High pressure nonlinear enhancement based near-infrared few-cycle pulse generation system and method

Abstract

The present application belongs to the field of ultrafast laser and nonlinear optics, and particularly relates to a near-infrared few-cycle pulse generation system and method based on high-pressure nonlinear enhancement. The method fills inert gas into a hollow-core anti-resonant optical fiber and adjusts the gas pressure to the high-pressure interval of 20 bar-90 bar, enhances the nonlinear coefficient of the gas to realize the dispersion and nonlinear dynamic balance of the pulse in the waveguide, thereby compensating for the insufficient peak power of the low-energy pump pulse. The pulse feedback module collects the output time-domain intensity waveform and extracts the main peak energy proportion and the splitting factor as feedback indexes. The control module diagnoses the offset state of the self-compression focal point relative to the truncated end face according to the feedback indexes, controls the small-step pressure regulation of the gas pressure regulation module to anchor the focal point on the truncated end face, and directly outputs high-quality and high-coherence near-infrared few-cycle pulses without external dispersion compensation devices.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast laser and nonlinear optics technology, specifically relating to a near-infrared few-period pulse generation system and method based on high-voltage nonlinear enhancement. Background Technology

[0002] Short-period ultrashort pulses have significant applications in attosecond science, high-harmonic generation, and precision spectroscopy. Due to the limited emission bandwidth of laser gain media, the pulse width directly output by the oscillator is typically on the order of tens to hundreds of femtoseconds. To obtain shorter pulses, nonlinear pulse post-compression technology based on gas-filled hollow waveguides is widely used. This involves filling the waveguide with an inert gas or molecular gas, causing the pulse to broaden its spectrum during transmission through nonlinear effects such as self-phase modulation, followed by dispersion compensation to achieve pulse width compression.

[0003] Currently, research on nonlinear compression in the field of ultrafast optics mainly focuses on 1 The 1550nm near-infrared band (primarily based on ytterbium-doped laser systems) offers millijoule-level high-energy output with extremely high peak power. To avoid gas ionization breakdown and self-focusing effects when processing such high-energy pulses, the system typically operates at extremely low gas pressures (e.g., below 1 bar or a few bar). However, the situation is quite different in the 1550nm near-infrared band (primarily based on erbium-doped laser systems). 1550nm femtosecond lasers have significant advantages such as low cost, high stability, high repetition rate, and being in the communication band, resulting in a large installed base. However, due to gain characteristics, their single-pulse energy is generally low, typically in the range of hundreds of nanojoules to microjoules.

[0004] When attempting to directly apply such 1550nm low-energy pulses to hollow-core fiber pulse compression, the following technical difficulties arise. First, the initial pulse energy is low, and the peak power is insufficient. Under normal low-pressure operating conditions, the nonlinear coefficient provided by the gas medium is extremely small, resulting in insufficient accumulation of nonlinear phase shift and severely inadequate spectral broadening, making few-cycle compression impossible. Second, to increase the internal power density of the fiber under low-energy conditions, the conventional approach is to reduce the core diameter of the hollow fiber. However, if traditional capillary hollow fiber is used, its transmission loss is inversely proportional to the cube of the core diameter; as the core diameter decreases, the confinement loss increases sharply, leading to severe attenuation of the pulse energy during transmission. In contrast, hollow-core antiresonant fiber possesses a broadband antiresonant light guiding mechanism, maintaining extremely low transmission loss even with a small core diameter, making it a more ideal waveguide medium for handling low-energy pulses. Third, even when using hollow-core antiresonant fiber, the fiber typically exhibits significant anomalous dispersion in the 1550nm band. The self-compression evolution of solitons is controlled by the soliton order. For low-energy pulses, the large anomalous dispersion makes the dispersion length short, while the weak nonlinear coefficient under low peak power and low pressure results in an extremely long nonlinear length. The soliton order is often less than 1, and the group velocity dispersion effect is completely autonomously controlled. The pulse will only continue to broaden in the fiber and cannot be compressed.

[0005] Therefore, achieving nonlinear spectral broadening, coherence preservation, and pulse-limited compression of near-infrared low-energy femtosecond pulses in gas-filled high-pressure hollow antiresonant fibers is an extremely challenging task. Blindly applying high pressure to the fiber to compensate for insufficient nonlinearity, while increasing the gas's nonlinear refractive index, simultaneously exacerbates the self-steepening effect and intensifies the poor coupling between higher-order dispersion and nonlinearity. This strong accumulation of nonlinearity leads to severe modulation instability, causing premature soliton breakage. Soliton breakage not only causes the compressed pulse to split into multiple sub-pulses in the time domain with a large amount of basis noise, severely degrading beam quality, but also leads to a sharp degradation of the pulse's time-frequency coherence. Furthermore, blindly applying pressure can easily cause the self-compression focus to deviate from the truncated end face, causing the pulse to complete compression prematurely in the middle of the fiber and then broaden again, completely losing the coherent evolution advantage of few-period pulses. Summary of the Invention

[0006] The purpose of this invention is to provide a near-infrared few-period pulse generation system and method based on high-voltage nonlinear enhancement, so as to solve the problems of dispersion and nonlinear imbalance of near-infrared low-energy pump pulses in hollow anti-resonant optical fibers, and the resulting soliton breakage and coherence degradation.

[0007] The present invention achieves the above objectives through the following technical solutions: In a first aspect, this invention proposes a near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement, comprising: The pump source module outputs an ultrashort pump pulse with a center wavelength located in the anomalous dispersion region of the near-infrared band and a single pulse energy in a preset low-energy range. An energy coupling module, optically connected to the pump source module, is used to adjust the single pulse energy and then couple it to form an injection beam. The fiber evolution module includes a section of hollow anti-resonant fiber and high-voltage optical chambers at both ends for receiving the injected beam; The air pressure regulation module is connected to the air path of the optical fiber evolution module, and fills the hollow anti-resonant optical fiber with inert gas and regulates the air pressure. The pulse feedback module is optically connected to the fiber evolution module and is used to acquire the output time-domain intensity waveform and extract feedback indicators. The control module is communicatively connected to the pulse feedback module and the air pressure regulation module, respectively. It is used to diagnose the offset state of the self-compressed focal point relative to the cut-off end face according to the feedback index, and control the air pressure regulation module to adjust the air pressure to anchor the focal point to the cut-off end face, so that the fiber evolution module outputs near-infrared few-period pulses.

[0008] Furthermore, the center wavelength of the pump pulse output by the pump source module is located at 1.3 GHz. -2.0 The band, the single pulse energy is in a preset low energy range, the upper limit of the preset low energy range is defined by a first threshold, the first threshold is the critical single pulse energy value when the soliton order of the pump pulse is equal to 1 under normal reference pressure.

[0009] Furthermore, the hollow antiresonant fiber has a fixed physical cut-off length, its cladding has a multi-ring non-nested or nested structure, and its core diameter ranges from 20 mm. -50 .

[0010] Furthermore, the energy coupling module includes a half-wave plate, a polarization beam splitter, and a focusing lens group; The half-wave plate and the polarization beam splitter are combined to receive the ultrashort pump pulses output by the pump source module, adjust their single pulse energy, and convert the beam into a linear polarization state. The focusing lens group is located downstream of the half-wave plate and the polarization beam splitter, and is used to focus and couple the linearly polarized beam to form the injection beam, which is then injected into the core layer of the hollow anti-resonant fiber.

[0011] Furthermore, the feedback indicators include the full width at half maximum (FWHM) of the main peak, the energy percentage of the main peak, and the splitting factor; the control module is specifically configured as follows: Extract the full width at half maximum (FWHM) of the main peak from the time-domain intensity waveform, and calculate the intensity drop to the peak value. The proportion of the integrated energy within the time interval corresponding to the location to the total pulse energy is taken as the main peak energy proportion, and the ratio of the peak intensity of the secondary satellite pulse to the peak intensity of the main peak is taken as the splitting factor; and a second threshold for the boundary value used to determine the validity of the main peak energy is set in advance, and a third threshold for the boundary value used to determine the integrity of the soliton envelope is set in advance; When the full width at half maximum (FWHM) of the main peak does not reach the theoretical minimum value under the current energy and the splitting factor is lower than the third threshold, it is determined that the current state is undercompressible. The self-compressible focus lags behind the cut-off end face and sends a pressurization command to the pressure control module to enhance the nonlinear coefficient. When the splitting factor is greater than the third threshold and the main peak energy ratio is less than the second threshold, it is determined that the current state is overcompressed, the self-compressing focus is ahead of the cut-off end face, and a depressurization command is sent to the pressure control module to reduce the nonlinear coefficient. When the full width at half maximum (FWHM) of the main peak reaches a minimum, the energy percentage of the main peak is greater than or equal to the second threshold, and the splitting factor is less than or equal to the third threshold, it is determined that the self-compression focus has been anchored to the truncated end face, and the current air pressure is locked as the optimal steady-state operating air pressure.

[0012] Furthermore, the control module is also configured as follows: Before the acquisition and diagnosis of the feedback indicators, the initial energy of the pump pulse is obtained. Based on the physical length of the hollow anti-resonant optical fiber and the pre-established correspondence between pump energy, working air pressure and self-compression focal position, the initial preset air pressure value required to position the self-compression focal point near the cut end face is determined within the preset high pressure range. The air pressure regulation module is then controlled to adjust the air pressure inside the optical fiber to the initial preset air pressure value.

[0013] Secondly, this invention proposes a near-infrared few-period pulse generation method based on high-voltage nonlinear enhancement, implemented using the near-infrared few-period pulse generation system described above. The method includes: The ultrashort pump pulse output by the pump source module is coupled into the hollow anti-resonant fiber after the single pulse energy is adjusted by the energy coupling module. The center wavelength of the pump pulse is located in the anomalous dispersion region of the air-filled hollow anti-resonant fiber, and the single pulse energy is in the preset low energy range. The gas pressure control module fills the hollow anti-resonant optical fiber with working gas and adjusts the gas pressure to a preset high pressure range. This is done to compensate for the insufficient peak power of the pump pulse by enhancing the nonlinear coefficient of the gas, thereby forcing the soliton order of the pulse to be increased to a self-compression evolution range greater than 1. The pulse feedback module acquires the time-domain waveform output from the fiber optic cut-off end face and extracts feedback indicators. The control module diagnoses the spatial offset state of the self-compression focus relative to the cut-off end face based on the feedback indicators. The control module controls the air pressure regulation module to adjust the air pressure according to the diagnostic results, so as to dynamically tune the air pressure to anchor the self-compressed focus to the cut-off end face, so that the optical fiber can directly output near-infrared short-cycle pulses.

[0014] Furthermore, the preset low-energy range is 100 nJ-2. .

[0015] Furthermore, the high-pressure range is 20 bar to 90 bar, and the inert gas is argon or krypton.

[0016] The beneficial effects of this invention are as follows: This invention significantly enhances the gas nonlinear coefficient by increasing the internal gas pressure of the hollow antiresonant fiber to a high-pressure range of 20-90 bar, effectively compensating for the physical defect of insufficient peak power in near-infrared low-energy pump pulses. This achieves a precise dynamic balance between anomalous dispersion and nonlinearity, forcing the soliton order to rise to a self-compression evolution range greater than 1. Simultaneously, a closed-loop gas pressure tuning mechanism based on time-domain waveform quality criteria is established. Using the main peak energy ratio and splitting factor as feedback indicators, the spatial offset of the self-compression focus relative to the fiber truncated end face is diagnosed in real time. By increasing or decreasing the pressure in small steps, the focus is precisely anchored to the truncated end face. This closed-loop mechanism effectively suppresses modulation instability caused by poor coupling between high-order dispersion and nonlinearity, physically preventing soliton breakage and achieving perfect preservation of pulse time-frequency coherence. The system directly outputs high-quality near-infrared pulses with pulse widths on the order of few periods, concentrated energy, and high coherence without the need for external dispersion compensation devices. The system has a compact structure and significantly improved stability and practicality. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement in an embodiment of the present invention; Figure 2 This is a flowchart illustrating the near-infrared few-period pulse generation method based on high-voltage nonlinear enhancement in an embodiment of the present invention. Figure 3 This is a spectral spatial evolution diagram of a near-infrared low-energy pulse propagating in a hollow anti-resonant optical fiber under a conventional low-pressure environment (10 bar) in Comparative Example 1 of the present invention. Figure 4 This is a time-domain intensity waveform of the optical fiber cut-off end face under a conventional low-pressure environment (10 bar) in Comparative Example 1 of the present invention. Figure 5 This is a spectral spatial evolution diagram of a near-infrared low-energy pulse propagating in a hollow anti-resonant optical fiber under an undercompressed state caused by insufficient air pressure (55 bar) in Comparative Example 2 of the present invention. Figure 6This is a time-domain intensity waveform of the output at the cut end of the optical fiber in the undercompressed state caused by insufficient air pressure (55 bar) in Comparative Example 2 of the present invention. Figure 7 This is a spectral spatial evolution diagram of the near-infrared low-energy pulse propagating in a hollow anti-resonant optical fiber under overcompression and soliton fracture state caused by excessive air pressure (65 bar) in Comparative Example 3 of the present invention. Figure 8 This is a time-domain intensity waveform of the optical fiber at the cut end face under the overcompression and soliton fracture state caused by excessive air pressure (65 bar) in Comparative Example 3 of the present invention. Figure 9 This is a spectral spatial evolution diagram of a near-infrared low-energy pulse propagating in a hollow anti-resonant fiber under the optimal state (59 bar) based on dispersive nonlinear balance and closed-loop tuning in an embodiment of the present invention. Figure 10 This is a time-domain intensity waveform diagram of the output at the fiber severance end face under the optimal state (59 bar) based on the dispersion nonlinear balance and closed-loop tuning in an embodiment of the present invention. Figure 11 This is a diagram showing the mapping relationship between the self-compressed pulse width and focal position of the near-infrared low-energy pulse under different input energies and working air pressures in an embodiment of the present invention. Figure 12 This is another structural schematic diagram of the near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement in an embodiment of the present invention. Detailed Implementation

[0018] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0019] It is worth noting that short-period ultrashort pulses have significant applications in attosecond science, high-harmonic generation, and precision spectroscopy. However, limited by the finite emission bandwidth and gain characteristics of erbium-doped laser gain media, the width of near-infrared pulses directly output by oscillators is typically in the tens to hundreds of femtosecond range, and the single-pulse energy is generally low, making it difficult to achieve effective soliton self-compression using conventional hollow-core fiber nonlinear compression techniques. This application constructs a system architecture combining a dispersive nonlinear balance mechanism and closed-loop feedback tuning control to actively lock the optimal coherent evolution state of the pulse, achieving direct short-period output of near-infrared low-energy pump pulses without the need for external dispersion compensation devices.

[0020] Example 1

[0021] This embodiment provides a near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement, which is suitable for applications that nonlinearly compress low-energy near-infrared femtosecond pulses to obtain ultrashort pulses on the order of few periods. It is especially suitable for the efficient compression of low-energy pump pulses with single-pulse energy in the range of hundreds of nanojoules to microjoules in the 1550nm band output by erbium-doped fiber lasers or optical parametric amplifiers.

[0022] like Figure 1 As shown, the near-infrared few-period pulse generation system based on high-pressure nonlinear enhancement in this embodiment includes a pump source module, an energy coupling module, an optical fiber evolution module, a gas pressure regulation module, a pulse feedback module, and a control module. The connection relationships and functions between the modules are described below.

[0023] The pump source module outputs an ultrashort pump pulse with a center wavelength located in the anomalous dispersion region of the near-infrared band and a single pulse energy in a preset low-energy range.

[0024] Specifically, the pump source module can be an erbium-doped fiber laser or an optical parametric amplifier, with the center wavelength of its output pulse preferably being 1550 nm. This wavelength falls within the anomalous dispersion region of the gas-filled hollow antiresonant fiber, providing the necessary dispersion conditions for soliton self-compression. The single-pulse energy of the pump pulse is within a preset low-energy range, the upper limit of which is defined by a first threshold, which is the critical single-pulse energy value when the soliton order is equal to 1 under normal reference pressure.

[0025] It should be noted that the first threshold in this invention is not a fixed constant, but rather a physical and dynamic critical energy value that defines whether the pulse is in a nonlinear decay state. This threshold is set and calculated based on the excitation conditions for soliton self-compression. A prerequisite for soliton self-compression in an anomalous dispersion fiber is that the soliton order N > 1, defined as the square root of the ratio of the dispersion length LD to the nonlinear length LNL. When the pulse energy is below a certain critical value, LNL > LD, resulting in N < 1. At this point, group velocity dispersion becomes dominant, and the pulse cannot be compressed. This invention defines this critical energy as the first threshold.

[0026] At a given conventional reference pressure P ref (That is, the safety upper limit pressure set in conventional operation in this field to prevent breakdown or avoid higher-order dispersion, for example, setting P) ref At 10 bar, the first threshold energy The following derivation and calculation are used to obtain the result: Dispersion length formula: ; Nonlinear length formula: ; Critical condition: Let the soliton order ; The critical peak power required to reach the critical state can be calculated from this. : ; Combining the initial time-domain distribution function of the pump pulse (for a hyperbolic secant pulse, The first threshold energy can then be accurately calculated. : ; in, The group velocity dispersion coefficient is given at a normal reference pressure. The effective nonlinear coefficient under normal reference pressure, The characteristic width of the pulse.

[0027] In a specific example of this embodiment, a hollow antiresonant fiber with a core diameter of 35 μm is used, filled with 10 bar of argon gas as a conventional reference pressure, and the pump pulse has a center wavelength of 1550 nm and an initial pulse width of approximately 300 fs. The first threshold energy is calculated by substituting these values ​​into the above formula. The initial energy is approximately 2 μJ. When a pump pulse of 500 nJ is used, the initial energy is lower than the first threshold and falls into the low-energy range that cannot meet the self-compression condition of the excited soliton. Therefore, the high-voltage nonlinear enhancement mechanism of this embodiment is required for compensation.

[0028] The energy coupling module is optically connected to the pump source module and is used to adjust the single pulse energy before coupling to form an injection beam.

[0029] In a preferred embodiment, the energy coupling module includes a half-wave plate, a polarization beam splitter, and a focusing lens group. The half-wave plate and polarization beam splitter are combined and positioned downstream of the pump source module's optical path to receive pump pulses, precisely adjust their single-pulse energy, and convert the beam to a linearly polarized state. The focusing lens group is positioned downstream of the half-wave plate and polarization beam splitter to focus and couple the linearly polarized beam into an injection beam, which is then injected into the core of the hollow anti-resonant fiber.

[0030] The fiber evolution module includes a section of hollow anti-resonant fiber and high-voltage optical chambers at both ends for receiving the injected beam.

[0031] Hollow-core antiresonant fiber has a fixed physical cut-off length, with its two ends connected to a front-end high-voltage optical cavity and a rear-end high-voltage optical cavity, respectively. Employing a broadband antiresonant light guiding mechanism, hollow-core antiresonant fiber maintains extremely low transmission loss even with a small core diameter. This overcomes the drawback of traditional capillary hollow-core fiber, where transmission loss increases sharply with the cube of the core diameter at small core diameters, making it an ideal waveguide medium for processing low-energy pulses.

[0032] In a preferred embodiment, the cladding of the hollow-core antiresonant fiber has a multi-ring non-nested or nested structure, with a core diameter ranging from 20μm to 50μm. It should be noted that the solution of this invention is not applicable to traditional capillary hollow-core fibers, because low-energy pulses require a smaller core diameter to increase power density, while capillary fibers suffer extremely high losses at small core diameters. This invention specifies that the waveguide medium is a hollow-core antiresonant fiber, relying precisely on its unique light-guiding mechanism to ensure low-loss transmission at small core diameters.

[0033] The high-pressure optical chamber adopts a high-pressure resistant sealed design, and the optical path window adopts a near-infrared anti-reflection thick window. The output end face of the rear chamber is the final output interface of the pulse, and no external dispersion compensation device is set downstream.

[0034] The air pressure regulation module is connected to the air path of the optical fiber evolution module, and inert gas is filled into the hollow anti-resonant optical fiber and the air pressure is regulated.

[0035] The gas pressure control module is connected to the front-end and rear-end high-pressure optical chambers via gas paths, precisely adjusting and stabilizing the gas pressure inside the optical fiber according to the instructions of the control module. In a preferred embodiment, the gas pressure control module regulates the gas pressure inside the hollow antiresonant fiber to a high-pressure range of 20-90 bar, preferably using argon or krypton as the inert gas. By increasing the gas pressure to this high-pressure range, the effective nonlinear coefficient of the gas medium is significantly enhanced, compensating for the insufficient peak power of the pump pulse from a physical level, forcing a sharp shortening of the nonlinear length, and increasing the soliton order to a self-compression evolution range greater than 1.

[0036] The pulse feedback module is optically connected to the fiber evolution module and is used to acquire the output time-domain intensity waveform and extract feedback indicators.

[0037] The pulse feedback module is located downstream of the output of the fiber evolution module and is used to acquire the time-domain intensity waveform of the pulse directly output from the fiber optic truncated end face in real time. In a preferred embodiment, the pulse feedback module includes an autocorrelator or a frequency-resolved optical switch, capable of accurately measuring the time-domain characteristics of the output pulse.

[0038] The control module is connected to the pulse feedback module and the air pressure regulation module respectively. It is used to diagnose the offset state of the self-compressed focal point relative to the cut-off end face according to the feedback index, and control the air pressure regulation module to adjust the air pressure to anchor the focal point to the cut-off end face, so that the fiber evolution module outputs near-infrared short-period pulses.

[0039] The control module receives feedback indicators extracted in real time by the pulse feedback module, including the full width at half maximum (FWHM) of the main peak, the energy percentage of the main peak, and the splitting factor.

[0040] Regarding the extraction and calculation of feedback indicators: The control module extracts the full width at half maximum (FWHM) of the main peak from the time-domain waveform and calculates the intensity decrease to 1 / e of the peak value.2 The proportion of the integrated energy within the corresponding time interval to the total pulse energy is used as the main peak energy proportion, and the ratio of the secondary satellite pulse peak intensity to the main peak peak intensity is used as the splitting factor. A second threshold is pre-set for the boundary value used to determine the validity of the main peak energy, and a third threshold is set for the boundary value used to determine the integrity of the soliton envelope.

[0041] Calculation of the main peak energy proportion: Obtain the time-domain intensity waveform of the pulse at the fiber output end. Searching for the highest peak of the main peak In the waveform, look for areas where the intensity decreases to either side of the main peak. Two time nodes and Calculate the integral energy within this interval as a percentage of the total test time window. The proportion of total internal integral energy, i.e. .

[0042] Regarding the calculation of the splitting factor: in the time-domain intensity waveform In the process, the peak intensity of the second highest peak (satellite pulse) after the first main peak is sought. Calculate the ratio of its intensity to the peak intensity of the first main peak, i.e. .

[0043] The control module diagnoses the offset state of the self-compression focus relative to the truncated end face based on feedback indicators as follows: When the full width at half maximum (FWHM) of the main peak does not reach the theoretical minimum value under the current energy and the splitting factor is lower than the third threshold, it is determined that the current state is undercompressible. The self-compressible focus lags behind the cut-off end face and sends a small step pressurization command to the pressure control module to enhance the nonlinear coefficient. When the splitting factor is greater than the third threshold and the main peak energy ratio is less than the second threshold, it is determined that the current state is overcompressed. The self-compression focus leads the cutoff end face and sends a small step depressurization command to the pressure control module to reduce the nonlinear coefficient. When the full width at half maximum (FWHM) of the main peak reaches its minimum value, the energy percentage of the main peak is greater than or equal to the second threshold, and the splitting factor is less than or equal to the third threshold, it is determined that the self-compression focus has been anchored at the cut-off end face, and the current air pressure is locked as the optimal steady-state operating air pressure.

[0044] It should be noted that the second and third thresholds are fixed boundary values ​​pre-set based on the physical tolerance of subsequent application scenarios. The second threshold is preferably 30%, which is set based on the following: at extreme compression ratios, when the proportion of the main peak energy is greater than or equal to 30%, the peak power multiplication effect brought about by pulse compression can cover the basis energy loss in nonlinear evolution, ensuring that the output pulse has an effective peak intensity to drive the generation of higher harmonics. The third threshold is preferably 0.15, which is set based on the following: in attosecond science and strong field physics, when the satellite pulse intensity is less than 15% of the main peak, the background noise generated in nonlinear interactions is negligible, ensuring that the pulse envelope maintains soliton integrity.

[0045] Physically, an increase in air pressure leads to enhanced nonlinearity, causing the self-compressing focal point to move forward towards the fiber optic input (i.e., overcompression); conversely, a decrease in air pressure causes the focal point to move backward towards the fiber optic output (i.e., undercompression). Based on this definite physical monotonicity, the control module can explicitly execute the logic for boosting or depressurizing based on feedback indicators.

[0046] As a preferred implementation, the control module is also equipped with a feedforward setting function: before the acquisition and diagnosis of feedback indicators, the initial energy of the pump pulse is obtained. Based on the physical length of the hollow anti-resonant fiber and a pre-established correspondence between pump energy, working air pressure, and self-compression focal point position, an initial preset air pressure value is determined within the high-pressure range to position the self-compression focal point near the truncated end face. The air pressure regulation module is then controlled to adjust the air pressure inside the fiber to this initial preset air pressure value. This correspondence can be pre-calibrated through numerical simulation of the nonlinear Schrödinger equation and stored in the control module. During actual operation, the initial preset air pressure value can be quickly determined by looking up a table, improving the system response speed and optimization efficiency.

[0047] Through the logical interaction control of the above modules, this embodiment successfully solves the technical problem of dispersion and nonlinear imbalance of near-infrared low-energy pump pulses in hollow anti-resonant optical fibers, and realizes the direct output of short-period pulses without external dispersion compensation devices.

[0048] As an example of a structural implementation, such as Figure 12As shown, the pump source module can be implemented using a near-infrared low-energy femtosecond laser; the energy coupling module can be implemented by polarization and energy control components; the fiber evolution module includes a front gas chamber, a hollow fiber, and a rear gas chamber, with both ends of the hollow fiber sealed in the front and rear gas chambers respectively; the gas pressure regulation module can be implemented by a pneumatic adjustment and control unit; the pulse feedback module can be implemented by the target detection end; the functions of the control module are integrated into the pneumatic adjustment and control unit. In this example, the near-infrared low-energy femtosecond laser outputs an ultrashort pump pulse, which is regulated by the polarization and energy control components and converted to a linear polarization state before being injected into the hollow fiber; the pneumatic adjustment and control unit fills the front and rear gas chambers with inert gas and adjusts the gas pressure to a high-pressure range; the pulse is directly emitted from the output end face of the rear gas chamber to the target detection end, where the time-domain waveform is acquired; the pneumatic adjustment and control unit dynamically tunes the gas pressure according to the waveform feedback index to anchor the self-compressed focus at the output end face.

[0049] Example 2

[0050] like Figure 2 As shown, this embodiment proposes a near-infrared few-period pulse generation method based on high-voltage nonlinear enhancement, implemented using the near-infrared few-period pulse generation system as described in Embodiment 1. The method includes the following steps: S1: The ultrashort pump pulse output by the pump source module is coupled into the hollow anti-resonant fiber after the single pulse energy is adjusted by the energy coupling module. The center wavelength of the pump pulse is located in the anomalous dispersion region of the air-filled hollow anti-resonant fiber, and the single pulse energy is in the preset low energy range. S2: The gas pressure control module fills the hollow anti-resonant fiber with working gas and adjusts the gas pressure to a preset high pressure range. This is done to compensate for the insufficient peak power of the pump pulse by enhancing the gas nonlinear coefficient, thereby forcing the soliton order of the pulse to be increased to a self-compression evolution range greater than 1. S3: The pulse feedback module acquires the time-domain waveform output from the fiber optic cut-off end face and extracts the feedback index. The control module diagnoses the spatial offset state of the self-compression focus relative to the cut-off end face based on the feedback index. S4: The control module controls the air pressure regulation module to adjust the air pressure according to the diagnostic results, and uses dynamic tuning of the air pressure to anchor the self-compressed focus to the cut-off end face, so that the optical fiber can directly output near-infrared short-cycle pulses.

[0051] In a preferred embodiment, the preset low-energy range is 100 nJ⁻². .

[0052] In a preferred embodiment, the high-pressure range is 20 bar to 90 bar, and the inert gas is argon or krypton.

[0053] In a preferred embodiment, the hollow antiresonant fiber has a core diameter of 35 mm. A six-ring, non-nested structure.

[0054] It should be noted that the aforementioned core diameter is 35. The six-ring non-nested structure is a preferred embodiment of the present invention and is not a limitation on the scope of protection. For hollow anti-resonant fibers with other core diameters and cladding structures, as long as they can provide anomalous dispersion and low loss at the target operating wavelength, the air pressure tuning mechanism of the present invention is equally applicable. When the fiber structure changes, its intrinsic dispersion value and transmission loss change accordingly. By simply redetermining the corresponding optimal operating air pressure value using the method provided by the present invention, a dynamic balance between dispersion and nonlinearity can be achieved, anchoring the self-compression focus to the fiber truncated end face.

[0055] The working principle and physical evolution process of this invention will be illustrated below with specific parameters.

[0056] In this example, the pump source is an erbium-doped fiber laser with a center wavelength of 1550 nm, which outputs an ultrashort pulse with an initial pulse width of approximately 300 fs and a single pulse energy of 500 nJ. The hollow-core antiresonant fiber adopts a six-ring non-nested structure with a core diameter of 35 μm, and the physical cut-off length of the fiber is set to 1 meter. Argon gas is filled inside as the working gas.

[0057] Comparative Example 1 (Conventional Low-Pressure Environment): The air pressure inside the optical fiber is set to 10 bar. For example... Figure 3 As shown, under this conventional gas pressure, the nonlinear refractive index of the gas medium is extremely small. Combined with the low energy of 500 nJ, the nonlinear length within the optical fiber is much greater than the dispersion length, and the soliton order N < 1. The spectrum exhibits almost no nonlinear broadening throughout its propagation from 0 to 150 cm. Figure 4 As shown, at the 100cm fiber optic cutoff end, the full width at half maximum (FWHM) of the time-domain pulse is still as high as approximately 260 fs, indicating severe pulse broadening at the exit point. Although the main peak energy does not split, the pulse peak power is extremely low due to the lack of compression. This comparison illustrates that conventional low-voltage techniques cannot achieve few-cycle compression of low-energy pulses.

[0058] Comparative Example 2 (undercompressible state due to insufficient air pressure): The air pressure inside the optical fiber is set to 55 bar. For example... Figure 5 As shown, the increase in air pressure significantly enhances the nonlinear coefficient, resulting in a marked broadening of the spectrum. However, the cumulative rate of the nonlinear phase shift is insufficient to allow the pulse to evolve to its limit within 100 cm⁻¹, and the location of maximum spectral broadening (i.e., the theoretical self-compression focus) is located after the 100 cm⁻¹ cutoff line. Figure 6 As shown, the waveform is extracted at the 100cm fiber optic cutoff end, and the pulse width is compressed to approximately 9.2 fs. Although significant compression is achieved, the pulse at this location has not yet evolved to its narrowest limit, remaining in an undercompressed state and not reaching the theoretical minimum pulse width at the current energy.

[0059] Comparative Example 3 (Overcompression and Soliton Fragmentation Due to Excessive Gas Pressure): The gas pressure inside the optical fiber was set to 65 bar. For example... Figure 7 As shown, excessively high air pressure causes a sharp increase in the nonlinear coefficient, and the self-compression focus is advanced to approximately 85 cm. When the pulse continues to propagate to the 1-meter truncated end face, excessive nonlinear accumulation induces higher-order dispersion coupling and modulation instability. Figure 8 As shown, waveforms extracted at the 100cm fiber optic cutoff point reveal significant soliton breakage in the pulse envelope, severe time-domain waveform asymmetry, and pulse width degradation to approximately 13.9 fs. The satellite pulse intensity is significantly increased, and the splitting factor exceeds the quality criterion threshold of 0.15. This comparative example demonstrates that blindly applying pressure not only fails to optimize compression but also damages pulse quality.

[0060] Example (based on the optimal state of dispersive nonlinear balance, coherence preservation, and closed-loop tuning in this invention): The air pressure inside the optical fiber is precisely adjusted to 59 bar. For example... Figure 9 As shown, under precise nonlinear rescaling at this pressure, the accumulation rates of dispersion and nonlinearity achieve perfect coordination, resulting in an extremely symmetrical and dramatic maximum broadening of the spectrum. This maximum broadening region precisely coincides with the dashed line of the 100cm physical truncation of the optical fiber, thus achieving precise alignment between the self-compressing focus and the truncated end face. Figure 10 As shown, an extremely sharp soliton pulse was directly output from the 100cm output facet, with its full width at half maximum (FWHM) reaching the limit of 6.2 fs, successfully achieving near-single-cycle near-infrared ultrashort pulse direct output. Quality verification of this output waveform was performed using the following criteria: the intensity decreased from the highest peak value to 1 / e... 2 Within the enclosed integral interval, the main peak energy accounts for 31.5%, satisfying the second threshold condition of greater than or equal to 30%, thus ensuring effective peak power. At the same time, the relative intensity of its tail secondary fluctuation is only 7.3%, satisfying the third threshold anti-split condition of less than or equal to 0.15. The pulse did not experience high-order breakage, and coherence was perfectly maintained.

[0061] To further reveal the core parameter matching rules and physical boundaries of this invention, Figure 11This figure illustrates the global parameter mapping relationship between the output evolution state of a pulse with a center wavelength in the near-infrared band propagating in a hollow anti-resonant optical fiber and the changes in input pulse energy and working gas pressure. The horizontal axis represents the initial pulse energy injected into the fiber, and the vertical axis represents the working gas pressure inside the fiber. The background gradient and contour lines in the figure represent the narrowest pulse width when the pulse evolves to the theoretical self-compressing focal point. This mapping diagram contains three main features: First, the optimal compression limit contour (solid black line in the diagram) represents the parameter contour line where the pulse is compressed to near the 5fs few-period limit. This curve intuitively reveals the inverse matching law between pressure and energy, that is, as the input pulse energy decreases, the working pressure must be nonlinearly increased to compensate for the insufficient peak power. Second, the self-compression focal position contour (dashed gray line in the diagram) represents the theoretical self-compression transmission distance required for the pulse to evolve to the corresponding narrowest pulse width. Given the fixed physical truncation length of the optical fiber, this contour line set is used to guide the system to deduce the corresponding optimal working pressure. Third, the boundary of the higher-order soliton fracture region (gray shaded area in the upper right corner of the diagram) represents the higher-order nonlinear runaway region. When the energy-pressure combination falls into this region, excessive nonlinear accumulation and higher-order dispersion coupling will cause severe higher-order soliton fission of the pulse, generating a large number of satellite pulses and floor noise. The lower edge of this region sets a strict upper limit safety threshold for the system's pressure tuning.

[0062] It should be further noted that the spectral spatial evolution diagrams shown in the above embodiments and comparative examples of the present invention ( Figure 3 , Figure 5 , Figure 7 , Figure 9 ) and output time-domain intensity waveform ( Figure 4 , Figure 6 , Figure 8 , Figure 10 The results were verified by solving a physical dynamic numerical model based on the multimode generalized nonlinear Schrödinger equation. This numerical model fully considers and incorporates the actual waveguide dispersion of the hollow anti-resonant fiber (accurate to higher-order dispersion terms), the nonlinear transient response of the inert gas (Kerr effect), and its associated delayed Raman response. By performing high-precision step integral solutions to the equation, not only was the theoretical validity of the pressure dynamic tuning mechanism demonstrated, but a precise reference threshold was also provided for subsequent searches for the self-compressing focus.

[0063] The comparison results of the above comparative examples and embodiments fully demonstrate that the present invention not only achieves dispersion nonlinear balance by charging high pressure and successfully activates the compression potential of low-energy near-infrared pulses, but more importantly, it achieves perfect spatial anchoring of the self-compressed focus through a precise air pressure tuning mechanism, successfully avoiding under-compression and over-pressure breakage. Finally, it obtains a few-period pulse output with excellent beam quality and pulse width and high coherence without external dispersion compensation devices.

[0064] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0065] In addition, the functional modules in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0066] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement, characterized in that, include: The pump source module outputs an ultrashort pump pulse with a center wavelength located in the anomalous dispersion region of the near-infrared band and a single pulse energy in a preset low-energy range. An energy coupling module, optically connected to the pump source module, is used to adjust the single pulse energy and then couple it to form an injection beam. The fiber evolution module includes a section of hollow anti-resonant fiber and high-voltage optical chambers at both ends for receiving the injected beam; The air pressure regulation module is connected to the air path of the optical fiber evolution module, and fills the hollow anti-resonant optical fiber with inert gas and regulates the air pressure. The pulse feedback module is optically connected to the fiber evolution module and is used to acquire the output time-domain intensity waveform and extract feedback indicators. The control module is communicatively connected to the pulse feedback module and the air pressure regulation module, respectively. It is used to diagnose the offset state of the self-compressed focal point relative to the cut-off end face according to the feedback index, and control the air pressure regulation module to adjust the air pressure to anchor the focal point to the cut-off end face, so that the fiber evolution module outputs near-infrared few-period pulses.

2. The near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement according to claim 1, characterized in that, The pump pulse output by the pump source module has a center wavelength of 1.3 GHz. -2.0 The band, the single pulse energy is in a preset low energy range, the upper limit of the preset low energy range is defined by a first threshold, the first threshold is the critical single pulse energy value when the soliton order of the pump pulse is equal to 1 under normal reference pressure.

3. The near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement according to claim 1, characterized in that, The hollow antiresonant fiber has a fixed physical cut-off length, its cladding has a multi-ring non-nested or nested structure, and its core diameter ranges from 20. -50 .

4. The near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement according to claim 1, characterized in that, The energy coupling module includes a half-wave plate, a polarization beam splitter, and a focusing lens group; The half-wave plate and the polarization beam splitter are combined to receive the ultrashort pump pulses output by the pump source module, adjust their single-pulse energy, and convert the beam into a linear polarization state. The focusing lens group is located downstream of the half-wave plate and the polarization beam splitter, and is used to focus and couple the linearly polarized beam to form the injection beam, which is then injected into the core layer of the hollow anti-resonant fiber.

5. The near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement according to claim 1, characterized in that, The feedback indicators include the full width at half maximum (FWHM) of the main peak, the energy percentage of the main peak, and the splitting factor; the control module is specifically configured as follows: Extract the full width at half maximum (FWHM) of the main peak from the time-domain intensity waveform, and calculate the intensity drop to the peak value. The proportion of the integrated energy within the time interval corresponding to the location to the total pulse energy is taken as the main peak energy proportion, and the ratio of the peak intensity of the secondary satellite pulse to the peak intensity of the main peak is taken as the splitting factor. A second threshold is pre-set for the boundary value used to determine the validity of the main peak energy, and a third threshold is set for the boundary value used to determine the integrity of the soliton envelope; When the full width at half maximum (FWHM) of the main peak does not reach the theoretical minimum value under the current energy and the splitting factor is lower than the third threshold, it is determined that the current state is undercompressible. The self-compressible focus lags behind the cut-off end face and sends a pressurization command to the pressure control module to enhance the nonlinear coefficient. When the splitting factor is greater than the third threshold and the main peak energy ratio is less than the second threshold, it is determined that the current state is overcompressed, the self-compressing focus is ahead of the cut-off end face, and a depressurization command is sent to the pressure control module to reduce the nonlinear coefficient. When the full width at half maximum (FWHM) of the main peak reaches a minimum, the energy percentage of the main peak is greater than or equal to the second threshold, and the splitting factor is less than or equal to the third threshold, it is determined that the self-compression focus has been anchored to the truncated end face, and the current air pressure is locked as the optimal steady-state operating air pressure.

6. The near-infrared few-period pulse generation system based on high-voltage nonlinear enhancement according to claim 5, characterized in that, The control module is also configured to: Before the acquisition and diagnosis of the feedback indicators, the initial energy of the pump pulse is obtained. Based on the physical length of the hollow anti-resonant optical fiber and the pre-established correspondence between pump energy, working air pressure and self-compression focal position, the initial preset air pressure value required to position the self-compression focal point near the cut end face is determined within the preset high pressure range. The air pressure regulation module is then controlled to adjust the air pressure inside the optical fiber to the initial preset air pressure value.

7. A method for generating near-infrared few-period pulses based on high-voltage nonlinear enhancement, implemented using the near-infrared few-period pulse generation system as described in any one of claims 1-6, characterized in that, The methods include: The ultrashort pump pulse output by the pump source module is coupled into the hollow anti-resonant fiber after the single pulse energy is adjusted by the energy coupling module. The center wavelength of the pump pulse is located in the anomalous dispersion region of the air-filled hollow anti-resonant fiber, and the single pulse energy is in the preset low energy range. The gas pressure control module fills the hollow anti-resonant optical fiber with working gas and adjusts the gas pressure to a preset high pressure range. This is done to compensate for the insufficient peak power of the pump pulse by enhancing the nonlinear coefficient of the gas, thereby forcing the soliton order of the pulse to be increased to a self-compression evolution range greater than 1. The pulse feedback module acquires the time-domain waveform output from the fiber optic cut-off end face and extracts feedback indicators. The control module diagnoses the spatial offset state of the self-compression focus relative to the cut-off end face based on the feedback indicators. The control module controls the air pressure regulation module to adjust the air pressure according to the diagnostic results, so as to dynamically tune the air pressure to anchor the self-compressed focus to the cut-off end face, so that the optical fiber can directly output near-infrared short-cycle pulses.

8. The near-infrared few-period pulse generation method based on high-voltage nonlinear enhancement according to claim 7, characterized in that, The preset low-energy range is 100 nJ-2. .

9. The near-infrared few-period pulse generation method based on high-voltage nonlinear enhancement according to claim 7, characterized in that, The high-pressure range is 20 bar to 90 bar, and the inert gas is argon or krypton.

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