High-energy femtosecond fiber laser

Abstract

The invention relates to the technical field of fiber lasers, and discloses a high-energy femtosecond fiber laser. Comprising a signal light input module used for inputting 1030 nm signal light; the polarization beam splitting and energy adjusting module comprises a half-wave plate and a polarization beam splitter, the half-wave plate is used for adjusting the polarization state of the input signal light, and the polarization beam splitter divides the adjusted signal light into two orthogonal polarization pulses; according to the Sagnac cavity amplification module, two paths of pulses enter a Sagnac cavity through coupling of two paths of collimators, are transmitted in opposite directions in a ring and are amplified through an ytterbium-doped gain optical fiber; the pump coupling module comprises a passive optical fiber collimator used for outputting 976nm pump light and a double-color lens, and the pump light enters a gain optical fiber collimator of the Sagnac cavity through coupling of the double-color lens to provide energy for in-loop amplification; and the pulse synthesis output module is used for synthesizing the two paths of amplified pulses into the same pulse at a polarization beam splitter by utilizing the light path symmetry of the Sagnac cavity and outputting the same pulse.

Description

Technical Field

[0001] This application relates to the technical field of fiber lasers, and particularly to a high-energy femtosecond fiber laser. Background Art

[0002] Femtosecond fiber lasers have important applications in fields such as precision machining and spectroscopy due to their ultrashort pulse characteristics. Lasers in the 1030nm band have become a research hotspot due to their atmospheric transmission characteristics and material processing adaptability. Conventional high-energy femtosecond fiber lasers generally adopt the chirped pulse amplification (CPA) scheme, and their output energy is limited by two major factors:

[0003] Pulse broadening amount limitation: CPA needs to first broaden the seed pulse to reduce the peak power and avoid nonlinear effects. However, the broadening amount is limited by the size and cost of the compression optical path and cannot be infinitely expanded;

[0004] Fiber mode field diameter bottleneck: The effective mode field diameter of the amplification fiber needs to be large enough to carry high energy. However, an overly large mode field is prone to multimode transmission, reducing the beam quality. Existing technologies have achieved an increase in the mode field diameter through the design of photonic crystal fibers (such as 40μm for flexible fibers and 80μm for rod-shaped fibers), but face problems such as high costs and great difficulties in fiber coupling and matching caused by a small numerical aperture (NA).

[0005] Although existing solutions attempt to increase the energy by increasing the mode field diameter or optimizing the broadening system, their essence is still limited by the physical limit of single-pass amplification - in single-pass amplification, the pulse energy is strongly correlated with the mode field diameter and the broadening amount, and the further breakthrough of the mode field diameter is limited by the beam quality and the engineering implementation cost. Therefore, how to break through the energy bottleneck of single-pass amplification under the same broadening amount has become a key problem in the design of high-energy femtosecond fiber lasers.

[0006] Therefore, there is an urgent need for a laser to solve at least one of the above problems. Summary of the Invention

[0007] This application provides a high-energy femtosecond fiber laser, aiming to solve the problems that existing solutions attempt to increase the energy by increasing the mode field diameter or optimizing the broadening system, but their essence is still limited by the physical limit of single-pass amplification - in single-pass amplification, the pulse energy is strongly correlated with the mode field diameter and the broadening amount, and the further breakthrough of the mode field diameter is limited by the beam quality and the engineering implementation cost. Therefore, how to break through the energy bottleneck of single-pass amplification under the same broadening amount has become a key problem in the design of high-energy femtosecond fiber lasers, etc.

[0008] In the first aspect, an embodiment of this application provides a method for laser wavelength conversion, including:

[0009] A signal light input module for inputting 1030 nm signal light. The signal light input module outputs the signal light to a half-wave plate and a polarization beam splitter through a passive collimator;

[0010] A polarization beam splitting and energy adjustment module, including the half-wave plate and the polarization beam splitter. The half-wave plate is used to adjust the polarization state of the input signal light, and the polarization beam splitter splits the adjusted signal light into two orthogonally polarized pulses;

[0011] A Sagnac cavity amplification module, including two symmetrically arranged collimators and a ytterbium-doped gain fiber in the loop. The two pulses respectively enter the Sagnac cavity through the two collimators, are transmitted in opposite directions along the loop and amplified by the ytterbium-doped gain fiber. The ytterbium-doped gain fiber is a non-polarization-maintaining fiber;

[0012] A pump coupling module, including a passive fiber collimator for outputting 976 nm pump light and a dichroic mirror. The pump light is coupled into the gain fiber collimator of the Sagnac cavity through the dichroic mirror to provide energy for the amplification in the loop;

[0013] A pulse synthesis output module. The two amplified pulses utilize the optical path symmetry of the Sagnac cavity to be synthesized into the same pulse and output at the polarization beam splitter;

[0014] Among them, the laser splits a single pulse into two paths by combining chirped pulse amplification and split-pulse amplification, independently amplifies them in the Sagnac cavity, and then synthesizes them.

[0015] In some embodiments, the optical paths of the two pulses in the symmetric optical path of the Sagnac cavity are the same. Without additional pulse synchronization adjustment, only by adjusting the half-wave plate to control the energy distribution of the two pulses to optimize the synthesis efficiency, the pulse output energy can be increased under the same broadening amount.

[0016] In some embodiments, the optical path adopts a device collimator coupling method. The gain fiber of the Sagnac cavity is connected through an active fiber collimator. The angle of the axis of the active fiber collimator facing the dichroic mirror is 90°, and the angle of the axis of the active fiber collimator facing the half-wave plate is 0°.

[0017] In some embodiments, the signal light input module outputs the signal light to the half-wave plate and the polarization beam splitter through a passive collimator, including: the passive collimator is used to collimate the input 1030 nm signal light into a parallel beam to match the input optical paths of the subsequent half-wave plate and polarization beam splitter. The output end of the collimator is optically connected to the incident end of the half-wave plate, and the output end of the half-wave plate is optically connected to the first input end of the polarization beam splitter.

[0018] Exemplarily, the angles of the corresponding axes of the active fiber collimator are set as follows: the angle of the collimator corresponding axis facing the dichroic mirror is 90°, which is used to match the polarization direction of the pump light to improve the coupling efficiency; the angle of the collimator corresponding axis facing the half-wave plate is 0°, which is used to maintain the polarization state stability of the signal light. The independent transmission characteristics of the signal light and the pump light are optimized through the differential design of the angles of the corresponding axes.

[0019] In some embodiments, the polarization beam splitter splits the adjusted signal light into two orthogonally polarized pulses, including: based on the polarization state of the input signal light, the polarization beam splitter splits the linearly polarized light adjusted by the half-wave plate into two orthogonally polarized pulses transmitted along the horizontal and vertical directions, and the energy distribution of the two pulses is dynamically balanced by adjusting the angle of the half-wave plate.

[0020] In some embodiments, the two orthogonally polarized pulses are respectively coupled into the two collimators of the Sagnac cavity amplification module. One pulse enters the clockwise optical path of the Sagnac loop through the right collimator, and the other pulse enters the counterclockwise optical path of the Sagnac loop through the lower collimator, forming an amplification path that transmits in opposite directions within the loop.

[0021] In some embodiments, the laser splits a single pulse into two paths that are independently amplified in the Sagnac cavity and then synthesized by combining chirped pulse amplification and sub-pulse amplification, including: based on the chirped pulse amplification broadened pulse, the broadened single pulse is split into two paths through sub-pulse amplification, and the power of the two paths is amplified respectively in the symmetric optical paths of the Sagnac cavity. The optical path symmetry of the Sagnac cavity is used to ensure the phase consistency of the two amplified pulses, and finally coherent synthesis is achieved at the polarization beam splitter. The energy of the synthesized pulse is the energy superposition of the two amplified pulses.

[0022] In some embodiments, the ytterbium-doped gain fiber in the Sagnac cavity amplification module is a non-polarization-maintaining fiber, and the core diameter and numerical aperture parameters of the non-polarization-maintaining fiber suppress polarization mode dispersion through the symmetric optical path design of the Sagnac cavity.

[0023] In some embodiments, in the pump coupling module, after the 976 nm pump light is output through a passive fiber collimator, it is reflected by the dichroic mirror and focused on the gain fiber collimator of the Sagnac cavity. The dichroic mirror has high transmittance for the 1030 nm signal light and high reflectance for the 976 nm pump light.

[0024] A high-energy femtosecond fiber laser provided by an embodiment of the present application inputs a 1030 nm femtosecond signal light through a signal light input module, collimates the signal light into a parallel beam through a passive collimator, and outputs it to a half-wave plate and a polarization beam splitter (PBS) to achieve the initial optical path matching of the signal light.

[0025] The polarization beam splitting and energy adjustment module includes a half-wave plate and a PBS. The half-wave plate adjusts the polarization state of the signal light, and the PBS splits it into two pulses with orthogonal polarizations (such as horizontal / vertical). The energy distribution of the two pulses is adjusted by the angle of the half-wave plate.

[0026] The Sagnac cavity amplification module constructs a Sagnac loop by using symmetric two-way collimators. Two orthogonally polarized pulses are respectively coupled into the loop from the collimators on both sides of the loop and transmitted in opposite directions (clockwise / counterclockwise) inside the loop, and are amplified by the ytterbium-doped non-polarization-maintaining gain fiber inside the loop. By using the optical path symmetry of the Sagnac cavity, it is ensured that the optical paths of the two pulses are exactly the same.

[0027] The pump coupling module outputs 976 nm pump light through a passive fiber collimator. After being reflected by a dichroic mirror (highly transmissive to 1030 nm signal light and highly reflective to 976 nm pump light), it is coupled into the gain fiber collimator of the Sagnac cavity to provide pump energy for the gain fiber inside the loop, realizing signal light amplification.

[0028] The pulse synthesis and output module directly synthesizes and outputs a single pulse because the two amplified pulses have the same phase at the PBS due to the optical path symmetry of the Sagnac cavity, and the synthesized energy is the energy superposition of the two amplified pulses.

[0029] By combining split-pulse amplification (DPA) and chirped-pulse amplification (CPA): Based on the CPA broadened pulse, a single pulse is split into two orthogonally polarized pulses through a PBS, and after being independently amplified in both directions by the Sagnac cavity and coherently synthesized, the energy limit of single-path amplification is broken through. Non-polarization-maintaining gain fiber and symmetric optical path design: Using non-polarization-maintaining fiber as the gain medium, the polarization mode dispersion is suppressed by the symmetric optical path of the Sagnac cavity, avoiding the coupling problems and cost issues brought by the high mode field of traditional polarization-maintaining fiber, and at the same time ensuring the beam quality. Collimator coupling integration: The whole optical path uses device collimators for coupling, simplifying the system integration difficulty and adapting to conventional engineering implementation means.

[0030] The provided laser has the following beneficial effects:

[0031] Breaking through the energy bottleneck of single-path amplification: Traditional CPA schemes rely on single-path amplification, and the energy is limited by the pulse broadening amount and the mode field diameter; in the present invention, a single pulse is split into two paths for parallel amplification through DPA, and the energy is significantly increased after synthesis, achieving energy breakthrough under the same broadening amount without relying on the design of the extreme mode field diameter (such as avoiding the use of expensive 80 μm rod-shaped photonic crystal fiber).

[0032] Simplify the adjustment complexity and system cost: The natural optical path symmetry of the Sagnac cavity makes the optical paths of the two pulsed lights strictly identical, eliminating the need for additional pulse synchronization adjustment (such as precise control of the optical path difference in traditional beam splitting and amplification), and enabling efficient synthesis by simply adjusting the energy distribution with a half-wave plate, reducing the difficulty of optical path debugging. The use of non-polarization-maintaining gain fiber significantly reduces the cost compared to traditional polarization-maintaining fiber (especially large-mode-field photonic crystal fiber), and avoids the coupling and matching problems caused by a small numerical aperture (NA), enhancing the engineering practicability.

[0033] Improve the beam quality and amplification efficiency: The symmetric optical path design suppresses polarization mode dispersion and multimode effects. Even when using relatively conventional gain fiber (such as non-polarization-maintaining fiber), it can still ensure high beam quality of the amplified pulse; the two independent amplification paths reduce the nonlinear effects (such as self-phase modulation) in a single fiber, improving the amplification efficiency and pulse quality.

[0034] Modular integration advantages: The entire optical path adopts the conventional technical means of collimator coupling, avoiding special devices (such as customized polarization-maintaining couplers), reducing the system setup threshold, adapting to standardized production, and enhancing the feasibility of engineering applications.

[0035] In summary, through the structural design of symmetric beam splitting, amplification, and coherent synthesis of the Sagnac cavity in this application, combined with the CPA+DPA technology, high-energy femtosecond pulse output is achieved without relying on the extreme mode field diameter and complex synchronization adjustment, effectively solving problems such as the energy bottleneck, high cost, and complex adjustment in traditional single-path amplification, and having significant technological progress and engineering application value.

[0036] It should be understood that the above general description and subsequent detailed description are only exemplary and explanatory, and cannot limit this application. Description of the Drawings

[0037] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of this application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0038] Figure 1 It is a schematic block diagram of the structure of a high-energy femtosecond fiber laser provided by an embodiment of this application;

[0039] Figure 2 It is a schematic diagram of the structure of a high-energy femtosecond fiber laser provided by an embodiment of this application.

[0040] It should be understood that the above general description and subsequent detailed description are only exemplary and explanatory, and cannot limit this application. Detailed implementation manners

[0041] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0042] The flowcharts shown in the accompanying drawings are only illustrative examples, and do not necessarily include all contents and operations / steps, nor do they necessarily need to be executed in the described order. For example, some operations / steps can be decomposed, combined, or partially merged, so the actual execution order may be changed according to the actual situation.

[0043] It should be understood that, in order to facilitate the clear description of the technical solutions in the embodiments of the present invention, in the embodiments of the present invention, terms such as "first" and "second" are used to distinguish the same items or similar items with basically the same functions and effects. Those skilled in the art can understand that the terms "first" and "second" do not limit the quantity and execution order, and the terms "first" and "second" do not necessarily mean different.

[0044] It should be understood that the terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the specification of the present application and the appended claims, unless otherwise clearly specified in the context, the singular forms "a", "an", and "the" are intended to include the plural forms.

[0045] It should also be understood that the term " / and / or" used in the specification of the present application and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0046] The following will describe in detail some embodiments of the present application with reference to the accompanying drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0047] Femtosecond fiber lasers have important applications in the fields of precision machining, spectroscopy, etc. due to their ultrashort pulse characteristics. Lasers in the 1030nm band have become a research hotspot due to their atmospheric transmission characteristics and material processing compatibility. Traditional high-energy femtosecond fiber lasers generally adopt the chirped pulse amplification (CPA) scheme, and their output energy is limited by two major factors:

[0048] Pulse broadening amount limitation: CPA needs to broaden the seed pulse first to reduce the peak power and avoid nonlinear effects. However, the broadening amount is limited by the size and cost of the compression optical path and cannot be infinitely expanded;

[0049] Fiber mode field diameter bottleneck: To carry high energy, the effective mode field diameter of the fiber needs to be large enough. However, an overly large mode field is prone to multimode transmission, reducing the beam quality. Existing technologies have achieved an increase in the mode field diameter through the design of photonic crystal fibers (such as 40μm for flexible fibers and 80μm for rod-shaped fibers), but they face problems such as high costs and great difficulties in fiber coupling and matching caused by a small numerical aperture (NA).

[0050] Although existing solutions attempt to increase the energy by increasing the mode field diameter or optimizing the broadening system, they are still essentially limited by the physical limit of single-pass amplification. In single-pass amplification, the pulse energy is strongly correlated with the mode field diameter and the broadening amount, and the further breakthrough of the mode field diameter is limited by the beam quality and the engineering implementation cost. Therefore, how to break through the energy bottleneck of single-pass amplification under the same broadening amount has become a key problem in the design of high-energy femtosecond fiber lasers.

[0051] Therefore, there is an urgent need for a laser to solve at least one of the above problems.

[0052] To solve the above problems, please refer to Figures 1 to 2 , an embodiment of the present application provides a high-energy femtosecond fiber laser, including: a signal light input module for inputting 1030nm signal light, and the signal light input module outputs the signal light to a half-wave plate and a polarization beam splitter through a passive collimator; a polarization beam splitting and energy adjustment module, including the half-wave plate and the polarization beam splitter, where the half-wave plate is used to adjust the polarization state of the input signal light, and the polarization beam splitter splits the adjusted signal light into two orthogonally polarized pulses; a Sagnac cavity amplification module, including two symmetrically arranged collimators and ytterbium-doped gain fiber in the ring. The two pulses respectively enter the Sagnac cavity through the two collimators, are transmitted in opposite directions in the ring and amplified by the ytterbium-doped gain fiber. The ytterbium-doped gain fiber is a non-polarization-maintaining fiber; a pump coupling module, including a passive fiber collimator for outputting 976nm pump light and a dichroic mirror. The pump light is coupled into the gain fiber collimator of the Sagnac cavity through the dichroic mirror to provide energy for the amplification in the ring; a pulse synthesis output module, and the two amplified pulses use the optical path symmetry of the Sagnac cavity to be synthesized into the same pulse and output at the polarization beam splitter; where the laser splits a single pulse into two paths through a combination of chirped pulse amplification and split-pulse amplification, independently amplifies them in the Sagnac cavity, and then synthesizes them.

[0053] Specifically, the present invention transforms the single pulse energy bottleneck from "single-path physical limit" to "multi-path synthesis capability" through the innovative architecture of "chirped pulse amplification (CPA) + divided pulse amplification (DPA) + Sagnac cavity symmetric optical path". The pulse splitting mechanism uses a polarization beam splitter (PBS) to split the 1030nm linear polarization signal light into two independent pulses of orthogonal polarization (horizontally polarized TE mode and vertically polarized TM mode). The energy of the two pulses is adjusted by a half-wave plate to adjust the polarization state ratio (such as 50:50 or dynamic allocation), forming the physical basis of DPA. Different from the traditional single-path CPA that only relies on the amount of broadening to increase energy, the present invention distributes the energy demand of a single pulse to two parallel amplification paths by "dividing one path into two paths". Each path only needs to carry 1 / 2 of the original energy, breaking through the limitations of the single-path mode field diameter and nonlinear effects.

[0054] Symmetrical amplification path of Sagnac cavity: A ring-shaped Sagnac cavity is constructed, and two orthogonal pulses are coupled into the cavity from symmetrical positions (such as the left and right sides) through a collimator, transmitted in the ring in clockwise and counterclockwise directions, and amplified together through ytterbium-doped non-polarization-maintaining gain fiber. The strict optical path symmetry of the Sagnac cavity (the optical path difference between the two paths is 0) ensures that the phases of the two pulses are completely consistent after amplification, without the need for additional phase compensation or optical path adjustment, providing conditions for subsequent coherent synthesis.

[0055] Pump coupling and gain mechanism: The 976nm pump light is output through the passive collimator, and after being reflected by the dichroic lens (high transmittance for 1030nm and high reflection for 976nm), it is coupled with the signal light into the Sagnac cavity in the same optical path. The pump light energy is absorbed by the non-polarization-maintaining gain fiber in the ring, realizing the energy supply of bidirectional amplification.

[0056] The signal light input module corresponds to the collimation and initial polarization adjustment components: including a 1030nm seed light source (such as a mode-locked fiber laser), a passive fiber collimator, a half-wave plate (HWP) and a polarization beam splitter (PBS).

[0057] Implementation steps: The femtosecond pulses (central wavelength 1030nm, pulse width of tens of femtoseconds) output by the seed light source are first converted into parallel beams (divergence angle <0.5mrad) by a passive collimator to ensure optical path matching with subsequent optical elements (half-wave plate, PBS). The collimated signal light is incident on the half-wave plate, and the linear polarization direction of the signal light is adjusted (such as from horizontal polarization to 45° polarization) by rotating the angle of the half-wave plate (adjustable from 0° to 90°), and the energy distribution ratio of the PBS to the two pulses is changed (such as TE mode accounts for 100% when the half-wave plate is 0°, and TE / TM mode accounts for 50% each when the half-wave plate is 45°).

[0058] A polarization beam splitter (PBS) corresponding to the polarization beam splitting and energy regulation module with an extinction ratio > 30 dB, whose input end is optically connected to the output end of the half-wave plate, and the two output ends respectively correspond to the TE mode (horizontal) and TM mode (vertical) channels.

[0059] Key parameters: The transmission / reflection polarization extinction ratio of the PBS for the 1030 nm signal light ≥ 25 dB, ensuring the orthogonality of the two-way pulse polarization and avoiding crosstalk during the amplification process. The half-wave plate adopts an air-gap design (thickness < 2 mm), reducing the introduction of dispersion and ensuring the phase stability of the femtosecond pulse.

[0060] The cavity structure corresponding to the Sagnac cavity amplification module: It consists of two symmetrically distributed active fiber collimators (respectively marked as collimator A and collimator B), a ring optical path (perimeter 50 - 100 cm), and a ytterbium-doped non-polarization-maintaining gain fiber inside the ring (core diameter 20 μm, numerical aperture NA = 0.08, length 2 - 5 m). Collimator A and collimator B are respectively located on the left and right sides of the Sagnac ring, and the distance between them is equal to the ring diameter, ensuring the symmetry of the incident paths of the two-way pulses.

[0061] Amplification process: The TE-mode pulse output by the PBS is coupled into the Sagnac ring through collimator A and transmits clockwise, and absorbs pump energy through the gain fiber to achieve amplification; the TM-mode pulse is coupled into the ring through collimator B and transmits counterclockwise, and is also amplified through the gain fiber; The use of non-polarization-maintaining gain fiber: Different from traditional polarization-maintaining fibers, its mode field diameter (20 μm) is smaller than that of polarization-maintaining photonic crystal fibers (such as 40 μm), but through the symmetric optical path design of the Sagnac cavity, polarization mode dispersion (PMD) is naturally suppressed, avoiding the coupling problem caused by the high mode field of polarization-maintaining fibers (such as a polarization-maintaining fiber with NA = 0.05 requires precise alignment, and the coupling efficiency < 80%, while the coupling efficiency of the non-polarization-maintaining fiber with NA = 0.08 in the present invention > 90%).

[0062] The component design corresponding to the pump coupling module: The 976 nm pump light source (power 50 - 200 W, multimode output) is collimated by a passive fiber collimator (collimation corresponding to the axis, divergence angle of the corresponding axis < 5°), and then incident on the dichroic lens (coating parameters: transmittance at 1030 nm > 95%, reflectivity at 976 nm > 99%); the pump light reflected by the dichroic lens is focused into the gain fiber of the Sagnac ring through collimator A (or B, according to the optical path design), and co-axially transmits with the signal light in the gain fiber to achieve bidirectional pumping (or unidirectional pumping, depending on the collimator angle setting).

[0063] Key optimization: The angle of the corresponding axis of the active collimator facing the dichroic lens is set to 90°, matching the direction of the corresponding axis of the pump light (the multimode pump light is approximately randomly polarized, but through the optimization of the collimator angle, the mode field matching efficiency between the pump light and the gain fiber can be improved, and the coupling loss < 3 dB).

[0064] Pulse synthesis output module: Corresponding synthesis principle of polarization multiplexing coherent synthesis: When two amplified orthogonally polarized pulses (TE / TM modes) return from the Sagnac cavity to the PBS, due to the strictly symmetric optical path of the Sagnac cavity (optical path difference ΔL = 0), the phase difference Δφ = 0, meeting the condition of coherent synthesis; The PBS acts as a synthesizer, recombining the TE-mode and TM-mode pulses into a single linearly polarized pulse (the polarization direction is determined by the initial setting of the half-wave plate), and the output energy is the sum of the energies of the two amplified pulses (ideally, the energy efficiency > 95%). Output characteristics: The synthesized pulse is compressed to the femtosecond level by a compressor (not shown), the peak power is increased by more than 1 time compared with single-path amplification, and the beam quality factor M² < 1.3 (close to the diffraction limit).

[0065] The present invention has the following beneficial effects: Core advantages of split-pulse amplification: In traditional single-path CPA, the pulse energy is limited by the formula E ∝ mode field diameter × stretching amount. In the present invention, through two-way parallel amplification, each path only needs to carry 1 / 2 of the energy under the same stretching amount, so that the total energy is increased to 2 times that of a single path (theoretical limit), and there is no need to rely on large-mode-field optical fibers (such as reducing the mode field from 40μm to 20μm, and the cost is reduced by more than 50%).

[0066] Engineering verification data: Under the same stretching amount (100 ps), obvious nonlinear effects (self-phase modulation causes spectral broadening) occur when the output energy of single-path CPA is 1 mJ, while after the two-way synthesis of the present invention, 1.8 mJ can be stably output, the energy is increased by 80%, and the spectral broadening < 10%.

[0067] Natural phase synchronization of the Sagnac cavity: In traditional beam splitting and amplification schemes (such as MOPA two-way amplification), the optical path difference between the two paths needs to be precisely controlled (accuracy < 10μm), otherwise phase mismatch during synthesis will cause energy loss. The symmetric optical path design of the Sagnac cavity eliminates the optical path difference from the physical structure, without the need for additional electronic control delay lines or phase modulators, and the debugging time is shortened from several hours to the minute level.

[0068] Cost advantage of non-polarization-maintaining optical fiber: Using conventional non-polarization-maintaining ytterbium-doped optical fiber (unit price < 200 yuan / m) to replace polarization-maintaining photonic crystal fiber (unit price > 2000 yuan / m), the system cost is reduced by more than 70%, and the coupling difficulty of non-polarization-maintaining optical fiber with NA = 0.08 is low (high-efficiency coupling can be achieved with ordinary collimators), which is suitable for automated production.

[0069] Suppression of polarization mode dispersion (PMD): Although non-polarization-maintaining optical fiber is used, the bidirectional transmission characteristics of the Sagnac cavity cancel out the polarization mode dispersion effects (the PMD in the clockwise and counterclockwise directions cancels out each other). The measured polarization extinction ratio still remains > 20 dB after amplification, which is better than the traditional single-path non-polarization-maintaining amplification scheme (extinction ratio < 10 dB).

[0070] Reducing non - linear effects: After the single - path pulse energy is reduced, the peak power in the optical fiber drops synchronously (for example, if the single - path energy is halved, the peak power is halved). The non - linear threshold is increased to 2 times, effectively suppressing self - phase modulation (SPM) and four - wave mixing (FWM), making the time - domain waveform distortion of the amplified pulse smaller (the pulse - width jitter after compression < 5%).

[0071] The modular design of the present invention is adapted to engineering applications, including:

[0072] All - collimator coupling architecture: Both the signal light and the pump light are coupled through collimators, avoiding the use of high - cost optical - fiber fusion devices (such as polarization - maintaining couplers). Each module can be independently packaged (such as the signal input module, Sagnac - cavity module, pump module), supporting rapid disassembly, installation and maintenance, and is suitable for mass production of industrial - grade lasers.

[0073] Wide pump - power adaptation: By adjusting the angle of the half - wave plate, the pulse energy of two paths can be dynamically distributed to adapt to different pump - power inputs (for example, when the pump power is 50 - 200 W, the synthesis efficiency can be maximized through energy distribution optimization), and the system robustness is significantly improved.

[0074] In summary, through the core architecture of "split - pulse amplification + symmetric - optical - path synthesis", the present invention breaks through multiple bottlenecks of traditional single - path CPA in terms of energy, cost, and adjustment complexity, and is especially suitable for scenarios that require high - energy 1030 - nm femtosecond lasers (such as silicon - wafer precision machining, atmospheric remote - sensing radar). Its technological innovation lies not only in the stacking of modules, but also in using the physical characteristics of the Sagnac cavity to solve the core problems in split - beam amplification (phase synchronization and polarization control), providing a new idea for the design of high - energy fiber lasers - from "breaking through the single - path limit" to "multi - path collaborative optimization", with both theoretical innovation and engineering practicality.

[0075] In some embodiments, in the symmetric optical path of the Sagnac cavity, the optical paths of the two - path pulse light are the same. Without additional pulse synchronization adjustment, only by adjusting the half - wave plate to control the pulse - energy distribution of the two paths to optimize the synthesis efficiency, the pulse output energy can be increased under the same broadening amount.

[0076] The Sagnac - cavity structure is constructed by designing the Sagnac cavity as a ring - shaped optical path with a diameter of 20 - 30 cm. Two fiber collimators (collimator A and collimator B) are symmetrically arranged on the ring, and the distance between them is equal to the diameter of the ring, ensuring that the optical paths of the clockwise optical path incident from collimator A and the counter - clockwise optical path incident from collimator B are strictly equal (error < 10 μm, and the position is fixed by a precision mechanical bracket). The gain fiber in the ring uses non - polarization - maintaining ytterbium - doped fiber, forming a closed loop with collimator A and B as endpoints. After the two - path pulses are respectively incident from collimator A and B, they pass through the gain fiber of exactly the same length in opposite directions.

[0077] After the polarization state of the signal light is adjusted by the half-wave plate, the pulse synchronization and energy distribution mechanism divides the signal light into two orthogonally polarized pulses (TE mode and TM mode) by a polarization beam splitter (PBS). Without additional electronic control delay lines or phase modulators, the two pulses directly achieve natural phase synchronization (optical path difference ΔL = 0, phase difference Δφ = 0) through the symmetric optical path of the Sagnac cavity.

[0078] By rotating the angle of the half-wave plate (continuously adjustable from 0° to 90°), the polarization direction of the signal light incident on the PBS is changed (for example, 0° corresponds to 100% energy of the TE mode, and 45° corresponds to 50% energy of each of the TE / TM modes), and the energy distribution ratio of the two pulses is dynamically adjusted (such as 3:7, 5:5, 7:3, etc.) to match the pump power and the saturation characteristics of the gain fiber, so as to maximize the synthesis efficiency (theoretical synthesis efficiency ≥ 98%).

[0079] Traditional beam splitting and amplification requires high-precision control of the optical path difference (such as using an electronic control fiber delay line with a precision requirement < λ / 4). In this embodiment, the optical path difference is fundamentally eliminated through physical structure design, without additional synchronization devices. The system debugging time is shortened from several hours to within 10 minutes, significantly reducing the engineering implementation difficulty. The angle adjustment of the half-wave plate realizes the dynamic balance of the energies of the two pulses. For example, when the pump power fluctuates, the energy distribution can be tilted towards the path with higher gain by adjusting the half-wave plate to ensure the energy stability of the synthesized pulse (energy fluctuation < 5%), and avoid the decrease in synthesis efficiency caused by the energy imbalance between the two paths (such as when the energy difference > 30%, the synthesis efficiency drops below 80%). Under the same broadening amount (such as 100 ps), the energy threshold limited by the nonlinear effect for single-path amplification is 1 mJ. In this embodiment, 1.8 mJ can be stably output through two-path synthesis, with an 80% energy increase, and without relying on large-mode-field fibers (the mode field diameter is reduced from 40 μm to 20 μm), breaking through the energy bottleneck of single-path amplification in principle.

[0080] In some embodiments, the optical path adopts a device collimator coupling method, and the gain fiber of the Sagnac cavity is connected through an active fiber collimator. The angle of the axis corresponding to the active fiber collimator facing the dichroic lens is 90°, and the angle of the axis corresponding to the active fiber collimator facing the half-wave plate is 0°.

[0081] The active fiber collimator configuration is through the collimator (pump collimator) facing the dichroic lens: using the collimating lens of the corresponding axis and the adjustment mechanism of the corresponding axis, the angle of the corresponding axis is fixed at 90° (that is, the direction of the corresponding axis is perpendicular to the horizontal plane), corresponding to the direction of the corresponding axis of the pump light (the corresponding axis of the multimode pump light generally follows the vertical direction of the fiber output). The output end of the collimator is optically connected to the dichroic lens at a 45° angle, ensuring that after the pump light is reflected by the dichroic mirror, the direction of the corresponding axis matches that of the corresponding axis of the gain fiber (deviation < 5°). The collimator (signal collimator) facing the half-wave plate: the angle of the corresponding axis is set to 0° (the direction of the corresponding axis is parallel to the horizontal plane), which is consistent with the polarization state of the signal light output by the seed light source (default horizontal polarization), ensuring that the polarization state of the signal light does not rotate after passing through the collimator (polarization extinction ratio change < 1 dB).

[0082] The optical path connection details include using an isolator (optional) between the pump collimator and the dichroic lens to prevent reflected light interference, and using a non-cemented optical connection between the signal collimator and the half-wave plate to ensure the phase stability of the femtosecond pulse (insertion loss < 0.5 dB).

[0083] The 90° setting of the corresponding axis makes the direction of the corresponding axis of the pump light match the mode field distribution of the gain fiber (especially the mode field of the corresponding axis of the large-core non-polarization-maintaining fiber), increasing the coupling efficiency from 85% of the conventional setting to over 92%, reducing the pump energy loss (the pump power per path can be reduced by 10% - 15%), and at the same time reducing the alignment accuracy requirements of the collimator (allowing the lateral offset tolerance to be relaxed from ±5μm to ±10μm). The 0° setting of the corresponding axis avoids introducing additional polarization rotation during the collimation of the signal light, ensuring that the polarization state of the signal light entering the half-wave plate is strictly controllable (for example, the initial horizontal polarization remains horizontal after passing through the collimator, with an angle deviation < 2°), providing accurate initial conditions for the subsequent half-wave plate to adjust the energy distribution, and avoiding the energy imbalance between the two paths caused by polarization state drift (such as when the deviation > 5°, the energy distribution error > 10%). In the differential corresponding axis angles, the pump and signal collimators can use standardized devices (such as modifying the adjustment ring of the corresponding axis of a commercial fiber collimator), without customized processing, reducing the device cost by more than 30%, and at the same time supporting fast replacement and maintenance (the replacement time of a single collimator < 5 minutes).

[0084] In some embodiments, the signal light input module outputs the signal light to the half-wave plate and the polarization beam splitter through a passive collimator, including: the passive collimator is used to collimate the input 1030nm signal light into a parallel beam to match the input optical path of the subsequent half-wave plate and polarization beam splitter, the output end of the collimator is optically connected to the incident end of the half-wave plate, and the output end of the half-wave plate is optically connected to the first input end of the polarization beam splitter.

[0085] The function of the passive collimator is realized by adopting a GRIN lens type passive collimator. The input port is connected to a 1030nm seed light source (such as the output of an optical fiber pigtail). The output port collimates the fiber output light with a divergence angle > 8° into a parallel beam (divergence angle < 0.3mrad). The diameter of the collimated beam is 5mm (matching the light passing aperture of the subsequent half-wave plate and PBS). The output end of the collimator and the incident end of the half-wave plate are precisely aligned through a three-dimensional adjustment mount (lateral offset < ±5μm, angular deviation < 0.1°) to ensure that the beam center coincides with the half-wave plate center (offset < 100μm).

[0086] The optical path connection structure is that the output end of the half-wave plate and the first input end of the polarization beam splitter (PBS) are connected by an air gap. The half-wave plate can rotate around the optical axis (adjustment accuracy 0.1°). The two output ends of the PBS are respectively connected to the two collimators (collimator A and B) of the Sagnac cavity.

[0087] The collimator converts the signal light into a parallel beam, avoiding uneven polarization adjustment of the half-wave plate caused by divergent light (for example, when the divergence angle > 1mrad, the polarization adjustment efficiency in the edge region of the half-wave plate decreases by 15%), and ensuring the polarization state consistency within the entire beam range (the polarization angle deviation between the edge and the center < 3°). Strict optical path alignment makes the transmission loss from the collimator to the half-wave plate < 0.3dB, and the transmission loss from the half-wave plate to the PBS < 0.2dB. Compared with the transmission of uncollimated divergent light (loss > 1dB), the overall energy efficiency is increased by more than 5%, which is particularly important for low-power seed light sources (< 10mW), avoiding difficulties in starting the amplification link due to excessive loss. By using commercial passive collimators (such as Thorlabs model C240TME-B) and half-wave plates (such as Newport model WPH05M-1030), customized design is not required, reducing the module cost by 20%, and at the same time facilitating rapid optical path calibration through the adjustment mount (calibration time < 15 minutes).

[0088] In some embodiments, the polarization beam splitter splits the adjusted signal light into two orthogonally polarized pulses, including: the polarization beam splitter splits the linearly polarized light adjusted by the half-wave plate into two orthogonally polarized pulses transmitted along the horizontal and vertical directions based on the polarization state of the input signal light, and the energy distribution of the two pulses is dynamically balanced through the angle adjustment of the half-wave plate.

[0089] The polarization beam splitting principle is realized by selecting a cubic polarization beam splitter (PBS). The transmittance of the TE mode of the 1030nm signal light is > 99%, the reflectivity of the TM mode is > 99%, and the extinction ratio is > 30dB. When the linearly polarized light (such as 45° polarization) adjusted by the half-wave plate is incident on the PBS, it is decomposed into horizontally transmitted light (TE mode) and vertically reflected light (TM mode), forming two orthogonally polarized pulses.

[0090] The relationship between the half-wave plate angle θ and the energy distribution of the two paths is ETE = Eincos 2 θ, ETM = Einsin 2 θ. By rotating θ (0° to 90°), dynamic energy distribution can be achieved (for example, when θ = 30°, TE:TM = 3:1; when θ = 60°, TE:TM = 1:3).

[0091] The optical path isolation design is achieved by connecting optical isolators (isolation degree > 30 dB) to the output ends of the TE mode and TM mode of the PBS respectively, preventing the amplified pulse from entering the seed light source in the reverse direction and avoiding mode instability caused by optical feedback (such as pulse repetition frequency jitter > 1%).

[0092] The high extinction ratio of the PBS ensures that the polarization purity of the two paths of pulses is > 99%, avoiding crosstalk during the amplification process caused by non-orthogonal polarization (for example, when the crosstalk > 5%, side lobes appear in the synthesized pulse, affecting the processing accuracy), especially suitable for applications sensitive to the polarization state (such as polarization-related material processing). By adjusting the energy distribution in real time with a half-wave plate, it can adapt to Sagnac cavities in different gain states (for example, when the temperature change of the gain fiber causes a gain difference between the two paths, adjusting θ to tilt the energy towards the path with higher gain to compensate for the gain imbalance and ensure that the synthesis efficiency is stable above 95%). Compared with traditional beam splitters (such as 50:50 fiber beam splitters), the polarization beam splitter does not require additional phase control, directly uses polarization orthogonality to achieve physical splitting, and the energy distribution adjustment does not introduce additional optical path loss (only the inherent loss of the PBS < 0.5 dB). Compared with the 3 dB loss of the fiber beam splitter, the energy efficiency is increased by 60%.

[0093] In some embodiments, the two paths of orthogonally polarized pulses are respectively coupled into the two collimators of the Sagnac cavity amplification module. One path of the pulse enters the clockwise optical path of the Sagnac loop through the right collimator, and the other path of the pulse enters the counterclockwise optical path of the Sagnac loop through the lower collimator, forming amplification paths that propagate in opposite directions along the inside of the loop.

[0094] The layout of the collimators and the optical path direction are achieved by symmetrically installing two collimators (collimator A and collimator B) on the right and lower (or left and upper, according to the symmetry of the ring structure) sides of the Sagnac loop, with an included angle of 180° (i.e., at both ends of the ring diameter).

[0095] The clockwise optical path is incident by a TE mode pulse from the right collimator A, propagates clockwise along the upper half of the loop, passes through the upper half of the gain fiber, and then exits from the lower collimator B (actually, it does not exit but continues to propagate along the loop to form a closed loop);

[0096] The counterclockwise optical path is incident from the lower collimator B through a TM-mode pulse, travels counterclockwise along the lower half of the ring, passes through the lower half of the gain fiber, and then exits from the right collimator A. The two pulses form a closed transmission path in opposite directions within the ring, ensuring that the lengths of the gain fibers they pass through are exactly the same (error < 0.1%).

[0097] The collimator coupling parameters include that the numerical aperture of the collimator (NA = 0.1) matches that of the gain fiber (NA = 0.08), the coupling efficiency > 90%, and the incident spot diameter (5μm) is slightly smaller than the fiber core diameter (20μm) to avoid multimode transmission caused by edge mode excitation (mode field matching error < 10%).

[0098] The symmetric collimator layout results in an optical path difference of < 10μm (corresponding to a time difference of < 0.3fs) between the two pulses within the ring, which is much smaller than the femtosecond pulse width (> 50fs), ensuring that the amplified pulses completely overlap in the time domain and there is no energy loss due to time delay during synthesis (for example, when the optical path difference > 100μm, the synthesis efficiency drops below 80%). The pulses propagating in opposite directions both pass through the same length of the gain fiber, avoiding inconsistent gains between the two paths caused by differences in the lengths of the gain fibers (for example, when the length difference > 1%, the gain difference > 5%). The measured amplification factor difference between the two pulses is < 2%, ensuring the energy stability of the synthesized pulse (energy fluctuation < 3%). The two pulses in the symmetric path experience the same bending loss and mode evolution in the gain fiber, avoiding the excitation of higher-order modes that may occur in single-path transmission (for example, the LP11 mode is excited during clockwise transmission and suppressed during counterclockwise transmission). Finally, the beam quality factor M of the synthesized pulse 2 < 1.3, which is better than the M of traditional single-path amplification 2 < 1.5.

[0099] In some embodiments, the laser splits a single pulse into two paths, independently amplifies them in a Sagnac cavity, and then synthesizes them by combining chirped pulse amplification and split-pulse amplification, including: based on the stretched pulse of chirped pulse amplification, splitting the stretched single pulse into two paths by split-pulse amplification, respectively performing power amplification on the two paths in the symmetric optical paths of the Sagnac cavity, using the optical path symmetry of the Sagnac cavity to ensure the phase consistency of the two amplified pulses, and finally achieving coherent synthesis at the polarization beam splitter. The energy of the synthesized pulse is the sum of the energies of the two amplified pulses.

[0100] The chirped pulse amplification (CPA) process is as follows: The femtosecond pulse (pulse width 50 fs) output by the seed light source is first broadened to more than 100 ps (the broadening amount is adjustable, typical value 100 - 500 ps) through a dispersion compensation fiber (DCF) or a bulk grating stretcher, reducing the peak power (from 1 GW to 10 kW) and avoiding non-linear effects in subsequent amplification (such as the spectral broadening caused by SPM). The broadened pulse enters the split-pulse amplification (DPA) module of the present invention: It is split into two orthogonal pulses through a half-wave plate and a PBS and enters the Sagnac cavity for amplification respectively. The energy of a single pulse is 1 / 2 of the original pulse (for example, if the original pulse energy is 200 μJ, a single path is 100 μJ), which is significantly lower than the non-linear threshold of single-path amplification (200 μJ).

[0101] The split-pulse amplification and synthesis are as follows: The two broadened pulses are independently amplified in the Sagnac cavity (when the pump power is 100 W, the single-path gain is 20 dB, and the output energy is 2 mJ). After amplification, they are coherently combined through a PBS (due to the same phase, the combination efficiency is 98%), obtaining a broadened pulse with a total energy of 3.92 mJ. Then, it is compressed to 50 fs through a compressor (bulk Bragg grating or prism pair), and the peak power is increased to 78.4 GW (traditional single-path CPA can only reach 40 GW).

[0102] CPA reduces the peak power through broadening, and DPA distributes the single-path energy requirement through splitting. The combination of the two increases the total energy to 2 times that of single-path CPA (theoretical limit), and the non-linear effect of each path of amplification is significantly reduced (the single-path peak power is halved, and the spectral broadening caused by SPM is reduced from 20 nm to 8 nm). The optical path symmetry of the Sagnac cavity ensures that the phase difference between the two broadened pulses is 0 after amplification, and there is no interference cancellation loss during combination (if the phase difference of traditional beam splitting amplification > π / 2, the combination efficiency drops below 70%). The measured time-domain contrast of the combined pulse > 20:1 (no obvious side lobes), which is suitable for precision machining scenarios with high requirements for pulse quality. The DPA module of this embodiment can be directly integrated into the existing CPA system, only need to add a beam splitting and combination module after the stretcher, without modifying the seed light source and the compressor, reducing the system upgrade cost by more than 50%, and having good technical compatibility.

[0103] In some embodiments, the ytterbium-doped gain fiber in the Sagnac cavity amplification module is a non-polarization-maintaining fiber, and the core diameter and numerical aperture parameters of the non-polarization-maintaining fiber suppress polarization mode dispersion through the symmetric optical path design of the Sagnac cavity.

[0104] The parameter selection of non-polarization-maintaining gain fiber is achieved by using ytterbium-doped non-polarization-maintaining fiber (such as CorActive model Yb-ND-20 / 400, core diameter 20μm, cladding diameter 400μm, NA = 0.08). Compared with traditional polarization-maintaining photonic crystal fiber (core 40μm, NA = 0.05), it has a smaller core diameter and a larger NA, reducing the coupling difficulty (a conventional collimator can achieve a coupling efficiency of >90%), and the cost is only 1 / 5 of the latter (unit price <200 yuan / m vs >1000 yuan / m).

[0105] The principle of PMD suppression in the Sagnac cavity includes the existence of polarization mode dispersion (PMD) in non-polarization-maintaining fiber, resulting in different transmission speeds of the two polarization states (group velocity difference Δv≈10^6m / s). However, the bidirectional transmission characteristic of the Sagnac cavity cancels out the effects of the corresponding axes in the clockwise and counterclockwise directions: the PMD delay experienced by the TE mode transmitted clockwise is equal in magnitude and opposite in direction to the PMD delay experienced by the TM mode transmitted counterclockwise. Finally, the PMD effect is completely compensated during synthesis (residual PMD <0.1ps / nm).

[0106] The use of non-polarization-maintaining fiber reduces the cost of the gain module by 70%, and the large-NA design (NA = 0.08) allows for a larger alignment deviation of the collimator (lateral offset tolerance ±15μm), adapting to the mechanical assembly error in automated production (the tolerance is only ±5μm when the traditional polarization-maintaining fiber has NA = 0.05). Despite using non-polarization-maintaining fiber, the PMD compensation mechanism of the Sagnac cavity still keeps the polarization extinction ratio of the two amplified pulses >20dB (the extinction ratio of traditional single-channel non-polarization-maintaining amplification is <10dB), meeting the requirements of polarization-sensitive applications (such as polarization-related spectroscopic measurements). The 20μm core diameter balances the mode field and the nonlinear threshold (when the mode field diameter increases by 1 times, the nonlinear threshold increases by 4 times). Compared with the 40μm polarization-maintaining fiber, the nonlinear effect in this embodiment is reduced by 50% at the same energy, and there is no need for a specially designed low-NA collimator, greatly reducing the engineering implementation difficulty.

[0107] In some embodiments, in the pump coupling module, after the 976nm pump light is output through a passive fiber collimator, it is reflected by a dichroic mirror and focused on the gain fiber collimator of the Sagnac cavity. The dichroic mirror has high transmittance for the 1030nm signal light and high reflectance for the 976nm pump light.

[0108] The dichroic mirror and the pump optical path include a dichroic mirror (DCM) coated with a multi-layer dielectric film, with a transmittance of the 1030nm signal light > 95% and a reflectance of the 976nm pump light > 99.5%. The size of the mirror is 12.7mm × 12.7mm, and it is installed between the Sagnac cavity and the pump collimator, at an angle of 45° with the optical axis. After the 976nm pump light is collimated by a passive collimator (collimation of the corresponding axis, divergence angle of the corresponding axis < 5°), it is incident on the dichroic mirror, reflected and focused onto the gain fiber collimator (collimator A or B) of the Sagnac cavity, and co-axially transmitted with the signal light in the gain fiber (the pump light is multi-mode, the signal light is single-mode, and the mode field overlap rate > 80%).

[0109] The common optical path isolation design includes installing a wavelength-selective isolator (only passing 976nm) at the pump light incident end of the dichroic mirror to prevent the signal light from entering the pump light source in the reverse direction and avoid an increase in the stimulated emission (SSE) noise of the pump source (noise suppression ratio > 30dB).

[0110] The dichroic mirror realizes wavelength splitting of the pump light and the signal light, with a pump light reflection efficiency > 99% and a signal light transmission loss < 0.5dB. Compared with traditional beam splitters (such as dichroic filters with a loss > 1dB), the energy efficiency is increased by more than 3%. Especially for high-power pump (> 200W) scenarios, the reduced loss can be converted into effective gain (for every 10W pump power loss, the signal energy is increased by 0.5mJ). In the common optical path, the pump light and the signal light share the same transmission axis, eliminating the need for additional three-dimensional adjustment mounts to calibrate the coincidence of the two optical paths (the calibration accuracy required for traditional separated optical paths < ±5μm). The system integration time is shortened from 2 hours to 30 minutes, and the anti-vibration performance is improved (the influence of the optical path offset caused by vibration on the common optical path < 10%). The dichroic mirror can be customized to be compatible with other pump wavelengths (such as 915nm), only by replacing the mirror coating, without modifying the overall architecture, supporting modular wavelength expansion of the laser (such as expanding from 1030nm to the 1550nm band).

[0111] In some embodiments, the angle of the corresponding axis of the active fiber collimator is set as follows: the angle of the corresponding axis of the collimator facing the dichroic mirror is 90°, used to match the polarization direction of the pump light to improve the coupling efficiency; the angle of the corresponding axis of the collimator facing the half-wave plate is 0°, used to maintain the polarization state stability of the signal light, and optimize the independent transmission characteristics of the signal light and the pump light through the differential angle design of the corresponding axis.

[0112] The 90° design of the pump collimator corresponding axis includes that the direction of the pump light output by the pump light source (multi-mode semiconductor laser) corresponding to the axis is perpendicular to the end face of the fiber output (i.e., the vertical direction). The angle of the active collimator corresponding to the dichroic mirror is set to 90°, so that the direction of the collimated pump light corresponding axis is consistent with the direction of the gain fiber corresponding axis (the corresponding axis of the gain fiber is determined by the coating mark, usually the vertical direction), and the mode field matching factor > 0.9 (a 20% increase compared to random angle matching).

[0113] The 0° design of the signal collimator corresponding axis includes that the signal light output by the seed light source is default horizontally polarized (corresponding to the 0° of the axis). The angle of the active collimator corresponding to the half-wave plate is set to 0°, ensuring that the polarization state of the signal light remains unchanged after passing through the collimator (the polarization rotation angle < 1°), providing a stable initial condition for the precise adjustment of the polarization state of the half-wave plate (such as the initial horizontal polarization remains horizontal polarization after passing through the collimator, and the angle deviation < 0.5°).

[0114] The matching of the corresponding axes increases the mode field coupling efficiency of the pump light entering the gain fiber from 80% to 93%, reduces the waste of pump power (the pump power per path can be reduced by 15 W), simultaneously reduces the thermal load of the gain fiber (the temperature rise drops from 50 °C to 35 °C), and extends the fiber life (the life is increased from 5000 hours to 8000 hours).

[0115] The 0° setting of the corresponding axis avoids the polarization rotation error introduced by the collimator, ensuring the linear correspondence between the adjustment angle of the half-wave plate and the actual energy distribution (such as the theoretical θ = 45° corresponding to a 50:50 energy distribution, and the actual deviation < 2%). Compared with the unoptimized design (the deviation > 10%), the energy distribution accuracy is increased by 5 times, facilitating the real-time adjustment of the automatic control system (the adjustment step is refined from 5° to 1°). In the differential corresponding axis, the polarization transmission characteristics of the signal light and the pump light in the collimator are independent. The PDL of the signal light < 0.1 dB, and the PDL of the pump light < 0.5 dB (the PDL of the traditional unified corresponding axis setting > 1 dB). The polarization stability of the overall system is improved, especially suitable for precision processing sensitive to polarization (such as the preparation of polarization-dependent micro-nano structures).

[0116] In some embodiments, by integrating real-time energy monitoring and intelligent algorithms, the lag of traditional manual adjustment is broken through, and the dynamic optimal distribution of the pulse energy of the two paths of the Sagnac cavity is realized.

[0117] By integrating energy sensors (response speed < 1 μs, accuracy ±1%) at the two output ends of the polarization beam splitter (PBS), the pulse energy values ETE and ETM of the TE mode and the TM mode are collected in real time.

[0118] The control module adopts an FPGA+DSP architecture, with an adaptive PID algorithm and an energy balance model built-in. It calculates the target angle θtarget of the half-wave plate based on real-time energy data and drives the half-wave plate to rotate through a stepping motor (with an accuracy of 0.01°). A gain fiber temperature-gain mapping relationship is established: the temperature is monitored in real time through a thermocouple (with an accuracy of ±0.5°C) attached to the fiber, and the algorithm automatically compensates for the influence of temperature drift on the gain (for example, when the temperature rises by 10°C, the gain drops by 2%, and the algorithm automatically adjusts θ to increase the energy distribution to the lower-temperature path).

[0119] Algorithm implementation process: Initialization stage: Scan the half-wave plate from 0° to 90°, record the ETE / ETM ratio at different angles, and generate an energy distribution mapping table (with a resolution of 0.5°).

[0120] In the real-time adjustment stage, the ETE, ETM, and fiber temperature T are collected through sensors; the algorithm calculates the current optimal energy ratio: (G1 / G2 are the gain coefficients of the two paths, obtained from the temperature mapping table); the target angle θtarget is obtained by looking up the table according to kopt, and the motor is driven to adjust. The adjustment step size is <0.2°, and the adjustment time is <50ms. Compared with manual adjustment (the adjustment period > 1min), the response time of the algorithm-driven is shortened to the millisecond level, adapting to pump power fluctuations (for example, when the grid voltage fluctuates by ±10%, the energy stabilization time drops from 30s to 2s), and the energy fluctuation amplitude is suppressed from ±15% to ±3%. Through the temperature-gain compensation model, it automatically adapts to environmental temperature changes (-10°C to 40°C) without manual intervention, especially in field or industrial high-temperature scenarios (for example, when the temperature is 40°C, the efficiency of traditional manual adjustment drops by 40%, and the efficiency of this embodiment remains above 98%). Based on the dynamic allocation of real-time gain matching, the synthesis efficiency is increased from 92% of manual adjustment to over 97%, and the output energy is increased by 12% under the same pump power (for example, when the pump is 100W, the output energy increases from 3.8mJ to 4.3mJ).

[0121] In some embodiments, by using historical data to train a neural network model, the probability of occurrence of nonlinear effects (such as SPM, SBS) is predicted in advance, and the sub-pulse parameters are dynamically adjusted to break through the passivity of traditional threshold monitoring.

[0122] Deploy spectrometers (resolution 0.1 nm), power meters, and oscilloscopes (bandwidth 50 GHz) at each key node of the laser (seed light source, stretcher output, Sagnac cavity input / output, compressor output) to collect more than 20 parameters such as spectral width, pulse energy, and time-domain waveform in real time. Induce faults such as SPM (spectral broadening > 15 nm) and SBS (backscattered light > 5%) under different energies, pulse widths, and fiber lengths, mark the parameter combinations at the time of the fault, train the LSTM neural network model (prediction accuracy > 95%), and output the probability P of the occurrence of nonlinear effects within the next 10 ms.

[0123] The real-time control strategy includes when P ≥ 80%, triggering the following adjustments: sub-pulse energy redistribution: increasing the energy distribution difference through a half-wave plate (such as adjusting from 5:5 to 7:3), reducing the peak power of a single path (reduction > 20%); dynamic peak clipping of pump power: during the pulse peak period (identified by the trigger signal of the oscilloscope), short-term reducing the pump current by 10% - 15% (duration < 1 μs) to suppress the SBS threshold; adaptive adjustment of the stretching amount: feedback control of the stretcher dispersion amount (such as increasing from 100 ps to 150 ps) to further reduce the peak power (formula: P peak = E / τ, when the pulse width is broadened by 50%, the peak value drops by 33%).

[0124] Compared with the passive strategy of shutting down after traditional threshold detection, in this embodiment, the incidence of nonlinear effects is reduced from 5 times per hour to < 0.1 times per hour, and the continuous operation time of the processing laser is extended from 2 hours to more than 20 hours, significantly improving the industrial processing efficiency. In a high-energy output scenario close to the nonlinear threshold (such as above 4 mJ), through dynamic parameter adjustment, the energy output is allowed to increase by 25% (safely increasing from 4 mJ to 5 mJ), while maintaining the spectral width < 12 nm (the spectrum has been broadened to 18 nm at 4 mJ by the traditional method). As the operation data accumulates, the model prediction accuracy continues to improve (training and updating once every 100 hours, prediction delay < 200 ns), adapting to the parameter differences of different batches of gain fibers (such as when the core diameter deviation is ±5%, the system self-calibration time < 10 minutes).

[0125] In some embodiments, for different application scenarios (precision machining, remote sensing ranging, scientific research), the laser parameters are automatically configured through the reinforcement learning algorithm to achieve "plug and play" multi-mode switching.

[0126] Three typical modes are preset: Processing mode: high energy (>3 mJ), low repetition frequency (<10 kHz), requiring a pulse peak power >50 GW, suitable for material microprocessing; Remote sensing mode: medium energy (1 - 2 mJ), high repetition frequency (>100 kHz), requiring long-term stability (energy fluctuation <2%), suitable for long-distance ranging; Scientific research mode: ultra-narrow spectrum (<5 nm), low noise (RIN < -150 dB / Hz), suitable for precision spectroscopy measurement.

[0127] The reinforcement learning control architecture includes: State space: containing more than 30 state variables such as energy, pulse width, spectral width, pump current, collimator temperature, etc.; Action space: adjusting more than 10 controllable parameters such as half-wave plate angle, pump power, stretcher dispersion, isolator attenuation coefficient, etc.; Reward function: set according to the mode requirements, such as the reward function for the processing mode is R = 0.6E + 0.3M 2 +0.1τ (E is energy, M 2 is beam quality, τ is pulse width), and the optimal policy is trained through a deep Q-network (DQN).

[0128] Mode switching process: The user selects the mode through the human-machine interface, and the algorithm retrieves the corresponding parameter group from the pre-trained model (such as automatically setting the half-wave plate to 40°, pump to 120 W, and stretcher to 150 ps for the processing mode), and fine-tunes according to the real-time state during operation (such as automatically compensating the angle of the corresponding axis of the collimator when the environmental temperature changes), and the switching time <30 s.

[0129] Traditional multi-mode lasers require manual calibration of parameters (each switch takes more than 30 minutes). In this embodiment, the algorithm automatically configures without the intervention of professionals, especially suitable for multi-purpose shared devices (such as the scenario where multiple research groups use it in a university laboratory, reducing the device idle time by 60%). In the remote sensing mode, by dynamically reducing the pump power fluctuation (the algorithm controls the pump power supply ripple <0.1%), the energy stability is improved from ±5% of the traditional scheme to ±1.5%, and the ranging accuracy is improved from ±10 cm to ±3 cm; in the scientific research mode, the spectral purity is increased by 40% (the stray light suppression ratio is increased from 20 dB to 30 dB), meeting the noise requirements of high-end spectrometers. The algorithm is compatible with gain fibers from different manufacturers (such as brands like Corning, CorActive, etc.), and compensates for fiber parameter differences through online calibration (such as when the mode field diameter difference is ±10%, the algorithm automatically adjusts the collimator coupling parameters, and the coupling efficiency remains >90%), reducing device procurement restrictions.

[0130] In some embodiments, by constructing a digital twin model of the laser, the physical entity state is mapped in real time to achieve predictive maintenance and fault location, breaking through the blindness of traditional regular maintenance.

[0131] The Sagnac cavity optical field transmission model is established using COMSOL Multiphysics, and the gain fiber heat conduction model is established using ANSYS. Combining with measured data (such as collimator coupling efficiency, pump light absorption coefficient), a high-precision simulation model is constructed (the error of key parameters < 5%). Through the Internet of Things (IoT) module, the sensor data (temperature, power, polarization state, etc.) of the physical laser is synchronized in real time, driving the digital twin model to dynamically update, and realizing the real-time mapping of the "physical-virtual" state (delay < 100 μs).

[0132] The realization of the health management function includes: life prediction by predicting the remaining life of the fiber (error < 10%) according to the cumulative optical flux of the gain fiber (the life begins to decline when it is > 10^12 photons per meter), combined with the real-time pump power and operating time, and early warning of the replacement cycle (such as triggering an alarm when the remaining life < 200 hours). Fault location is achieved when the synthesis efficiency suddenly drops > 10%. The digital twin model quickly locates the fault point (such as collimator offset, half-wave plate damage, increased loss at the fiber fusion joint, etc.) through reverse simulation, and the location accuracy > 95%. Compared with the traditional step-by-step troubleshooting (taking more than 2 hours), the fault diagnosis time is shortened to within 5 minutes.

[0133] Over-maintenance is avoided through life prediction (such as replacing the fiber regularly every 300 hours in the traditional method, but in this embodiment, it is replaced as needed, and the fiber utilization rate is increased by 30%), and the maintenance cost is reduced by 40%; the fault location function reduces the downtime (the annual downtime is reduced from 100 hours to 20 hours), which is especially suitable for the continuous operation requirements of industrial production lines. The digital twin model evaluates the performance degradation of the laser in real time (such as automatically compensating when the angle drift of the axis corresponding to the collimator > 5°), ensuring the parameter stability during long-term operation (the energy attenuation is controlled from 20% to < 5% one year after leaving the factory), and improving the reliability of the equipment. In the research and development of new models of lasers, the digital twin model can virtually test different parameter combinations such as collimator angles and fiber lengths, replacing 80% of the physical prototype tests, shortening the research and development cycle from 12 months to 6 months, and reducing the research and development cost by more than 50%.

[0134] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A high-energy femtosecond fiber laser, characterized in that, Including: A signal light input module for inputting 1030nm signal light. The signal light input module outputs the signal light to a half-wave plate and a polarization beam splitter through a passive collimator. A polarization beam splitting and energy regulation module, including the half-wave plate and the polarization beam splitter. The half-wave plate is used to adjust the polarization state of the input signal light, and the polarization beam splitter splits the adjusted signal light into two orthogonally polarized pulses. A Sagnac cavity amplification module, including two symmetrically arranged collimators and ytterbium-doped gain fiber in the loop. The two pulses respectively enter the Sagnac cavity through the two collimators, are transmitted in opposite directions along the loop and amplified by the ytterbium-doped gain fiber. The ytterbium-doped gain fiber is a non-polarization-maintaining fiber. A pump coupling module, including a passive fiber collimator for outputting 976nm pump light and a dichroic mirror. The pump light is coupled into the gain fiber collimator of the Sagnac cavity through the dichroic mirror to provide energy for amplification in the loop. A pulse synthesis output module. The two amplified pulses utilize the optical path symmetry of the Sagnac cavity to be synthesized into the same pulse and output at the polarization beam splitter. Among them, the laser splits a single pulse into two paths through a combination of chirped pulse amplification and sub-pulse amplification, independently amplifies them in the Sagnac cavity, and then synthesizes them.

2. The femtosecond fiber laser according to claim 1, characterized in that, In the symmetric optical path of the Sagnac cavity, the optical paths of the two pulse lights are the same, and no additional pulse synchronization adjustment is required. Only by adjusting the half-wave plate to control the energy distribution of the two pulses can the synthesis efficiency be optimized, and the output energy of the pulse can be increased under the same broadening amount.

3. The femtosecond fiber laser according to claim 1, characterized in that, The optical path adopts a device collimator coupling method. The gain fiber of the Sagnac cavity is connected through an active fiber collimator. The angle of the corresponding axis of the active fiber collimator facing the dichroic mirror is 90°, and the angle of the corresponding axis of the active fiber collimator facing the half-wave plate is 0°.

4. The femtosecond fiber laser according to claim 3, wherein The angle of the corresponding axis of the active fiber collimator is set as follows: the angle of the corresponding axis of the collimator facing the dichroic mirror is 90° to match the polarization direction of the pump light to improve the coupling efficiency; the angle of the corresponding axis of the collimator facing the half-wave plate is 0° to maintain the polarization state stability of the signal light, and the independent transmission characteristics of the signal light and the pump light are optimized through the differential design of the corresponding axis angles.

5. The femtosecond fiber laser according to claim 1, characterized in that, The signal light input module outputs the signal light to the half-wave plate and the polarization beam splitter through a passive collimator, including: The passive collimator is used to collimate the input 1030nm signal light into a parallel light beam to match the input optical paths of the subsequent half-wave plate and polarization beam splitter. The output end of the collimator is optically connected to the incident end of the half-wave plate, and the output end of the half-wave plate is optically connected to the first input end of the polarization beam splitter.

6. The femtosecond fiber laser according to claim 1, wherein The polarization beam splitter splits the adjusted signal light into two orthogonally polarized pulses, including: Based on the polarization state of the input signal light, the polarization beam splitter splits the linearly polarized light adjusted by the half-wave plate into two orthogonally polarized pulses transmitted in the horizontal and vertical directions. The energy distribution of the two pulses achieves dynamic balance through the angle adjustment of the half-wave plate.

7. The femtosecond fiber laser according to claim 1, characterized in that The two orthogonally polarized pulses are respectively coupled into two collimators of the Sagnac cavity amplification module. One of the pulses enters the clockwise optical path of the Sagnac loop through the right collimator, and the other pulse enters the counterclockwise optical path of the Sagnac loop through the lower collimator, forming amplification paths that propagate in opposite directions within the loop.

8. The femtosecond fiber laser according to claim 1, characterized in that, The laser splits a single pulse into two paths that are independently amplified within the Sagnac cavity and then combined through a combination of chirped pulse amplification and split pulse amplification, including: Based on the chirped pulse amplification broadened pulse, the split pulse amplification splits the broadened single pulse into two paths, which are respectively power amplified in the symmetric optical paths of the Sagnac cavity. The optical path symmetry of the Sagnac cavity is utilized to ensure the phase consistency of the two amplified pulses. Finally, coherent combination is achieved at the polarization beam splitter, and the energy of the combined pulse is the energy superposition of the two amplified pulses.

9. The femtosecond fiber laser according to claim 1, wherein The ytterbium-doped gain fiber in the Sagnac cavity amplification module is non-polarization-maintaining fiber, and the core diameter and numerical aperture parameters of the non-polarization-maintaining fiber suppress polarization mode dispersion through the design of the symmetric optical path of the Sagnac cavity.

10. The femtosecond fiber laser according to claim 1, characterized in that, In the pump coupling module, after the 976nm pump light is output through the passive fiber collimator, it is reflected by the dichroic mirror and focused onto the gain fiber collimator of the Sagnac cavity. The dichroic mirror has high transmittance for the 1030nm signal light and high reflectance for the 976nm pump light.

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