A method and apparatus for generating high-energy, few-cycle blue light
By performing nonlinear frequency doubling and multiple pulse width compressions within a helium-filled cavity, the problem of simultaneously achieving high energy and ultra-short pulse width in existing technologies has been solved, enabling the efficient generation of high-energy, short-cycle blue lasers.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HUAZHONG UNIV OF SCI & TECH
- Filing Date
- 2023-02-20
- Publication Date
- 2026-05-19
AI Technical Summary
Existing technologies struggle to balance high energy and ultrashort pulse width, posing a challenge to generating 400 nm few-period blue lasers with sub-6 femtosecond and microjoule-level pulses.
Frequency doubling is performed using a nonlinear crystal, and pulse width compression is achieved by using a helium-filled cavity, spectral broadening and dispersion management using a solid sheet, and negative chirp processing is combined to avoid higher-order dispersion and gas ionization, thus realizing multiple pulse width compression.
It has achieved the generation of high-energy, short-cycle blue lasers with high pulse width compression efficiency, good beam quality, low energy loss, and output energy reaching the level of hundreds of microjoules.
Smart Images
Figure CN116449630B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of ultrafast laser technology, and more specifically, relates to a method and apparatus for generating high-energy, short-period blue laser light. Background Technology
[0002] High-energy, few-period pulses with ultrafast time resolution and high peak intensity can facilitate many ultrafast and strong-field applications. Compared to commonly used 800 nm pulses, few-period blue lasers have higher photon energy and shorter wavelengths, offering many beneficial properties. Sub-microjoule, sub-10 femtosecond 400 nm pulses have been used in many time-resolved spectroscopy experiments to study the ultrafast dynamics of some chemical materials. For the study of strong-field ionization, it has been theoretically predicted that few-period 400 nm pulses can be used to study electron vortices in the multiphoton ionization state of inert gas atoms. The higher harmonic yield is highly wavelength-dependent; the reported higher harmonic yield of a 400 nm laser is 54 times that of an 800 nm laser. Furthermore, when a 400 nm pulse is compressed to 6.5 fs, isolated attosecond pulses can be expected. Therefore, using tens of microjoules of few-period 400 nm pulses can drive new spectral regions, namely near-ultraviolet to extreme ultraviolet attosecond resolution pumped-probe spectroscopy experiments.
[0003] Several approaches have been proposed to generate short-period 400nm pulses. Among these, frequency up-conversion based on nonlinear crystals is the most direct. However, the inherent contradiction between phase-matching bandwidth and conversion efficiency, along with the strong bandwidth dependence on the fundamental pulse, significantly reduces the applicability of this approach. Pulse durations generated by these approaches are typically around 10 femtoseconds. However, a few works in this category have achieved pulse durations of 6 femtoseconds through more sophisticated design. Nevertheless, due to the required tight focusing structure and low pulse compression efficiency, the pulse energy is currently limited to the level of one or two microjoules, or even sub-microjoules.
[0004] As a widely used pulse compression method, this type of scheme based on hollow-core fiber (HCF) has also been applied to generate 400 nm few-period pulses. However, for the most conventional HCF post-compression scheme, due to the easy introduction of higher-order dispersion during spectral broadening, the pulse can only be compressed to the sub-10 femtosecond level. Furthermore, by spectral splitting of the 800 nm supercontinuum output from HCF and compression with a chirped mirror, a 400 nm pulse of 6 microjoules and 4.4 femtoseconds was obtained. However, the weak energy of the supercontinuum at the short-wavelength end limits the energy scalability of this scheme. Resonant dispersive wave emission in HCF has proven to be a method that can potentially generate 3 femtosecond, 20 microjoule blue lasers. However, how to completely separate the blue light portion from the ultrawideband supercontinuum (covering a spectral range of 350-2500 nm) without introducing higher-order dispersion remains a technical challenge. In summary, with existing technology, it is difficult to achieve both high energy and short pulse width, and generating 400 nm pulses in the sub-6 femtosecond, hundred microjoule range is still very challenging. Summary of the Invention
[0005] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a method and apparatus for generating high-energy short-period blue laser, which solves the technical problem that the existing methods for generating short-period blue laser cannot simultaneously achieve ultra-short pulse width (sub-6 femtosecond) and high energy (hundreds of microjoules).
[0006] To achieve the above objectives, in a first aspect, the present invention provides a method for generating high-energy, few-period blue laser light, comprising the following steps:
[0007] S1. After converting the post-compressed fundamental frequency pulse from P-polarization to S-polarization, a nonlinear crystal is used for frequency doubling, and the generated frequency-doubled pulse is separated.
[0008] S2. A focusing lens is used to focus the latest frequency-doubled pulse. A solid sheet placed near the focal point of the focusing lens is used to broaden the spectrum of the focused frequency-doubled pulse. After collimating the spectrally broadened frequency-doubled pulse, dispersion management is performed to compress the pulse width of the frequency-doubled pulse and output an updated frequency-doubled pulse.
[0009] S3. Repeat step S2 at least once to obtain a high-energy, short-period blue laser.
[0010] Steps S1-S3 are all performed inside a cavity filled with helium.
[0011] More preferably, when post-compressing the fundamental frequency pulse, the process of introducing negative chirp is performed in a helium-filled cavity.
[0012] More preferably, the nonlinear crystal is a substrate-free nonlinear crystal.
[0013] More preferably, the aforementioned solid sheet is placed near the focal point of the focusing lens at a Brewster angle to maximize the efficiency of energy transfer.
[0014] More preferably, along the optical path, the solid sheet is placed in front of the focal point of the focusing lens to further broaden the pulse width of the focused frequency-doubled pulse, thereby avoiding helium ionization at the focal point and preventing energy absorption.
[0015] More preferably, there are one or more solid sheets; when there are multiple solid sheets, the multiple solid sheets are placed sequentially along the optical path direction.
[0016] Secondly, the present invention provides a high-energy, few-period blue laser generating device, comprising:
[0017] A frequency doubling pulse generator is used to convert the post-compressed fundamental frequency pulse from P-polarization to S-polarization, then perform frequency doubling using a nonlinear crystal, and finally separate the generated frequency doubling pulse.
[0018] N compression modules are placed sequentially along the optical path. The compression module is used to focus the frequency-doubled pulse using a focusing lens, broaden the spectrum of the focused frequency-doubled pulse by placing a solid sheet near the focal point of the focusing lens, collimate the spectrally broadened frequency-doubled pulse, and then perform dispersion management to compress the pulse width of the frequency-doubled pulse.
[0019] Where N is greater than or equal to 2; the frequency multiplier pulse generator and the compression module are both placed in a cavity filled with helium.
[0020] More preferably, the compression module includes: a focusing lens, a solid sheet, a concave mirror, and a frequency doubling pulse dispersion manager, which are arranged sequentially along the optical path.
[0021] A focusing lens is used to focus the frequency-doubled pulse;
[0022] A solid sheet is placed near the focal point of the focusing lens to broaden the spectrum of the focused frequency-doubled pulse;
[0023] Concave mirrors are used to collimate the broadened frequency-doubled pulses;
[0024] The frequency doubling pulse dispersion manager is used to manage the dispersion of the aligned frequency doubling pulse to shorten the pulse width.
[0025] More preferably, along the optical path, the solid sheet is placed in front of the focal point of the focusing lens to further broaden the pulse width of the focused frequency-doubled pulse, thereby avoiding helium ionization at the focal point and preventing energy absorption.
[0026] More preferably, there are one or more solid sheets; when there are multiple solid sheets, the multiple solid sheets are placed sequentially along the optical path direction.
[0027] More preferably, the above-mentioned high-energy, few-period blue laser generating device further includes a post-compression module placed along the optical path before the frequency-doubling pulse generator, comprising:
[0028] A fundamental frequency pulse spectral broadener is used to broaden the spectrum of a fundamental frequency pulse.
[0029] A fundamental frequency pulse dispersion manager is used to compress fundamental frequency pulses by managing the dispersion of the spectrally broadened fundamental frequency pulses.
[0030] The portion of the fundamental frequency pulse dispersion manager that introduces negative chirp is placed within the helium-filled cavity.
[0031] More preferably, the nonlinear crystal is a substrate-free nonlinear crystal.
[0032] More preferably, the aforementioned solid sheet is placed near the focal point of the focusing lens at a Brewster angle to maximize the efficiency of energy transfer.
[0033] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:
[0034] 1. This invention provides a method and apparatus for generating high-energy, few-period blue laser. The method involves frequency doubling a post-compression-processed, high-field-strength, short-pulse fundamental frequency pulse using a nonlinear crystal, followed by at least two pulse width compressions to obtain the high-energy, few-period blue laser. Both the frequency doubling and pulse width compression processes are performed within a helium-filled cavity. The helium-filled cavity eliminates the need for pre-compensation for chirp during the frequency doubling process, maintaining a short pulse width while doubling the frequency. The pulse width compression process utilizes a solid-state thin film for spectral broadening and employs a dispersion tube based on a frequency-doubled pulse dispersion manager. Theoretically, because helium has extremely low linear dispersion and nonlinear coefficients, as well as extremely high ionization energy, the pulse compression process takes place in a cavity filled with helium. This ensures that the solid sheet is not damaged, contributes to spectral broadening, and provides a near-vacuum environment in terms of both dispersion and laser-induced ionization. This makes dispersion management of frequency-doubled pulses easier, improves pulse compression efficiency, and avoids energy loss caused by gas ionization and limitations on input energy, allowing for higher output energy. This enables the generation of blue lasers with higher energies in the hundreds of microjoules and ultrashort pulse widths in the sub-6 femtosecond range.
[0035] 2. Furthermore, the high-energy, few-period blue laser generation method and apparatus provided by this invention, by post-compressing the fundamental frequency pulse and using a thinner nonlinear crystal for frequency doubling, can simultaneously achieve both pulse width and frequency doubling efficiency of the frequency-doubled pulse. Compared to directly obtaining ultrashort blue lasers through frequency doubling, it has lower requirements for fundamental frequency pulse compression and higher frequency doubling efficiency. On the other hand, the shorter frequency-doubled pulse reduces the difficulty of subsequent pulse compression, resulting in a compressed pulse with ultrashort duration and high energy concentration.
[0036] 3. Furthermore, the high-energy, short-period blue laser generation method and apparatus provided by the present invention generates a high-field-strength, short-pulse-width fundamental frequency pulse (800 nm) through post-compression. During the post-compression processing of the fundamental frequency pulse, the process of introducing negative chirp is performed in a helium-filled cavity, thereby controlling the nonlinear phase accumulation introduced during the dispersion management of the fundamental frequency pulse, reducing nonlinear phase accumulation, and thus avoiding beam quality degradation caused by self-focusing of the high-intensity fundamental frequency pulse. This enables the generation of a high-intensity fundamental frequency pulse with sufficient bandwidth and good beam quality. Attached Figure Description
[0037] Figure 1 A flowchart of the high-energy, short-period blue laser generation method provided by the present invention;
[0038] Figure 2 This is a schematic diagram of the high-energy, short-period blue laser generating device provided in an embodiment of the present invention;
[0039] Figure 3 The output power variation curves recorded under helium-filled cavity and vacuum conditions are provided in the embodiments of the present invention.
[0040] Figure 4 The images show the spectrum of the final output pulse of the high-energy, few-period blue laser generator provided in this embodiment of the invention and the reconstructed pulse envelope; wherein, (a) is a schematic diagram of the spectrum of the final output pulse of the high-energy, few-period blue laser generator; and (b) is a schematic diagram of the reconstructed pulse envelope.
[0041] Figure 5 This is a schematic diagram of the far-field spot of the final output pulse of the high-energy, short-period blue laser generating device provided in an embodiment of the present invention. Detailed Implementation
[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.
[0043] To achieve the above objectives, the present invention provides a method for generating high-energy, few-period blue laser light, such as... Figure 1 As shown, it includes the following steps:
[0044] S1. After converting the post-compressed fundamental frequency pulse from P-polarization to S-polarization, a nonlinear crystal is used for frequency doubling, and the generated frequency-doubled pulse is separated.
[0045] Preferably, in the pulse dispersion management process following the fundamental frequency pulse compression, the glass plate and window with positive chirp are placed at Brewster's angle, and the process of introducing negative chirp is carried out in a cavity filled with helium (the pressure is generally set to 1 bar) to reduce the nonlinear phase accumulation introduced during the dispersion management of the fundamental frequency pulse, thereby generating a high-intensity fundamental frequency pulse with sufficient bandwidth and good beam quality. Due to the presence of the frequency-doubled pulse dispersion manager and the solid sheet that needs to be placed at Brewster's angle, the frequency-doubled pulse is required to be P-polarized. Therefore, it is often necessary to convert the fundamental frequency pulse transmitted by conventional P-polarization to S-polarization.
[0046] Preferably, to avoid nonlinear phase accumulation caused by the high-intensity fundamental frequency pulse passing through the substrate (when the substrate is in front of the nonlinear crystal) or to broaden the pulse width of the frequency-doubled pulse (when the substrate is behind the nonlinear crystal), a substrate-free nonlinear crystal (such as a thinner BBO crystal) is used for frequency doubling. A dichroic mirror or beam splitter is used to separate the generated frequency-doubled pulse. Specifically, in one optional embodiment, the polarization of the post-compressed fundamental frequency pulse is changed from P-polarization to S-polarization. A substrate-free nonlinear crystal of suitable thickness is used to double the fundamental frequency light, and a dichroic mirror with suitable bandwidth (high reflectivity for blue light, high transmittance for red light, and a low-dispersion coating on the reflective surface) is used to separate the fundamental and frequency-doubled pulses, ultimately obtaining a P-polarized frequency-doubled pulse.
[0047] It's worth noting that by using a post-compressed fundamental frequency pulse and then frequency doubling it with a nonlinear crystal, both the pulse width and frequency doubling efficiency of the frequency-doubled pulse can be balanced. Compared to directly obtaining ultrashort blue laser light through frequency doubling, this method results in lower compression of the fundamental frequency pulse and higher frequency doubling efficiency. Furthermore, the shorter frequency-doubled pulse reduces the difficulty of subsequent pulse compression, leading to a compressed pulse with ultrashort pulse length and high energy concentration.
[0048] S2. The latest frequency doubling pulse is focused using a focusing lens. The focused frequency doubling pulse is spectrally broadened by a solid thin film (usually an optical glass plate with a thickness of hundreds of micrometers) placed near the focal point of the focusing lens. After collimating the spectrally broadened frequency doubling pulse, dispersion management is performed to compress the pulse width of the frequency doubling pulse and output an updated frequency doubling pulse.
[0049] Preferably, the aforementioned solid sheet is placed near the focal point of the focusing lens at a Brewster angle to maximize the efficiency of energy transfer.
[0050] Preferably, along the optical path, the solid sheet is placed in front of the focal point of the focusing lens to further broaden the pulse width of the focused frequency-doubled pulse, thereby avoiding helium ionization at the focal point and preventing energy absorption.
[0051] Preferably, there can be one or more solid sheets; when there are multiple solid sheets, the multiple solid sheets are placed sequentially along the optical path direction.
[0052] It should be noted that the selection of the number and thickness of the solid film depends on the tightness of the focusing and the light intensity at the focal point. If it is tight focusing, a single thicker film can be selected and placed in front of the focal point; if it is loose focusing, more and thinner films can be selected and placed near the focal point, either in front or behind.
[0053] S3. Repeating step S2 at least once can further enhance spectral broadening while ensuring beam quality and simplicity of dispersion management, thereby obtaining high-energy, few-period blue laser. Preferably, in an optional implementation, repeating step S1 once is sufficient to obtain high-energy, few-period blue laser.
[0054] Steps S1-S3 are all performed within a helium-filled cavity (typically at a pressure of 1 bar) to ensure the solid sheet is not damaged and to contribute to spectral broadening. It should be noted that helium has extremely low material dispersion and extremely high ionization energy. This not only effectively prevents damage to the solid sheet but also provides a near-vacuum environment in terms of both dispersion and laser-induced ionization, making dispersion compensation for the frequency-doubled pulse easier, improving pulse compression efficiency, and avoiding energy loss and limitations on input energy caused by gas ionization. Furthermore, performing the frequency-doubled pulse generation process in step S1 within a helium-filled cavity allows the frequency-doubled pulse to be directly compressed without passing through a window, eliminating the need for additional chirp pre-compensation. This significantly simplifies the device's complexity, reduces reflection loss introduced by the lens, and improves compression efficiency.
[0055] Secondly, the present invention provides a high-energy, few-period blue laser generating device, comprising:
[0056] A frequency-doubled pulse generator is used to convert the post-compressed fundamental frequency pulse from P-polarization to S-polarization, perform frequency doubling using a nonlinear crystal, and separate the generated frequency-doubled pulse. Preferably, a substrate-free nonlinear crystal (such as a thin BBO crystal) is used for frequency doubling. A dichroic mirror or beam splitter is used to separate the generated frequency-doubled pulse. Specifically, in one optional embodiment, the polarization of the post-compressed fundamental frequency pulse is converted from P-polarization to S-polarization, the fundamental frequency light is frequency-doubled using a substrate-free nonlinear crystal of appropriate thickness, and the fundamental and frequency-doubled pulses are separated using a dichroic mirror of appropriate bandwidth (high reflectivity for blue light, high transmittance for red light, and a low-dispersion coating on the reflective surface), ultimately obtaining a P-polarized frequency-doubled pulse. It should be noted that by using a post-compressed fundamental frequency pulse and performing frequency doubling with a nonlinear crystal, both the pulse width and frequency doubling efficiency of the frequency-doubled pulse can be considered simultaneously. Compared to directly obtaining ultrashort blue laser light through frequency doubling, it results in lower compression of the fundamental frequency pulse and higher frequency doubling efficiency. On the other hand, shorter frequency-harmonic pulses reduce the difficulty of subsequent pulse compression, resulting in compressed pulses with ultra-short lengths and high energy concentration.
[0057] N compression modules are placed sequentially along the optical path. The compression modules are used to focus the frequency-doubled pulse using a focusing lens, broaden the spectrum of the focused frequency-doubled pulse by placing a solid sheet near the focal point of the focusing lens, collimate the spectrally broadened frequency-doubled pulse, and perform dispersion management to compress the pulse width of the frequency-doubled pulse. Preferably, the solid sheet is placed near the focal point of the focusing lens at a Brewster angle to maximize the energy transfer efficiency.
[0058] In this configuration, N is greater than or equal to 2; both the frequency-doubled pulse generator and the compression module are placed within a helium-filled cavity. It should be noted that helium possesses extremely low material dispersion and extremely high ionization energy. This not only effectively prevents damage to the solid-state sheet but also provides a near-vacuum environment in terms of both dispersion and laser-induced ionization, making dispersion compensation for the frequency-doubled pulses easier and avoiding energy loss and limitations on input energy caused by gas ionization. Furthermore, placing the frequency-doubled pulse generator within a helium-filled cavity allows the frequency-doubled pulses to be directly compressed into subsequent pulses without passing through a window, eliminating the need for additional chirp pre-compensation. This significantly simplifies the device's complexity, reduces reflection losses introduced by the mirrors, and improves compression efficiency.
[0059] In one optional implementation, the compression module includes: a focusing lens, a solid sheet, a concave mirror, and a frequency doubling pulse dispersion manager arranged sequentially along the optical path direction;
[0060] A focusing lens is used to focus the frequency-doubled pulse;
[0061] A solid sheet is placed near the focal point of the focusing lens to broaden the spectrum of the focused frequency-doubled pulse;
[0062] A concave mirror can be a focusing mirror or a collimating mirror, used to collimate the broadened frequency-doubled pulse;
[0063] A frequency-doubled pulse dispersion manager (usually a chirped mirror) is used to manage the dispersion of the aligned frequency-doubled pulses in order to shorten the pulse width of the frequency-doubled pulses.
[0064] Preferably, along the optical path, the solid sheet is placed in front of the focal point of the focusing lens to further broaden the pulse width of the focused frequency-doubled pulse, thereby avoiding helium ionization at the focal point and preventing energy absorption.
[0065] Preferably, there are one or more solid sheets; when there are multiple solid sheets, the multiple solid sheets are placed sequentially along the optical path direction.
[0066] Preferably, the above-mentioned high-energy, few-period blue laser generating device further includes a post-compression module placed along the optical path before the frequency-doubling pulse generator, comprising:
[0067] A fundamental frequency pulse spectral broadener is used to broaden the spectrum of a fundamental frequency pulse.
[0068] A fundamental frequency pulse dispersion manager is used to compress fundamental frequency pulses by managing the dispersion of the spectrally broadened fundamental frequency pulses.
[0069] It should be noted that the fundamental frequency pulse dispersion manager can be divided into two parts: introducing positive chirp and introducing negative chirp. The glass plate, typically used to introduce positive chirp, is placed at a Brewster angle to reduce the light intensity within the plate, thereby reducing the accumulation of nonlinear phase and Fresnel reflection. The process of introducing negative chirp is performed within a helium-filled cavity to further reduce the accumulation of nonlinear phase. Specifically, in the pulse dispersion management process following the fundamental frequency pulse compression, the glass plate and window for introducing positive chirp are placed at a Brewster angle, and the process of introducing negative chirp is performed within a helium-filled cavity (the pressure is typically set to 1 bar) to reduce the accumulation of nonlinear phase introduced during the dispersion management of the fundamental frequency pulse, thus enabling the generation of a sufficiently wide, high-intensity fundamental frequency pulse with good beam quality.
[0070] It should be noted that the selection of the number and thickness of the solid slices depends on the tightness of the focusing and the light intensity at the focal point. For tight focusing, a single, thicker slice can be selected and placed in front of the focal point; for loose focusing, more and thinner slices can be selected and placed near the focal point, either in front or behind. Furthermore, the spectral broadening of the fundamental pulse, the light intensity of the fundamental pulse, and the thickness of the nonlinear crystal are determined by the expected final output result.
[0071] To further illustrate the high-energy, few-period blue laser generating device provided by the present invention, the following detailed description is provided in conjunction with specific embodiments:
[0072] In this embodiment, N is 2, such as Figure 2 As shown, a high-energy, few-period blue laser generating device includes:
[0073] The post-compression module includes: a fundamental frequency pulse spectral broadener 1 and a fundamental frequency pulse dispersion manager 2; the fundamental frequency pulse spectral broadener 1 is used to broaden the spectrum of the fundamental frequency pulse; the fundamental frequency pulse dispersion manager 2 is used to manage the dispersion of the spectrally broadened fundamental frequency pulse, and finally realizes pulse compression.
[0074] Frequency multiplier pulse generator 3 is used to realize the conversion of the fundamental frequency pulse, frequency multiplication, and subsequent separation of the fundamental frequency and frequency multiplier pulses;
[0075] The two compression modules connected in series are designated as the first compression module and the second compression module, respectively. The first compression module includes a focusing lens 4, a solid-state thin film 5, a focusing lens 6, and a frequency-doubled pulse dispersion manager 7. The second compression module includes a focusing lens 8, a solid-state thin film 9, a focusing lens 10, and a frequency-doubled pulse dispersion manager 11. The focusing lens is used to focus the output frequency-doubled pulse beam; the solid-state thin film is used to broaden the spectrum of the frequency-doubled pulse, and when placed before the focal point of the focusing lens, it can also achieve a certain degree of pulse width broadening (which helps to reduce the focal intensity and ionization when a tight focusing structure is used and the focal light intensity is too high); the focusing lens is used to collimate the spectrally broadened frequency-doubled pulse beam; and the frequency-doubled pulse dispersion manager is used to compensate for the dispersion of the spectrally broadened frequency-doubled pulse.
[0076] The fundamental frequency pulse negative chirp introduction part, the frequency multiplier pulse generator 3, and the two subsequent compression modules in the post-compression module are all placed in the helium-filled cavity 12.
[0077] In this embodiment, all reflective lenses used are coated with broadband and low dispersion to ensure that broadband pulses do not broaden after being reflected by the lenses, and to avoid introducing high-order dispersion that is difficult to compensate for.
[0078] In summary, this embodiment generates a high-field-strength, short-pulse-width fundamental frequency pulse (800 nm) through post-compression and controls nonlinear phase accumulation during fundamental frequency pulse dispersion management, thus avoiding beam quality degradation caused by self-focusing of the high-intensity fundamental frequency pulse. Frequency-doubled pulse generation occurs within a helium-filled cavity, eliminating the need for pre-compensation chirp and maintaining a short pulse width. This allows for the generation of short-cycle blue laser light through two stages of helium-assisted solid-state thin-film spectral broadening and chirped mirror-based dispersion compensation. Helium plays a role in preventing thin-film damage during spectral broadening and contributes to the broadening process. Furthermore, due to helium's extremely low linear dispersion and nonlinear coefficients, as well as its extremely high ionization energy, the cavity environment closely approximates a vacuum environment in terms of dispersion and ionization. Therefore, compared to an atmospheric environment, dispersion compensation is easier, pulse compression efficiency is higher, and higher output energy is permitted.
[0079] To further illustrate the high-energy, few-period blue laser method and apparatus provided in the embodiments of the present invention, the following detailed description is provided in conjunction with the accompanying drawings and specific examples:
[0080] Experimental setup such as Figure 2 As shown, a 1 kHz, 1.8 mJ, 30 fs, 800 nm pulse is input to a fundamental frequency pulse spectral broadener 1 based on hollow fiber for spectral broadening. The fundamental frequency pulse dispersion manager 2 consists of a 3 mm thick fused silica sheet (positive chirp introduction portion) and a chirped mirror (introducing a negative chirp of -355 fs^2). The fundamental frequency pulse is compressed to 13.1 femtoseconds. By using two periscopes to form a frequency-doubled pulse generator 3, the polarization state conversion from P-polarization to S-polarization of the fundamental frequency pulse, the frequency doubling process, and the separation of the fundamental frequency pulse and the frequency-doubled pulse are realized. A 72 μm thick, cut-angle (θ = 29.2°), substrate-free BBO is integrated into the second periscope. Finally, a 131 μjoule, 12.5 femtosecond, P-polarized frequency-doubled pulse is obtained with a conversion efficiency of 10.7%. Both periscopes are placed inside a cavity filled with 1 bar of helium gas. Intracavity fundamental frequency pulse compression is crucial for reducing nonlinear phase accumulation in high-intensity (up to 350 GW / chirped mirror²) fundamental wave pulses, thereby suppressing beam quality degradation. Therefore, short SHG pulses with ideal beam quality are prepared for subsequent pulse compression. Furthermore, the frequency-doubled pulses can be directly introduced into the subsequent pulse compression section without passing through a window.
[0081] The pulse compression employs a two-stage structure. Each stage consists of two concave mirrors (focusing mirrors 4, 6, 8, and 10) with f = 500 mm UV-enhanced aluminum coating for beam focusing and collimation; a 200-micron-thick fused silica (FS) sheet placed at a Brewster angle before focusing for spectral broadening (solid sheets 5 and 9); and a chirped mirror (chirped mirrors 7 and 11) for dispersion compensation. In a vacuum environment, under a safe field strength of approximately 1 × 10^12 W / cm^2 (atmospheric conditions), the FS sheet exhibits unexpected damage. However, this damage is avoided by filling with 1 bar of helium. Figure 3 The figures show the changes in output power under helium-filled and vacuum conditions, respectively. The decrease in output power is caused by damage to the thin sheet.
[0082] The extremely low material dispersion of helium within the cavity limits the total group delay dispersion (GDD) of the two-stage compression to 10 fs^2. By injecting 1 bar of helium into the cavity, damage to the FS plate is avoided, and the output power is 85 milliwatts, which is close to the 87 milliwatts achieved when the FS plate is largely intact in a vacuum environment. The helium within the cavity not only prevents damage to the FS plate but also significantly suppresses ionization effects and material dispersion, which are precisely the advantages of a vacuum environment.
[0083] The selection of the number and thickness of the FS plates was based on the following considerations: using a single plate per stage facilitates intracavity operation and saves cavity space. When the FS plate thickness is 200 micrometers, it not only allows for a moderate increase in pulse width (considering only material dispersion, from 8 femtoseconds to 13 femtoseconds), but also allows for spectral broadening at relatively low in-plate intensity. This helps avoid damage to the FS plates. Finally, the B-integral of each thin plate is controlled at around 1.5 rad; a smaller B-integral maintains beam quality to some extent.
[0084] like Figure 4 As shown, the final compressed pulse has a pulse width of 5.5 femtoseconds. Furthermore, 96% of the pulse energy is concentrated in the main pulse. The pulse compression efficiency is 64.8%, while energy loss mainly comes from reflections from the four focusing lenses. Finally, the beam is focused using a lens with f = 400 mm to diagnose the far-field beam spot. The spot at the focal point is shown below. Figure 5 As shown, the beam maintains high performance in the central bright spot, with only a small amount of energy dispersed around it.
[0085] The above results demonstrate that, using the method and apparatus of this invention, a blue laser with an energy density of 85 microjoules, a wavelength of 5.5 femtoseconds, and a spectral span of 350-500 nm was generated, with the main pulse containing 96% of the energy. This is currently the highest-energy sub-6 femtosecond pulse known in this wavelength range. Furthermore, by controlling the nonlinear phase accumulation introduced during the compression of the fundamental frequency pulse, the beam quality under high fundamental frequency intensity was ensured. The use of helium not only solves the problem of thin-film damage in a vacuum but also, due to its high ionization energy and low dispersion characteristics, makes dispersion compensation of the frequency-doubled pulses easier, resulting in higher pulse compression efficiency (up to 64.8%) and allowing for higher energy pulses (up to the hundreds of microjoules). In summary, this invention provides an effective method for generating short-period blue lasers of hundreds of microjoules, and demonstrates the effectiveness of the method with an example.
[0086] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for generating high-energy, few-period blue laser light, characterized in that, Includes the following steps: S1. After converting the post-compressed fundamental frequency pulse from P-polarization to S-polarization, a nonlinear crystal is used for frequency doubling, and the generated frequency-doubled pulse is separated. S2. A focusing lens is used to focus the latest frequency-doubled pulse. A solid sheet placed near the focal point of the focusing lens is used to broaden the spectrum of the focused frequency-doubled pulse. After collimating the spectrally broadened frequency-doubled pulse, dispersion management is performed to compress the pulse width of the frequency-doubled pulse and output an updated frequency-doubled pulse. S3. Repeat step S2 at least once to obtain a high-energy, short-period blue laser. Steps S1-S3 are all performed in a helium-filled cavity to ensure that the solid sheet is not damaged, contribute to spectral broadening, and provide a near-vacuum environment in terms of both dispersion and laser-induced ionization. This makes dispersion management of the frequency-doubled pulse easier, improves pulse compression efficiency, and avoids energy loss and input energy limitations caused by gas ionization, allowing for higher output energy. This enables the generation of blue lasers with higher energies in the hundreds of microjoules and ultrashort pulse widths in the sub-6 femtosecond range.
2. The method for generating high-energy, few-period blue laser light according to claim 1, characterized in that, When the fundamental frequency pulse is post-compressed, the process of introducing negative chirp is performed in the helium-filled cavity.
3. The method for generating high-energy, few-period blue laser light according to claim 1, characterized in that, The nonlinear crystal is a substrate-free nonlinear crystal.
4. The method for generating high-energy, few-period blue laser light according to claim 1, characterized in that, Along the optical path, the solid sheet is placed in front of the focal point of the focusing lens to further broaden the pulse width of the focused frequency-doubled pulse, thereby avoiding helium ionization at the focal point and preventing energy absorption.
5. The method for generating high-energy, few-period blue laser light according to any one of claims 1-4, characterized in that, The solid sheet may be one or more; when there are multiple solid sheets, each solid sheet is placed sequentially along the optical path direction.
6. A high-energy, short-period blue laser generating device, characterized in that, include: A frequency doubling pulse generator is used to convert the post-compressed fundamental frequency pulse from P-polarization to S-polarization, then perform frequency doubling using a nonlinear crystal, and finally separate the generated frequency doubling pulse. N compression modules are placed sequentially along the optical path. The compression module is used to focus the frequency-doubled pulse using a focusing lens, broaden the spectrum of the focused frequency-doubled pulse by placing a solid sheet near the focal point of the focusing lens, collimate the spectrally broadened frequency-doubled pulse, and perform dispersion management to compress the pulse width of the frequency-doubled pulse. Wherein, N is greater than or equal to 2; the frequency doubling pulse generator and the compression module are both placed in a helium-filled cavity to ensure that the solid sheet is not damaged, contribute to spectral broadening, and provide a near-vacuum environment in terms of both dispersion and laser-induced ionization, making dispersion management of the frequency doubling pulse easier, improving pulse compression efficiency, and avoiding energy loss caused by gas ionization and limitations on input energy, allowing for higher output energy, and enabling the generation of blue lasers with higher energy in the hundreds of microjoules and ultrashort pulse widths in the sub-6 femtosecond range.
7. The high-energy, few-period blue laser generating device according to claim 6, characterized in that, The compression module includes: a focusing lens, a solid sheet, a concave mirror, and a frequency doubling pulse dispersion manager, which are arranged sequentially along the optical path. The focusing lens is used to focus the frequency-doubled pulse; The solid sheet is placed near the focal point of the focusing lens to broaden the spectrum of the focused frequency-doubled pulse; there are one or more solid sheets; when there are multiple solid sheets, the multiple solid sheets are placed sequentially along the optical path. The concave mirror is used to collimate the broadened frequency-doubled pulse; The frequency doubling pulse dispersion manager is used to manage the dispersion of the aligned frequency doubling pulse to shorten the pulse width of the frequency doubling pulse.
8. The high-energy, few-period blue laser generating device according to claim 6, characterized in that, Along the optical path, the solid sheet is placed in front of the focal point of the focusing lens to further broaden the pulse width of the focused frequency-doubled pulse, thereby avoiding helium ionization at the focal point and preventing energy absorption.
9. The high-energy, few-period blue laser generating device according to claim 6, characterized in that, The nonlinear crystal is a substrate-free nonlinear crystal.
10. The high-energy, few-period blue laser generating device according to any one of claims 6-9, characterized in that, Also includes: A post-compression module placed before the frequency-doubling pulse generator along the optical path direction; The post-compression module includes: A fundamental frequency pulse spectral broadener is used to broaden the spectrum of a fundamental frequency pulse. A fundamental frequency pulse dispersion manager is used to compress fundamental frequency pulses by managing the dispersion of the spectrally broadened fundamental frequency pulses. The portion of the fundamental frequency pulse dispersion manager that introduces negative chirp is placed within the helium-filled cavity.