Optical system for generating harmonics and method for improving efficiency of harmonics

Abstract

The invention provides an optical system for generating higher harmonics and a method for improving the efficiency of the higher harmonics. The system comprises a femtosecond laser light source, a pulse width compression module and a higher harmonic generation module. The femtosecond laser light source outputs femtosecond laser, the pulse width compression module compresses the pulse width of the femtosecond laser, the pulse width of the femtosecond laser in multi-period magnitude is compressed at least in the period magnitude, the atomic monatomic effect is enhanced, and the higher harmonic generation efficiency is improved. The higher harmonic generation module comprises a vacuum chamber and a first optical fiber structure in the vacuum chamber, and first gas is introduced into optical fibers. The femtosecond laser after pulse width compression is coupled into the first optical fiber structure and interacts with the first gas, and high-efficiency higher harmonics are output. By optimizing the structure of the first optical fiber and related parameters of the first gas, the generation efficiency of higher harmonics can be improved. Moreover, the higher harmonic efficiency is improved, so that the requirement on femtosecond laser pulse energy can be reduced, and the selection range of the output wavelength, the pulse width and the repetition frequency of the femtosecond laser light source is widened.

Description

Technical Field

[0001] Embodiments of the present application relate to the field of optical technology, and in particular to an optical system for generating high-order harmonics and a method for improving the efficiency of high-order harmonics. Background Art

[0002] The light source from vacuum ultraviolet to soft X-rays generated by the high-order harmonic (HHG) process can be used for the measurement of electron dynamics in atoms and molecules, the exploration of the structure and energy band characteristics of two-dimensional materials, and the principle verification of deep ultraviolet lithography. Generally speaking, the process of generating an HHG light source is to focus the light intensity of a visible light band femtosecond laser to 10 14 -10 16 Watts per square centimeter (W / cm 2 ) range, interacts with gases, and produces light sources from vacuum ultraviolet to soft X-rays through high-order nonlinear processes. With the increasing number of applications, HHG light sources have a problem that needs to be solved, which is the low generation efficiency (generally around 10 -6 Therefore, it is still difficult to apply it to high-power experiments, such as nonlinear extreme ultraviolet optics and high-precision damage-free diffraction imaging. Summary of the Invention

[0003] The embodiments of the present application provide an optical system for generating high-order harmonics and a method for improving the efficiency of high-order harmonics, aiming to improve the efficiency of high-order harmonic generation.

[0004] To achieve the above objectives, a first aspect of an embodiment of the present application provides an optical system for generating high-order harmonics, comprising: A femtosecond laser light source, used for outputting femtosecond laser; A pulse width compression module, connected to the femtosecond laser light source, for compressing the pulse width of the femtosecond laser; The high-order harmonic generation module is connected to the pulse width compression module, and includes a vacuum chamber and a first optical fiber structure arranged in the vacuum chamber. A first gas is introduced into the first optical fiber structure. The first optical fiber structure is used to output high-order harmonics based on the femtosecond laser after the pulse width is compressed.

[0005] The solution of the embodiment of the present application has at least the following beneficial effects: In an embodiment of the present application, a pulse width compression operation is first performed on the femtosecond laser output by a femtosecond laser light source, compressing the multi-cycle femtosecond laser pulse width to at least the cycle level. This operation can significantly enhance the single-atom effect of atoms on the laser field, thereby effectively improving the generation efficiency of high-order harmonic signals. Then, the low-cycle femtosecond laser after pulse width compression is coupled into a first optical fiber structure. In the first optical fiber structure, the low-cycle femtosecond laser interacts with the first gas to generate efficient high-order harmonics. In actual application scenarios, by optimizing the relevant parameters of the first optical fiber structure and the first gas, the macroscopic phase matching conditions for high-order harmonic generation can be optimized, further improving the generation efficiency of high-order harmonics. Moreover, the improvement of high-order harmonic efficiency can reduce the requirements for femtosecond laser pulse energy and broaden the selection range of the output wavelength, pulse width and repetition frequency of the femtosecond laser light source.

[0006] In a possible implementation of the embodiment of the present application, the first optical fiber structure has at least two gas inlets for introducing the first gas, and each of the gas inlets is arranged on a side surface in the longitudinal direction of the first optical fiber structure.

[0007] In a possible implementation of the embodiment of the present application, a high-order harmonic measurement module is further included, which is arranged in the vacuum chamber and at the output end of the high-order harmonic generation module, and is used to measure the high-order harmonics; the high-order harmonic measurement module is connected to the output end of the high-order harmonic generation module through a metal filter, and the metal filter is used to filter out excess femtosecond laser.

[0008] In a possible implementation of the embodiment of the present application, the higher harmonics measurement module includes a power measurement device, and the power measurement device is used to measure the power of the higher harmonics.

[0009] In a possible implementation of an embodiment of the present application, the high-order harmonic measurement module includes a spectrometer and an optical signal detection device. The spectrometer is used to separate the multiple wavelengths of light contained in the high-order harmonic light source output by the high-order harmonic generation module, and the optical signal detection device is used to measure the light of each wavelength band to obtain the intensity distribution of different wavelengths in the high-order harmonic light source.

[0010] In a possible implementation of the embodiment of the present application, a first reflector is further provided in the vacuum chamber, and a laser export window is provided on the vacuum chamber. The first reflector is provided at the output end of the high-order harmonic generation module, and is used to reflect the remaining femtosecond laser output from the high-order harmonic generation module that does not participate in the high-order harmonic generation to the laser export window, so as to export the remaining femtosecond laser out of the vacuum chamber through the laser export window.

[0011] In one possible implementation of the embodiment of the present application, the pulse width compression module includes a nonlinear spectral broadening submodule and a dispersion compensation element. The nonlinear spectral broadening submodule is used to perform spectral broadening on the femtosecond laser; the dispersion compensation element is used to perform pulse width compression on the femtosecond laser after spectral broadening. Implementation methods include but are not limited to: multi-pass cavity technology, hollow fiber technology, and filamentation technology.

[0012] A second aspect of the embodiments of the present application provides a method for improving high-order harmonic efficiency, the method being applied to the optical system described in the first aspect, the method comprising at least one of the following: Optimizing the spot size of the femtosecond laser output by the femtosecond laser light source; Optimizing the energy of the femtosecond laser output by the femtosecond laser light source; Optimizing the length of the first optical fiber structure; optimizing the inner diameter of the first optical fiber structure; Optimizing the air inlet position of the first optical fiber structure; Optimizing the type of the first optical fiber structure, where the type of the first optical fiber structure includes but is not limited to a photonic bandgap fiber or an antiresonant fiber; optimizing the pressure of the first gas; The composition of the first gas is optimized.

[0013] It should be understood that the above general description and the following detailed description are merely illustrative and are not restrictive of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or related technical descriptions. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0015] Figure 1 A schematic diagram of the structure of an optical system for generating high-order harmonics provided in an embodiment of the present application Figure 1 ; Figure 2 A schematic structural diagram of a high-order harmonic generation module provided in an embodiment of the present application; Figure 3 A schematic diagram of the structure of an optical system for generating high-order harmonics provided in an embodiment of the present application Figure 2 ; Figure 4 A schematic diagram of the structure of an optical system for generating high-order harmonics provided in an embodiment of the present application Figure 3 ; Figure 5 The calculation results of the HHG spectrum generated under the conditions of a central wavelength of 800 nm and different pulse widths provided in the examples of this application. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical methods and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0017] In the description of the embodiments of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0018] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the embodiments of this application, the meaning of "plurality" is two or more, unless otherwise clearly specified.

[0019] In the embodiments of the present application, unless otherwise expressly specified or limited, the terms "installed," "connected," "connected," "fixed," etc. should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections, electrical connections; direct connections, or indirect connections through an intermediate medium; and internal connections between two components or interactions between two components. Those skilled in the art will understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0020] In the embodiments of the present application, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Furthermore, a first feature being "above," "above," and "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0021] In the embodiments of the present application, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent the situation where A exists alone, A and B exist at the same time, or B exists alone. A and B may be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" and similar expressions refer to any combination of these items, including any combination of single or plural items. For example, at least one of a, b and c may represent: a exists alone, b exists alone, c exists alone, a and b exist at the same time, a and c exist at the same time, b and c exist at the same time, or a, b and c exist at the same time, wherein a, b and c may be single or multiple.

[0022] In the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of this application should not be interpreted as being preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0023] The light source from vacuum ultraviolet to soft X-rays generated by the high-order harmonic (HHG) process can be used for the measurement of electron dynamics in atoms and molecules, the exploration of the structure and energy band characteristics of two-dimensional materials, and the principle verification of deep ultraviolet lithography. Generally speaking, the process of generating an HHG light source is to focus the light intensity of a visible light band femtosecond laser to 10 14 -10 16 Watts per square centimeter (W / cm 2 ) range, interacts with gases, and produces light sources from vacuum ultraviolet to soft X-rays through high-order nonlinear processes. With the increasing number of applications, HHG light sources have a problem that needs to be solved, which is the low generation efficiency (generally around 10 -6 Therefore, it is still difficult to apply it to high-power experiments, such as nonlinear extreme ultraviolet optics and high-precision damage-free diffraction imaging.

[0024] In related technologies, in order to improve the efficiency of HHG generation, it is proposed to generate HHG through a loose focusing structure. Loose focusing is to use a longer focal length (several meters or tens of meters) to focus the femtosecond laser, and achieve a longer Rayleigh distance near the focus. Within the Rayleigh distance range, the laser field intensity changes weakly, and phase matching can be achieved over a longer distance; at the same time, the phase-matched gas pressure is low, which can effectively reduce the reabsorption effect of the gas on HHG, thereby significantly improving the efficiency of HHG generation. However, in order to use a loose focusing structure to generate high-efficiency HHG, a femtosecond laser with a pulse energy of the order of tens of millijoules (mJ) is required. Generally, the repetition rate of such a laser system is on the order of several hertz to tens of hertz, and the system is large, technically difficult, and expensive.

[0025] The embodiments of the present application aim to improve the HHG generation efficiency and provide an optical system for generating high-order harmonics and a method for improving the efficiency of high-order harmonics.

[0026] The technical solutions provided by the embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0027] See Figure 1 , which is a schematic diagram of the structure of an optical system for generating high-order harmonics provided in an embodiment of the present application Figure 1 .like Figure 1 As shown, the optical system includes a femtosecond laser light source, a pulse width compression module and a high-order harmonic generation module, wherein the femtosecond laser light source is used to output femtosecond laser; the pulse width compression module is connected to the femtosecond laser light source and is used to compress the pulse width of the femtosecond laser; the high-order harmonic generation module is connected to the pulse width compression module, and includes a vacuum chamber and a first optical fiber structure arranged in the vacuum chamber, a first gas is passed into the first optical fiber structure, and the first optical fiber structure is used to output high-order harmonics based on the femtosecond laser after the pulse width is compressed.

[0028] Exemplarily, the type of the first optical fiber structure may include, but is not limited to, the following: hollow core fiber (HCF), photonic bandgap fiber (PBGF), or anti-resonant fiber (ARF).

[0029] like Figure 2 As shown, it is a structural schematic diagram of a high-order harmonic generation module provided in an embodiment of the present application. The high-order harmonic generation module includes a vacuum chamber and a hollow-core optical fiber HCF arranged in the vacuum chamber. A first gas is passed into the hollow-core optical fiber HCF. The hollow-core optical fiber HCF is used to output high-order harmonics based on a femtosecond laser with compressed pulse width.

[0030] The femtosecond laser light source in the embodiments of this application can specifically employ a titanium sapphire femtosecond laser system, an ytterbium-doped femtosecond laser, an optical parametric chirped pulse amplification system, and the like. The femtosecond laser output requirements include a central wavelength in the visible to near-infrared range, pulses with millijoule energy, a repetition rate between 10 and 100 kHz, and a pulse width of tens to hundreds of femtoseconds.

[0031] The pulse compression module in the embodiments of the present application is used to compress the pulse width of a femtosecond laser emitted from a femtosecond laser source, for example, compressing a laser pulse width of tens of femtoseconds to a few femtoseconds. Specifically, the pulse compression module can compress the pulse width of a femtosecond laser using multi-pass cavity technology, hollow-core fiber technology, or filamentation technology.

[0032] Among them, pulse width compression based on multi-pass cavity technology is to make the laser travel back and forth multiple times in the multi-pass cavity, fully interacting with the medium (such as solid thin film) nonlinearly (such as self-phase modulation, SPM), broadening the spectrum and then compressing the pulse width through dispersion compensation.

[0033] Pulse width compression based on hollow-core fiber technology utilizes the nonlinear interaction (such as efficient self-phase modulation) between the gas medium (usually an inert gas) in the hollow-core fiber and the strong laser to broaden the pulse spectrum, and then combines the dispersion characteristics of the fiber (usually anomalous dispersion) or external dispersion compensation elements to achieve compression.

[0034] Pulse width compression based on filamentation technology uses the self-focusing and electric discrete focusing balance effect of femtosecond pulses when they are transmitted in transparent media (such as air, gas or liquid) to form optical filaments. Subsequently, the broadened spectrum is temporally realigned through external dispersion compensation elements (such as grating pairs, wedge pairs or chirped mirrors) to achieve pulse width compression.

[0035] Exemplarily, the pulse compression module includes a nonlinear spectral broadening submodule and a dispersion compensation element. The nonlinear spectral broadening submodule is used to broaden the spectrum of the femtosecond laser, while the dispersion compensation element is used to compress the pulse width of the femtosecond laser after spectral broadening. In this example, the pulse compression module employs a cascaded "nonlinear spectral broadening + dispersion compensation" architecture. High-intensity pulses are spectrally broadened in a nonlinear medium (such as a hollow-core fiber) via self-phase modulation (SPM), simultaneously introducing positive chirp (instantaneous frequency increases with time, with high-frequency components lagging behind). Subsequently, a dispersion compensation element (such as a chirped mirror or grating pair) provides anomalous dispersion (negative group velocity dispersion), accelerating the propagation of high-frequency components, gradually catching up with low-frequency components, thereby aligning them in the time domain and significantly compressing the pulse width, achieving a "broaden first, compensate later" pulse compression method. In this embodiment of the present application, specific implementation methods for pulse compression may include, but are not limited to, multi-pass cavity technology, hollow-core fiber technology, and filamentation technology.

[0036] In one specific example, the nonlinear spectral broadening submodule includes a second optical fiber structure, into which a second gas is introduced. A femtosecond laser enters the second optical fiber structure to perform spectral broadening. The core component of the nonlinear spectral broadening submodule is the second optical fiber structure, into which a second gas with a high ionization threshold (such as helium, neon, argon, or other inert gases) is introduced. When the femtosecond laser is injected into the second optical fiber structure, the intense light field excites the nonlinear Kerr effect in the gas molecules, triggering self-phase modulation (SPM). This instantaneous change in light intensity causes the refractive index of the medium to fluctuate with the light intensity, resulting in a phase difference between the different frequency components of the pulse, ultimately broadening the spectrum symmetrically about the central wavelength. Specifically, this second optical fiber structure can be a hollow-core fiber (HCF).

[0037] It should be noted that the second gas can interact with the femtosecond laser in a static or differential manner to generate spectrum broadening through the self-phase modulation effect.

[0038] In a specific example, the dispersion compensation element may include a chirped mirror, a wedge pair, or the like.

[0039] It should be noted that the first gas and the second gas in the embodiment of the present application can be an inert gas, a halide gas, nitrogen or a specific gas with unique nonlinear optical properties. The embodiment of the present application does not limit the specific types of the first gas and the second gas.

[0040] In an embodiment of the present application, a pulse width compression operation is first performed on the femtosecond laser output by a femtosecond laser light source to compress the multi-cycle femtosecond laser pulse width to at least the cycle level. This operation can significantly enhance the single-atom effect of atoms on the laser field, thereby effectively improving the generation efficiency of high-order harmonic signals. Then, the femtosecond laser with a small cycle level after pulse width compression is coupled into the first optical fiber structure. In the first optical fiber structure, the femtosecond laser with a small cycle level interacts with the first gas to generate efficient high-order harmonics. In actual application scenarios, by optimizing the relevant parameters of the first optical fiber structure and the first gas, the macroscopic phase matching conditions in the high-order harmonic process can be optimized, further improving the generation efficiency of high-order harmonics. Moreover, the improvement of high-order harmonic efficiency can reduce the requirements for femtosecond laser pulse energy and broaden the range of choices for femtosecond laser light sources.

[0041] It should be noted that the peak power of the laser that generates high-order harmonics is one order of magnitude higher than that during the self-phase modulation (SPM) in the pulse width compression stage. Studies have found that when the first gas is injected from the light inlet of the first optical fiber structure, a strong filamentation effect will be stimulated. This effect will cause the femtosecond laser spot to be distorted, which will not only significantly reduce the efficiency of laser coupling into the first optical fiber structure, but also seriously hinder the effective generation of high-order harmonics. To avoid this technical problem, in an embodiment of the present application, the air inlet is arranged on the side of the length direction of the first optical fiber structure. Injecting the first gas through this side air inlet can effectively avoid the filamentation effect caused by injection from the light inlet, thereby creating favorable conditions for the stable generation of high-order harmonics.

[0042] In a specific example, the first optical fiber structure has at least two gas inlets for introducing the first gas, and each gas inlet is arranged on a side surface in the longitudinal direction of the first optical fiber structure.

[0043] Specifically, the first fiber structure is equipped with no fewer than two gas inlets, distributed along the sides of the fiber structure's length. Of particular note, precise control of the distance between the two side inlets effectively optimizes the efficiency of high-harmonic generation. This design provides a means for fine-tuning gas distribution and laser-gas interaction, enabling optimal high-harmonic output performance under specific parameter combinations.

[0044] It should be noted that in practical applications, high-efficiency high-order harmonics can be achieved by optimizing the type of the first optical fiber structure, which may include but is not limited to hollow-core fiber (HCF), photonic bandgap fiber (PBGF), or antiresonant fiber (ARF).

[0045] See Figure 3 , which is a schematic diagram of the structure of an optical system for generating high-order harmonics provided in an embodiment of the present application Figure 2 In some embodiments of the present application, the optical system further includes a high-order harmonic measurement module, which is disposed within the vacuum chamber and at the output end of the high-order harmonic generation module, and is used to measure high-order harmonics. The high-order harmonic measurement module is connected to the output end of the high-order harmonic generation module via a metal filter, which is used to filter out excess femtosecond laser light.

[0046] Specifically, the light beam output by the high-order harmonic generation module is first processed through a metal filter to filter out excess femtosecond laser light, thereby separating and obtaining high-order harmonics. The obtained high-order harmonics are then accurately measured using a high-order harmonic measurement module. Based on the measured high-order harmonic data, the relevant parameters of the optical system are optimized to improve the efficiency of high-order harmonic generation. For example, fine-tuning can be performed on laser-related parameters (such as laser energy and spot size) and parameters of the first optical fiber structure (including the fiber inner diameter, air inlet drilling position, air inlet drilling spacing, and gas pressure). By optimizing and controlling these key parameters, the efficiency of high-order harmonic generation (HHG) can be effectively improved to a more ideal level.

[0047] It should be noted that the efficiency of high-order harmonic generation (HHG) can be improved by optimizing the first fiber structure. Optional first fiber structures include, but are not limited to, typical structures such as hollow-core fiber (HCF), photonic bandgap fiber (PBGF), and antiresonant fiber (ARF). Other fiber types that meet specific performance requirements can also be included. In actual application scenarios, the appropriate first fiber structure type can be flexibly selected based on specific experimental conditions, performance indicators, and application requirements to achieve optimal HHG generation.

[0048] In a specific example, the high-order harmonics measurement module includes a power measurement device, which is used to measure the power of the high-order harmonics. For example, the power measurement device can be a photodiode, a crystal detector, or the like.

[0049] In a specific example, the high-order harmonic measurement module includes a spectrometer and an optical signal detection device. The spectrometer is used to separate light of different wavelengths in the high-order harmonic light source output by the high-order harmonic generation module, and the optical signal detection device is used to measure light in each wavelength band to obtain the intensity distribution of different wavelengths in the high-order harmonic light source.

[0050] Exemplarily, the optical signal detection device includes a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (cMOS) camera.

[0051] The high-harmonic generation module can be used to obtain data such as the power energy and spectral intensity distribution of high-order harmonics (HHG), and then optimize the relevant parameters of the optical system to improve the efficiency of high-harmonic generation.

[0052] In some embodiments of the present application, a first reflector is further disposed within the vacuum chamber, and a laser extraction window is provided on the vacuum chamber. The first reflector is disposed at the output of the high-harmonic generation module and is configured to reflect the remaining femtosecond laser light from the high-harmonic generation module that does not participate in high-harmonic generation toward the laser extraction window, thereby extracting the remaining femtosecond laser light out of the vacuum chamber through the laser extraction window. Subsequently, by observing the extracted femtosecond laser spot morphology and transmittance data, targeted optimization of the system coupling efficiency can be performed.

[0053] The technical solution for achieving high-efficiency high-harmonic generation (HHG) based on an optical fiber structure, proposed in the embodiments of this application, is adaptable to laser systems operating at the millijoule level. This feature significantly reduces the performance requirements for the laser system: not only can it effectively control costs, but more importantly, femtosecond lasers operating at the millijoule level can support high repetition rate output—from the kilohertz level of Ti:sapphire laser systems to hundreds of kilohertz and even higher frequency bands of ytterbium-doped laser systems, thereby greatly expanding the application scenarios of high-efficiency HHG technology based on optical fiber structures. In addition, in response to the current situation where mainstream femtosecond laser systems generally output pulse widths on the order of multiple optical cycles (for example, commercial Ti:sapphire laser systems typically output 800 nm lasers with pulse widths of 30 femtoseconds (fs), while ytterbium-doped laser systems mostly output pulse widths on the order of hundreds of femtoseconds), this solution integrates pulse width compression technology to compress the femtosecond laser pulse width to the sub-optical cycle level, effectively improving HHG efficiency through the optimization of underlying optical parameters.

[0054] In summary, the embodiments of the present application provide an optical system for generating high-efficiency high-order harmonics, achieving a technological breakthrough through the coordinated optimization of microscopic mechanisms and macroscopic structures. At the microscopic level, pulse compression technology reduces the pulse width of femtosecond lasers to sub-cycle levels, significantly enhancing the nonlinear response of single atoms in the intense laser field and improving the efficiency of high-order harmonic generation through quantum interaction. At the macroscopic level, by precisely controlling the structural parameters of the optical fiber structure (including fiber length, inner diameter, gas inlet position and spacing), the phase matching process between the optical field and the gas medium is optimized, and the physical conditions for high-efficiency harmonic generation are established at the system level. The core advantage of the solution of the embodiments of the present application lies in the organic integration of pulse compression technology and hollow-core fiber technology, forming a dual optimization mechanism for microscopic quantum effect control and macroscopic light field matching. Furthermore, it is compatible with a variety of femtosecond laser systems (including typical systems such as Ti:sapphire lasers, ytterbium-doped fiber lasers, and optical parametric chirped pulse amplifiers), has relaxed laser pulse energy requirements, and is particularly suitable for high repetition rate scenarios (from kilohertz to hundreds of kilohertz), providing feasibility for industrial-grade high-frequency applications.

[0055] In order to better understand the solution provided by the embodiment of the present application, the solution is described below in conjunction with a specific application scenario.

[0056] See Figure 4 , which is a schematic diagram of the structure of an optical system for generating high-order harmonics provided in an embodiment of the present application Figure 3 .exist Figure 4 In this example, the femtosecond laser source uses a Ti:sapphire femtosecond laser, with a central wavelength of 800 nm, a single pulse energy of 9 mJ, a repetition rate of 1 kHz, and a pulse width of 35 fs. The laser light is first modulated by a half-wave plate (HWP) and a polarizer, then injected into a hollow-core fiber approximately 1 m long via a lens and a reflector. It interacts with the gas (which can be an inert gas) within the hollow-core fiber in a static or differential mode, broadening the spectrum through the phase modulation (SPM) effect. It should be noted that by optimizing the laser energy, spot size, gas type, and pressure, a broad spectrum supporting a short-cycle pulse width can be achieved. The SPM-modulated laser output is collimated by a concave mirror and then compressed by a chirped mirror (CM) and a pair of wedges, generating a short-cycle femtosecond laser with a pulse width of approximately 5 fs and a single pulse energy exceeding 1 mJ. This short-cycle femtosecond laser is injected into a hollow-core fiber (HCF) through a window (Window 1) in the vacuum chamber, generating high-order harmonics (HHG) through the HCF. It should be noted that the hollow-core fiber used here is different from the one used in the pulse compression stage. The hollow-core fiber used here requires two gas inlets (Gas Inlets) drilled along the fiber's length. This is because the peak laser power during HHG generation is an order of magnitude higher than the peak power during self-phase modulation (SPM). If gas is injected through the HCF's light inlet, the strong filamentation effect will cause the femtosecond laser spot to be distorted, severely affecting the efficiency of coupling into the HCF and preventing effective HHG generation. Therefore, gas must be injected from the side of the HCF to interact with the femtosecond laser to produce HHG. It can be understood that the efficiency of HHG generation can be improved by optimizing parameters such as the laser (energy, spot size) and the HCF (inner diameter, drilling parameters, and gas pressure).

[0057] A movable reflector is set at the rear end of the HCF to reflect the laser to the laser output window (Window2) for output. The coupling is optimized by observing the light spot and transmittance. After HHG is generated, the infrared is blocked by the IR rejector and the metal filter (including Figure 4The residual femtosecond laser is filtered out by MF1 and MF2 in the microscope, and the transmitted HHG light is focused by a toroidal mirror (TM) onto a photodiode (PD) for HHG power measurement. After the photodiode is removed, the light passes through a slit and enters a grating for spectrum measurement. A cMOS camera (a charge-coupled device (XUV-CCD) can also be used) is used to measure the HHG spectrum.

[0058] It should be noted that in the embodiments of this application, the phase modulation (SPM) process and the HHG generation process are two ultrafast phenomena that occur under different laser conditions. In the SPM process, the hollow-core fiber used has an inner diameter of 200-500 μm, a length of 1000 mm, and no side drilling is required. Its main function is to broaden the spectrum, and the pulse width will be further compressed with the help of chirped mirrors and wedge pairs. The hollow-core fiber used for HHG generation has an inner diameter of 100-200 μm, a length between 20-100 mm, and requires side drilling. This structure improves macroscopic phase matching conditions, such as increasing the interaction distance, reducing laser intensity variations, and optimizing gas pressure distribution, thereby achieving high-efficiency HHG.

[0059] See below Figure 5 , Figure 5 The calculated results of HHG spectra generated under different pulse widths with a central wavelength of 800 nm are shown. Figure 5 (a) shows the theoretical calculation results of the high-order harmonic spectrum generated in helium under the conditions of producing the same critical ionization rate at the center of the pulse peak of 30 fs and 5 fs. It can be seen that the efficiency of the harmonic signal can be significantly improved by using a femtosecond laser with a small cycle (5 fs). After considering the propagation effect of the 800 nm femtosecond laser in the hollow-core fiber, the calculated HHG spectrum results are shown in Figure 2. Figure 5 (b) Similarly, it can be seen that in the EUV band, the HHG yield driven by a femtosecond laser with a low cycle level (5 fs) is an order of magnitude higher than that driven by a femtosecond laser with a high cycle level (30 fs). Therefore, by driving the HHG process in a hollow-core fiber structure with a low cycle level femtosecond laser, efficiency is improved at both the microscopic and macroscopic levels.

[0060] An embodiment of the present application also provides a method for improving the efficiency of high-order harmonics, which is applied to the aforementioned optical system for generating high-order harmonics, and includes at least one of the following: optimizing the spot size of the femtosecond laser output by the femtosecond laser light source; optimizing the energy of the femtosecond laser output by the femtosecond laser light source; optimizing the length of the first optical fiber structure; optimizing the inner diameter of the first optical fiber structure; optimizing the air inlet position of the first optical fiber structure; optimizing the type of the first optical fiber structure, the type of the first optical fiber structure including but not limited to: hollow fiber, antiresonant fiber or photonic bandgap fiber; optimizing the gas pressure of the first gas; optimizing the composition of the first gas.

[0061] It should be understood that the core advantage of the solution implemented in the embodiments of this application lies in significantly improving system efficiency and performance by compressing the pulse width of the driving laser from tens of femtoseconds to at least the order of a cycle and exciting high-order harmonics (HHG) within the fiber structure. This is demonstrated in three key aspects: First, it reduces energy consumption and increases repetition rate. The improved HHG efficiency significantly reduces the pulse energy requirement of the driving laser in specific experiments, such as angle-resolved photoelectron spectroscopy (ARPES). This feature allows the laser repetition rate to be increased to the megahertz range while maintaining a high photon flux, laying the foundation for the development of high-repetition-rate extreme ultraviolet (EUV) light sources. Second, it enhances light source power to advance nonlinear optics research. The increased HHG efficiency directly translates into increased light source output power. Through optical focusing, the peak power of EUV pulses can be increased to the terawatt level. This not only provides a higher-intensity driving light source for research in attosecond pulse generation and strong-field ionization, but also accelerates breakthroughs in cutting-edge fields such as nonlinear EUV optics (such as EUV frequency combs and high-order harmonic interferometry). Third, it expands industrial and biomedical applications. The improved stability and practicality of high-power HHG light sources can overcome the bottleneck that has traditionally limited their application to basic scientific research. For example, in the industrial field, they can be used for defect detection in extreme ultraviolet lithography masks and high-speed online monitoring of advanced process chips. In biomedicine, they can provide new tools for dynamic protein structure analysis and ultrafast imaging of nanoparticle drug delivery.

[0062] Finally, it should be noted that the above embodiments are only specific implementation methods of the present application, which are used to illustrate the technical solutions of the present application rather than to limit them. The protection scope of the present application is not limited thereto. Although the present application has been described in detail with reference to the aforementioned embodiments, ordinary technicians in this field should understand that any technician familiar with this technical field can still modify the technical solutions recorded in the aforementioned embodiments within the technical scope disclosed in the present application, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be covered by the protection scope of the present application.

Claims

1. An optical system for generating high-order harmonics, characterized in that: include: A femtosecond laser light source, used for outputting femtosecond laser; A pulse width compression module, connected to the femtosecond laser light source, for compressing the pulse width of the femtosecond laser; The high-order harmonic generation module is connected to the pulse width compression module, and includes a vacuum chamber and a first optical fiber structure arranged in the vacuum chamber. A first gas is introduced into the first optical fiber structure. The first optical fiber structure is used to output high-order harmonics based on the femtosecond laser after the pulse width is compressed.

2. The optical system according to claim 1, wherein: The first optical fiber structure has at least two gas inlets for introducing the first gas, and each of the gas inlets is arranged on a side surface in the longitudinal direction of the first optical fiber structure.

3. The optical system according to claim 1, wherein: It also includes a high-order harmonic measurement module, which is arranged in the vacuum chamber and at the output end of the high-order harmonic generation module, and is used to measure the high-order harmonics; The high-order harmonic measurement module is connected to the output end of the high-order harmonic generation module through a metal filter, and the metal filter is used to filter excess femtosecond laser light.

4. The optical system according to claim 3, wherein: The high-order harmonics measurement module includes a power measurement device, and the power measurement device is used to measure the power of the high-order harmonics.

5. The optical system according to claim 3, wherein: The high-order harmonic measurement module includes a spectrometer and an optical signal detection device. The spectrometer is used to separate the multiple wavelengths of light contained in the high-order harmonic light source output by the high-order harmonic generation module, and the optical signal detection device is used to measure the light of each wavelength band to obtain the intensity distribution of different wavelengths in the high-order harmonic light source.

6. The optical system according to claim 1, wherein: A first reflecting mirror is further provided in the vacuum chamber, and a laser export window is provided on the vacuum chamber. The first reflecting mirror is provided at the output end of the high-order harmonic generation module, and is used to reflect the remaining femtosecond laser output from the high-order harmonic generation module that does not participate in the high-order harmonic generation to the laser export window, so as to export the remaining femtosecond laser out of the vacuum chamber through the laser export window.

7. The optical system according to claim 1, wherein: The pulse width compression module includes a nonlinear spectrum broadening submodule and a dispersion compensation element. The nonlinear spectrum broadening submodule is used to perform spectrum broadening on the femtosecond laser, and the implementation methods include but are not limited to: multi-pass cavity technology, hollow fiber technology, and filamentation technology; the dispersion compensation element is used to perform pulse width compression on the femtosecond laser after spectrum broadening.

8. A method for improving high-order harmonic efficiency, characterized in that: The method is applied to the optical system according to any one of claims 1 to 7, and the method includes at least one of the following: Optimizing the spot size of the femtosecond laser output by the femtosecond laser light source; Optimizing the energy of the femtosecond laser output by the femtosecond laser light source; Optimizing the length of the first optical fiber structure; optimizing the inner diameter of the first optical fiber structure; Optimizing the air inlet position of the first optical fiber structure; Optimizing the type of the first optical fiber structure, where the types of the first optical fiber structure include but are not limited to: hollow-core optical fiber, antiresonant optical fiber, and photonic bandgap optical fiber; optimizing the gas pressure of the first gas; The composition of the first gas is optimized.

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