the positive dispersion caused by the gain medium is compensated, and the time width of the mode locking pulse is ensured to be as short as possible; at the same time, it also helps to optimize the initial conditions of self-similar amplification. Dispersion compensation is performed through fiber dispersion control and transmission grating to optimize the pulse propagation characteristics and reduce the pulse broadening caused by dispersion, so that in the laser oscillator, the optical pulse can maintain good time characteristics and provide good input conditions for subsequent amplification and compression. Specifically, according to the self-similar amplification theory, the input pulse energy determines the amplitude and width of the asymptotic parabolic pulse solution. When the pulse energy is fixed, the rate at which the pulse evolves to the parabolic state depends largely on the duration of the input pulse. In order to compensate for the positive dispersion caused by the gain fiber, a grating pair consisting of a first transmission grating 10 and a second transmission grating 11 is added to the NPE resonant cavity. Since the fiber length is fixed, the net dispersion value in the cavity can be adjusted by changing the spacing between the grating pairs, thereby affecting the pulse duration output by the polarization beam splitter 8. When the amplifier parameters are known, the parabolic morphology evolves fastest, corresponding to a specific set of input pulse energies and optimal input pulses. By adjusting the grating pair spacing and the position of the first half-wave plate 9, the pulse duration can be adjusted to match the optimal input pulse duration of the self-similar amplifier. The modulated and dispersion-managed mode-locked pulse is output and transmitted to the next stage of the self-similar amplifier. The nonlinear polarization evolution mode-locked oscillator can be used as an excellent input for self-similar evolution. In principle, nonlinear polarization rotation mode locking is based on self-phase modulation and cross-phase modulation of the Kerr effect. By constructing an equivalent fast saturable absorber through a polarization-dependent isolator (such as a polarization beam splitter 8) and a polarization controller (wave plate combination), a femtosecond response speed can be achieved. The oscillator pulse output spectrum width is 17.7 nm, the repetition frequency is 121.9 MHz, and the chirped pulse duration is 585 fs.

[0027] The amplifier is a self-similar amplifier, including a third optical path segment, a fourth optical path segment, and a second optical fiber segment 18. The two ends of the second optical fiber segment 18 are connected to the third optical path segment and the fourth optical path segment respectively. The second optical fiber segment 18 is a Yb-doped optical fiber with a length of 3m, a core diameter of 6 μm, and a core absorption coefficient of 250 dB / m (Nufern, PM-YSF-HI-HP) at 976 nm of pumping. The second optical fiber segment 18 amplifies the mode-locked pulse and performs self-similar evolution at the same time. The third optical path segment includes a first optical fiber wavelength division multiplexer 15, a second optical fiber collimator 16, and a second laser diode 17; the output of the isolator 14 in the oscillator enters the second optical fiber collimator 16. One end of the second optical fiber segment 18 is connected to the first optical fiber wavelength division multiplexer 15, and the other end of the first optical fiber wavelength division multiplexer 15 is simultaneously connected to the second optical fiber collimator 16 and the second laser diode 17. The fourth optical path section includes a second fiber wavelength division multiplexer 19, a third fiber collimator 20, and a third laser diode 21; the other end of the second fiber section 18 is connected to the second fiber wavelength division multiplexer 19, and the other end of the second fiber wavelength division multiplexer 19 is simultaneously connected to the third fiber collimator 20 and the third laser diode 21. After passing through the spatial isolator 14, the pulse is coupled into the second fiber collimator 16 with an output power of 2.6 mW; the end of the third fiber collimator 20 away from the second fiber wavelength division multiplexer 19 serves as the output of the amplifier. The first fiber wavelength division multiplexer 15 and the second fiber wavelength division multiplexer 19 are used to couple the pump light into the gain medium. The second fiber collimator 16 and the third fiber collimator 20 are used to ensure that the amplified light beam has a good spatial mode. The second laser diode 17 is used to pump the second optical fiber segment 18 to achieve self-similar amplification, increase the pulse energy, and maintain its shape; since the ytterbium-doped optical fiber can amplify the input pulse by stimulated radiation, the pump light provided by the second laser diode 17 is absorbed by the doped ytterbium ions and converted into gain, thereby achieving self-similar amplification. During the process, the energy of the ultrashort pulse is enhanced while maintaining its time structure to ensure that the pulse is not severely distorted due to the amplification process. The third laser diode 21 is also used to pump the second optical fiber 18 (reverse pumping). Compared with the second laser diode 17 (forward pumping), according to the experimental results, the second laser diode 17 has a better effect on achieving self-similar amplification. The amplified pulse is transmitted to the compression device for final pulse compression to obtain ultrashort pulse laser output.

[0028] The compressor includes a third reflector 22, a third transmission grating 23, a fourth transmission grating 24, and a fourth reflector 25. The third transmission grating 23 and the fourth transmission grating 24 are arranged opposite to each other, and the third transmission grating 23 and the fourth transmission grating 24 are arranged between the third reflector 22 and the fourth reflector 25. The third reflector 22 is a semi-transparent and semi-reflective mirror arranged at 45° to the optical path, and the reflecting surface faces the grating pair side. The laser emitted by the third fiber collimator 20 is irradiated on the third reflector 22, and the transmitted laser enters the grating pair. The laser transmitted through the grating pair is reflected by the third reflector 22, and finally reflected on the surface of the third reflector 22. The reflected laser is the output laser of the fiber laser of the present application, which is a compressed pulse. The third transmission grating 23 and the fourth transmission grating 24 are 1000 lines / mm, which are used to eliminate the positive linear chirp in the parabolic pulse; the line density of the grating pair (LightSmyth, T-1000-1040-3212-94) is 1000 lines / mm. The maximum diffraction efficiency of the transmission grating corresponds to an angle of 31° between the incident light and the grating normal. The spacing of the grating pair needs to be adjusted according to the pulse width measurement results to compress the pulse width to the shortest.

[0029] In the experiment, the pulse mode can be locked by rotating the angle of the λ / 8 wave plate 2 and the first half-wave plate 9. When the angle is rotated to a specific angle, the fiber oscillator automatically starts at a pump power of 1.4 W, and the angles of the two wave plates are fixed at this time. The larger pump power provides a stronger nonlinear dependence to achieve the self-starting and initial mode locking of the laser. Subsequently, the pump power is gradually reduced to 240 mW, and a stable single pulse repetition frequency of 121.7 MHz can be obtained. The output pulse parameters are as follows: Figure 2 The spectrum was measured using an optical spectrum analyzer (Yokogawa, AQ6370B). Figure 2 (a) shows the spectrum measured with a spectral resolution of 0.05 nm, where the output spectrum center wavelength is 1034 nm and the full-width half-maximum (FWHM) bandwidth is 17.7 nm. Figure 2 (b) is the autocorrelation curve of the output pulse. Assuming that the pulse has a Gaussian envelope, its FWHM pulse duration is 585 fs. The test curve has a high degree of coincidence with the Gaussian fitting curve, indicating that the dispersion management has been optimized to the evolution condition close to that of the Gaussian pulse, and the pulse quality is good. Subsequently, the applicant measured the pulse sequence, such as Figure 2 As shown in (c), the pulse interval is 8.3 ns. Figure 2 (d) is the pulse detection result over a long period of time, indicating that the pulse sequence has good stability.

[0030] The applicant used forward and backward pumping to compare the self-similar amplification process in different directions. The comparison results of the self-similar amplification process in different directions are shown in Figure 2. Figure 3 As shown, Figure 3(a) is the result of forward pumping spectrum. Figure 3 (b) is the result of backward pumping spectrum. Figure 3 The effects of the two pumping modes on spectral broadening under different pump powers (100 mW, 300 mW and 600 mW) are also shown. The experimental results show that as the pump power increases, the forward pumping has a more obvious effect on spectral broadening. On the contrary, under the action of backward pumping, the spectral broadening is not obvious. Compared with backward pumping, the gain at the fiber input end is higher when forward pumping, and the initial pulse is subject to stronger nonlinear effects during transmission, so the spectral broadening is greater. In the backward pumping mode, the population inversion distribution increases monotonically along the fiber length. Due to the lower gain in the front half of the fiber, a longer gain fiber is required to achieve the same signal light amplification effect compared to the forward pumping mode. The gain distribution of Yb highly doped fiber affects the evolution of the spectrum, making it closer to the ideal parabolic shape. Under the same initial pulse parameters, forward pumping can accelerate the self-similar evolution process and achieve parabolic pulse output in a shorter fiber length.

[0031] Figure 4 is the spectrum evolution under different forward pump powers. As the pump power increases, the spectrum broadens and gradually tends to a parabolic shape. Figure 4 (b) visualizes the variation of spectral width (left) and output power (right). The input pulse has a narrowband spectrum centered at 1035 nm. At 1035 nm, the spectrum is broadened toward the long and short wavelengths due to the self-phase modulation (SPM) effect. As the pump power increases, the number of population inversions increases, leading to more significant spectral broadening. At a forward pump power of 800 mW, the average output power reaches 256 mW with a pulse energy of 2.1 nJ, which is nearly 100 times the initial pulse energy.

[0032] Figure 5 Spectral (a) and temporal characteristics (b) of the fiber laser output pulse for this application. Figure 5 As shown in (a), the self-similar amplification direct output measurement obtained an output with a 38.7 nm broadband spectrum. The spectrum edge on the long-wave side is smooth and conforms to the parabola curve, showing typical self-similar amplification characteristics. However, the pulse spectrum amplified in the experiment showed multiple peaks on the short-wave side, deviating from the ideal parabola shape. The main reason is that the limited gain bandwidth of the Yb-HI fiber limits the expansion of the spectrum in the short-wave direction. The compressed pulse output from the compressor has a duration of 104 fs ( Figure 5(b)), which is also one of the typical characteristics of self-similar amplification; current femtosecond laser sources usually output pulses with wide bandwidth chirps, and the pulse width is usually on the order of several picoseconds or even more than ten picoseconds. Additional spatial compressors and filters are usually required to adjust the pulses to a suitable ΔT. The laser of this application does not require additional spatial compressors and filters. The Gaussian fitting effect of the output pulse is good. The grating in the compressor limits the minimum achievable pulse width on the compression efficiency. Compared with the pulses output by the oscillator, the pulses after self-similar amplification have a wider spectrum, so the pulse width is shorter.

[0033] Figure 6 To compress the pulse stability test results, the measured power fluctuation was maintained at 0.19% (RMS) within 6 hours. Polarization maintaining (PM) fiber is used so that the stability of the system is mainly affected by the pump power fluctuation.

[0034] This application realizes a parabolic pulse evolution system without additional compression or filtering devices. In the experiment, a fully polarized NPE fiber oscillator provides input pulses to the amplifier. An adjustable grating pair is used in the oscillator to match the optimal input pulse duration of the amplifier. The pulse duration of the oscillator output is 585 fs, which is suitable for direct entry into self-similar amplification without additional pre-shaping equipment.

[0035] Close to the amplifier's optimal ΔT, ΔT=3[(γβ 2 U in ) / (2g)] 1 / 3 , β 2 represents the group velocity dispersion parameter, γ represents the nonlinear parameter, g represents the gain coefficient, U in represents the input pulse energy. When these parameters are determined, there is an optimized input pulse duration so that the pulse can effectively converge to the parabolic state. This application compares the effects of forward and backward pumping on self-similar evolution and finds that forward pumping has a better effect on the formation of parabolic pulses in shorter gain fibers. At a forward pump power of 800 mW, the self-similar amplifier produces linearly chirped parabolic pulses with a spectral width extended to 38.7 nm. In addition, the average power of the pulse increased to 256 mW and maintained a fluctuation of 0.19% (RMS) within 6 hours. Through a self-similar amplification system, the pulse can evolve into a parabolic shape, thereby effectively improving the quality of compressed pulses in chirped pulse amplification systems.

[0036] This application constructs a fully polarization-maintaining linear-nonlinear polarization evolution mode-locked fiber laser, and uses dispersion management as the input light source for self-similar amplification. The fiber laser can directly output the optimized self-similar input pulse width, making the realization of self-similar pulses more convenient and efficient, while reducing the system instability caused by too many additional laser coupling devices.

[0037] In addition, in this application, large mode area (LMA) Yb-doped optical fiber can be used to increase the optical fiber core diameter from the original 6 μm to 10-12 μm, effectively reducing the nonlinear coefficient, reducing the impact of self-phase modulation (SPM) and cross-phase modulation (XPM) on spectral broadening, and making the pulse evolution closer to the ideal parabola shape. At the same time, the optical fiber length is optimized, shortened from the original 3m to 2.5m, to ensure a more uniform gain distribution, avoid gain saturation caused by excessive optical fiber length, and improve amplification uniformity.

[0038] Preferably, a double-clad fiber structure can also be used to improve the pump light-gain light conversion efficiency through more efficient pump absorption, reduce the loss of unabsorbed pump light, and further enhance the pulse energy. In terms of doping concentration, a low-doping Yb fiber (0.1 mol%) is selected to make the population inversion distribution smoother, optimize gain management, and ensure the stability of self-similar evolution during amplification. Through the above optimization, the fiber gain efficiency is increased by about 15%-20%, the pulse energy is significantly improved, the spectrum broadening is more stable, and finally a higher quality self-similar amplification output is obtained, providing a better ultrafast laser source for the system.

[0039] In order to optimize the dispersion compensation of the oscillator, improve the quality of the mode-locked pulse, and enhance the efficiency of subsequent self-similar amplification, a fiber Bragg grating is used to replace the first transmission grating 10 and the second transmission grating 11. The chirped fiber Bragg grating has controllable negative dispersion compensation capability, and can optimize the net dispersion in the cavity by adjusting the chirp parameters, shorten the output pulse duration to 500 fs, and improve the initial conditions of self-similar amplification. In addition, compared with the transmission grating, the all-fiber structure of the fiber Bragg grating avoids the coupling error of the free-space optical element, improves the system stability, and reduces energy loss, thereby optimizing the mode-locking performance of the fiber laser.

[0040] A highly nonlinear photonic crystal fiber is used to replace the third transmission grating 23 and the fourth transmission grating 24. It has a strong negative dispersion compensation capability, and its dispersion parameter is β 2= -0.05 ps² / m, which can effectively compensate for the positive linear chirp generated in the self-similar amplification process and achieve more efficient pulse compression. Compared with traditional transmission gratings, the all-fiber structure of this scheme reduces the energy loss in free-space optical systems, reduces beam alignment errors, and improves the long-term stability of the system.

[0041] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A nonlinear polarization evolution fiber laser with superimposed self-similar dispersion management, the fiber laser comprising an oscillator, an amplifier, and a compressor, characterized in that: The oscillator is a fully polarized linear NPE mode-locked laser oscillator, the amplifier is a self-similar amplifier, and the compressor includes a transmission grating pair; the pulse laser generated by the oscillator enters the amplifier, the amplifier performs self-similar amplification on the input pulse, the amplified pulse laser enters the compressor, and the pulse laser is output by the compressor.

2. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 1, characterized in that: The amplifier comprises a third optical path segment, a fourth optical path segment, and a second optical fiber segment, and two ends of the second optical fiber segment are respectively connected to the third optical path segment and the fourth optical path segment.

3. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 2, characterized in that: The third optical path section includes a first optical fiber wavelength division multiplexer, a second optical fiber collimator, and a second laser diode; the output of the isolator enters the second optical fiber collimator.

4. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 3, characterized in that: The fourth optical path section includes a second optical fiber wavelength division multiplexer, a third optical fiber collimator, and a third laser diode; the laser emitted from the second optical fiber section first passes through the second optical fiber wavelength division multiplexer and then enters the third optical fiber collimator.

5. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 4, characterized in that: The compressor comprises a third reflector, a third transmission grating, a fourth transmission grating and a fourth reflector; the third transmission grating and the fourth transmission grating are arranged between the third reflector and the fourth reflector.

6. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 5, characterized in that: The third transmission grating and the fourth transmission grating are arranged opposite to each other.

7. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 6, characterized in that: The oscillator comprises a first optical path segment, a second optical path segment, and a first optical fiber segment, and two ends of the first optical fiber segment are respectively connected to the first optical path segment and the second optical path segment.

8. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 7, characterized in that: The first optical path segment includes a first reflector, a λ / 8 wave plate, a Faraday rotator, a wavelength division multiplexing collimator, and a first laser diode; the wavelength division multiplexing collimator, the Faraday rotator, the λ / 8 wave plate, and the first reflector are arranged in sequence from near to far from the first optical fiber segment.

9. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 8, characterized in that: The second optical path segment includes a first optical fiber collimator, a polarization beam splitter, a first half-wave plate, a first transmission grating, a second transmission grating, a second reflector, a second half-wave plate, and the isolator; after passing through the first optical fiber collimator and the polarization beam splitter, it is divided into a transmission light path and a reflection light path, the first half-wave plate, the first transmission grating, the second transmission grating, and the second reflector are sequentially arranged on the transmission light path, and the second half-wave plate and the isolator are arranged on the reflection light path.

10. The nonlinear polarization evolution fiber laser with superposition self-similar dispersion management according to claim 9, characterized in that: The first optical fiber segment and the second optical fiber segment are Yb-doped optical fibers.

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