A method for realizing laser spectrum widening based on stimulated rotational Raman scattering

Abstract

The application relates to a method for realizing laser spectrum widening based on stimulated rotational Raman scattering, belonging to the technical field of laser spectrum widening, which comprises the following steps: providing a monochromatic pumping light and a monochromatic signal light; superimposing the monochromatic pumping light and the monochromatic signal light in space and keeping time synchronization to form complex color light; injecting the complex color light into a transmission path containing a gas medium according to a set distance for transmission, carrying out stimulated rotational Raman scattering, and finally outputting wideband laser after spectrum widening; the wave number difference between the monochromatic pumping light and the monochromatic signal light is matched with the molecular rotational energy level difference of the gas medium; the method is used for solving the problems that the final-stage spectrum bandwidth of an existing high-power laser device is limited, the traditional stimulated rotational Raman scattering excitation threshold is high, the required transmission distance is long, and the widening effect is uncontrollable.

Description

Technical Field

[0001] This invention relates to the field of laser spectral broadening technology, and specifically to a method for achieving laser spectral broadening based on stimulated rotational Raman scattering. Background Technology

[0002] Large-aperture, high-power solid-state lasers can output laser energy on the order of megajoules (MJ), providing an important experimental platform for cutting-edge scientific research such as high-energy-density physics and laser inertial confinement fusion (ICF). In laser-driven inertial confinement fusion experiments, complex laser-plasma physics processes occur during the interaction between the laser and the fuel target or metal black cavity. Laser-plasma instabilities (LPI) instabilities affect the fusion process. For example, inter-beam energy transfer reduces the coupling efficiency between the laser and the target, hyperthermic electron preheating inhibits effective target compression, and stimulated scattering of the laser reduces coupling efficiency, thus disrupting compression symmetry. Therefore, the energy deficit generated during these laser-plasma instabilities limits the design space of target parameters and determines the characteristics of the driving laser.

[0003] Currently, broadband laser ablation can effectively suppress the development of laser plasma instability and significantly increase the instability occurrence threshold. However, existing large-scale high-power laser devices are limited by the gain narrowing effect during laser amplification and the phase matching bandwidth limitation of third-harmonic crystals. Their actual output ultraviolet laser bandwidth is usually less than 0.03% (about 0.1 nm), which is difficult to meet the physical requirements of broadband ablation.

[0004] Stimulated Rotational Raman Scattering (SRRS) in gaseous media is a potential method for achieving spectral broadening at the end of laser transmission. It utilizes the nonlinear coupling between a high-power laser and the rotational energy levels of gas molecules to generate multiple Stokes sidebands, thus forming a broadband spectrum. However, traditional spontaneous stimulated Raman scattering processes have significant limitations: firstly, their excitation threshold is extremely high, typically requiring monochromatic pump light power densities of tens of kilometres per second. On the one hand, achieving substantial spectral broadening often requires transmission distances of tens of meters or even longer. In practical large-aperture, high-power laser devices are limited by the damage threshold of optical components and the spatial layout, making it difficult to simultaneously meet such stringent power density and transmission distance requirements. Furthermore, stimulated scattering processes that rely solely on spontaneous noise have poor controllability in beam quality, broadening efficiency, and spectral morphology.

[0005] Therefore, we propose a method to efficiently and controllably excite stimulated rotational Raman scattering of a gas medium to achieve laser spectral broadening within the actual operating flux limitations and finite transmission distance of a large-aperture, high-power laser device. Summary of the Invention

[0006] The purpose of this invention is to provide a method for laser spectral broadening based on stimulated rotational Raman scattering, which solves the problems of limited final-stage spectral bandwidth in existing high-power laser devices, as well as the high excitation threshold, long required transmission distance, and uncontrollable beam quality and broadening effect of traditional stimulated rotational Raman scattering.

[0007] This invention is achieved through the following technical solution:

[0008] A method for laser spectral broadening based on stimulated rotational Raman scattering, specifically including:

[0009] It provides a monochromatic pump light and a monochromatic signal light;

[0010] Monochromatic pump light and monochromatic signal light are spatially overlapped and time-synchronized to form polychromatic light;

[0011] According to the set distance, polychromatic light is injected into the transmission path containing the gas medium for transmission, and stimulated rotational Raman scattering is performed to finally output a broadband laser with broadened spectrum.

[0012] The wavenumber difference between the monochromatic pump light and the monochromatic signal light matches the molecular rotational energy level difference in the gas medium.

[0013] Furthermore, the wavenumber difference between the monochromatic pump light and the monochromatic signal light matches the molecular rotational energy level difference of the gas medium, specifically including:

[0014] Define the center wavelength of the monochromatic pump light as The center wavelength of the monochromatic signal light is Furthermore, when the monochromatic pump light undergoes stimulated rotational Raman scattering alone, the wavenumber difference corresponding to the molecular rotational energy levels in the gas medium is: ;

[0015] Construct a correlation model between the center wavelength of the monochromatic pump light and the center wavelength of the monochromatic signal light;

[0016] The wavelength difference between the monochromatic pump light and the monochromatic signal light is calculated based on the correlation model;

[0017] If the calculated wavelength difference is within the preset fluctuation range of the actual wavelength difference, then the wavenumber difference value is determined to match the molecular rotational energy level difference of the gas medium.

[0018] If the calculated wavelength difference is not within the preset fluctuation range of the actual wavelength difference, the wavenumber difference value needs to be adjusted.

[0019] Furthermore, the correlation model between the center wavelength of the monochromatic pump light and the center wavelength of the monochromatic signal light is constructed, and the calculation formula is as follows:

[0020] .

[0021] Furthermore, the wavelength difference between the monochromatic pump light and the monochromatic signal light is calculated based on the correlation model. The calculation formula is:

[0022] .

[0023] Furthermore, a spatial beam combiner is used to combine the monochromatic pump light and the monochromatic signal light.

[0024] Furthermore, an optical fiber combiner is used to combine the monochromatic pump light and the monochromatic signal light in the optical fiber seed source stage to form polychromatic light;

[0025] After undergoing laser amplification and frequency conversion, the polychromatic light is injected into the transmission path.

[0026] Furthermore, when the gas medium is atmosphere or pure nitrogen, the wavenumber difference between the monochromatic pump light and the monochromatic signal light is taken as 75. Or 91 .

[0027] Furthermore, the deviation between the wavenumber difference of the monochromatic pump light and the monochromatic signal light and the rotational energy level difference does not exceed the bandwidth of the atomic polarization field excited by the monochromatic pump light and the signal.

[0028] Furthermore, the typical power density range of the monochromatic pump light is 0.5. -5 The power density range of the monochromatic signal light is 1. -100 The length range of the transmission path is 1. -50 .

[0029] Furthermore, the gaseous medium is atmosphere, pure nitrogen, hydrogen, deuterium, methane, or carbon dioxide.

[0030] The technical solution of the present invention has at least the following advantages and beneficial effects:

[0031] This invention discloses a method for laser spectral broadening based on stimulated rotational Raman scattering (SRRS). By introducing a signal light that satisfies a wavenumber matching relationship with the pump light for co-injection, the stimulated rotational Raman scattering effect in the gas medium is excited using a dual-light synergistic approach. This method significantly lowers the pump light power density threshold required for effective SRRS excitation, enabling efficient nonlinear process excitation and spectral broadening within the flux safety range of large-aperture, high-power laser devices. Simultaneously, the dual-light injection drastically shortens the transmission distance required to establish a stable stimulated scattering process, allowing for sufficient spectral broadening within a limited spatial layout of several meters to tens of meters.

[0032] Furthermore, since the stimulated scattering process is dominated by the externally injected signal light rather than relying on the random amplification of spontaneous noise, the beam quality and the bandwidth and morphology of the output spectrum have good controllability and stability, providing a reliable guarantee for the engineering application of broadband lasers.

[0033] In addition, by limiting the wavenumber deviation between the pump light and the signal light to no more than the bandwidth of the atomic polarization field of the gas medium, it is ensured that the light field and the molecular energy level always maintain an effective resonant coupling state during the excitation process, thus avoiding gain attenuation caused by wavelength detuning.

[0034] Furthermore, by limiting the pump light power density, signal light power density, and transmission distance to reasonable numerical ranges, a broad and feasible parameter design window is provided for high-power laser devices of different configurations, taking into account both the safe operating margin of optical components and the physical requirements of spectral broadening. Attached Figure Description

[0035] Figure 1 This is a flowchart of a method for laser spectral broadening based on stimulated rotational Raman scattering according to the present invention;

[0036] Figure 2 This is a schematic diagram of the first method for forming polychromatic light according to the present invention;

[0037] Figure 3 This is a schematic diagram of the second method for forming polychromatic light according to the present invention;

[0038] Figure 4 This is a schematic diagram of the polychromatic light spectral distribution before spectral broadening, as shown in the initial injection of this invention.

[0039] Figure 5 This is a schematic diagram of the spectrum of broadband light after the polychromatic light of the present invention undergoes spectral broadening through stimulated rotational Raman scattering. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0041] Example 1

[0042] like Figure 1 The method shown here for achieving laser spectral broadening based on stimulated rotational Raman scattering specifically includes:

[0043] It provides a monochromatic pump light and a monochromatic signal light;

[0044] When a single pump beam acts alone on a gaseous medium, the initiation of the stimulated rotational Raman scattering process depends on the spontaneous emission of quantum noise. This noise has a random phase and weak intensity, requiring a long transmission distance to excite and amplify the frequency components that resonate with the molecular rotational energy levels. Therefore, by adding a signal beam to form a dual-field injection architecture with the pump beam, the signal beam acts as a seed beam, carrying frequency information that precisely matches the target rotational energy level difference. This provides a definite frequency reference and an initial coherent atomic polarization field for the stimulated scattering process from the source.

[0045] Furthermore, the pump light plays the role of providing energy, acting as the "donor" of energy transfer; the signal light plays the role of providing the frequency reference and broadening seed, acting as the "receiver" and "guide" of energy transfer. The separation of their functions allows the two engineering parameters of energy supply and frequency broadening to be optimized independently: the pump light can adjust its energy according to the device's load capacity, and the signal light can adjust its wavelength and intensity according to the required broadening characteristics, without any mutual constraints.

[0046] Furthermore, the wavenumber difference between the pump light and the signal light was subsequently set to match the rotational energy level difference of the gas molecules. This means that when the two beams of light propagate in the medium, they resonate with the intrinsic rotational frequency of the gas molecules to excite the atomic polarization field. As a periodic driving force, the excitation of the molecular rotational energy level is in a state of resonant enhancement from the beginning, rather than relying on random matching of noise. Thus, the energy transfer channel from the pump light to the signal light and the higher-order Stokes light is pre-established at the beginning of light injection, which greatly improves the directivity and efficiency of energy conversion.

[0047] Furthermore, the power density range of the monochromatic pump light is 0.5. -5 The power density range of the monochromatic signal light is 1. -100 The length range of the transmission path is 1. -50 Specifically, the power density of the monochromatic pump light is 2.2. The power density of monochromatic signal light only needs to be 10 This monochromatic pump light power threshold is far lower than the tens of thousands required by traditional single-light injection schemes. This approach achieves a level of efficiency that allows for effective excitation within the safe flux range of large-aperture laser devices. Furthermore, traditional methods require a long exponential gain region from noise buildup to stable stimulated scattering, typically necessitating transmission distances of tens of meters. With this approach, a spectral bandwidth of 2% lower bound is achieved in a transmission distance of only 10 meters. This effect allows the spectral broadening stage to be flexibly embedded within the limited space of the final stage optical path of existing laser devices, without requiring large-scale modifications to the device.

[0048] Furthermore, since the stimulated process is dominated by the externally injected monochromatic signal light, rather than relying on random noise, a direct controllable relationship is established between the characteristics of the output spectrum and the parameters of the monochromatic signal light. Specifically, by adjusting the injection intensity of the monochromatic signal light, the intensity of energy transfer from the monochromatic pump light to higher-order Stokes sidebands can be precisely controlled, thereby adjusting the bandwidth of the final output spectrum. Additionally, by changing the wavelength of the monochromatic signal light to match different rotational energy level differences (such as 75° corresponding to S8), the output spectrum can be optimized. Or S10 corresponding to 91 The frequency comb spacing of the broadened spectrum can be flexibly selected.

[0049] It should be noted that the wavelengths of the monochromatic pump light and monochromatic signal light can be selected from the infrared band (such as 1053nm fundamental frequency light) or the ultraviolet band (such as 351nm third harmonic light) according to actual needs, as long as the matching relationship between the wavenumber difference and the rotational energy level difference is met; when the gas medium is changed from nitrogen to hydrogen or deuterium, only the wavelength of the monochromatic signal light needs to be adjusted accordingly to match the rotational energy level difference of the different media, and the method framework itself does not need to be changed.

[0050] Monochromatic pump light and monochromatic signal light are spatially overlapped and time-synchronized to form polychromatic light;

[0051] Since stimulated rotational Raman scattering is essentially the result of stimulated rotational Raman scattering of the pump light and the signal light in a medium, this nonlinear effect can only occur when the two beams overlap in space and time. If the pump light and the signal light are spatially separated and the two beams propagate independently, stimulated scattering cannot be formed, and the gas molecules will still experience two independent light fields. Therefore, spatial overlap ensures that the photons of the pump light and the signal light interact with the same group of molecules at the same spatial position, so that the electric field vectors of the two beams can nonlinearly excite stimulated scattering, thereby establishing an electromagnetic driving force with a definite difference frequency in the medium.

[0052] Since both the pump light and the signal light are pulsed lasers with pulse widths on the order of nanoseconds, if the two pulses are out of sync, the signal light will not arrive when the pump light arrives (or vice versa). The molecules in the medium will still only experience the effect of a single beam of light and will not be able to form a dual-light synergistic excitation. Only when the two pulses are strictly synchronized in time can stable stimulated scattering be formed during the pulse duration, so that the molecules are continuously driven by the laser throughout the entire pulse width, and the atomic polarization field can be effectively established and maintained. Therefore, time synchronization also ensures the timing matching of the energy transfer process: the energy of the pump light is continuously transferred to the signal light and higher-order Stokes light during the pulse duration. If the timing is out of sync, the energy transfer window will be compressed or even completely lost.

[0053] Furthermore, the combined effect of spatial coincidence and temporal synchronization enables the pump light and signal light to form stable stimulated scattering in the medium. This ensures the effectiveness of dual-light synergistic excitation of stimulated rotational Raman scattering from the source. Spatial coincidence ensures that the two beams interact throughout the entire beam cross-section, while temporal synchronization ensures that they continue to interact throughout the entire pulse width. The product of these two factors, i.e., the spatiotemporal overlap integral, directly determines the total amount of energy transferred. By ensuring spatial coincidence and temporal synchronization, the spatiotemporal window for energy transfer is maximized, thereby achieving the maximum spectral broadening effect for a given pump power and transmission distance.

[0054] According to the set distance, polychromatic light is injected into the transmission path containing the gas medium for transmission, and stimulated rotational Raman scattering is performed to finally output broadband laser.

[0055] Since stimulated rotational Raman scattering is an exponential gain process, polychromatic light travels a certain distance in a gaseous medium, and through the excitation of atomic polarization fields, energy transfer from pump light to signal light and generation of higher-order Stokes light are achieved. The transfer efficiency increases exponentially with the transmission distance. The set distance is the gain length, meaning that the longer the transmission distance, the more complete the energy transfer, the higher the order of the generated Stokes sidebands, and the greater the broadening of the final output spectrum. However, a longer transmission distance is not always better. An excessively long transmission distance can introduce competing nonlinear effects such as atmospheric turbulence, beam self-focusing, and stimulated Brillouin scattering, leading to beam quality degradation. Therefore, the selection of the set distance in this application needs to strike a balance between the broadening effect and the beam quality.

[0056] The use of gaseous media utilizes the discrete rotational energy levels of gas molecules as a medium for nonlinear spectral broadening. Compared with solid or liquid media, gaseous media have unique advantages such as extremely high damage threshold, moderate nonlinear coefficient, and good optical homogeneity, making them particularly suitable for transmission scenarios of large-aperture, high-power lasers. If atmospheric air at normal temperature and pressure is used as the transmission medium, there is no need for a vacuum system or sealed gas pool. The air path of the final stage of the laser device can be used directly to achieve "on-site" spectral broadening. Furthermore, since nitrogen molecules are the main component of the atmosphere (approximately 78%), their S8 and S10 rotational transitions have high gain and are matched with commonly used laser wavelengths, making them an ideal medium for exciting stimulated rotational Raman scattering.

[0057] When the gas medium is atmospheric, the final stage transmission space can be directly utilized. When the gas medium is pure nitrogen, a purer gain environment can be achieved in a sealed pipe. When the gas medium is hydrogen or deuterium, different rotational energy level differences can be matched to meet the monochromatic pump light requirements of different bands.

[0058] The wavenumber difference between the monochromatic pump light and the monochromatic signal light matches the molecular rotational energy level difference of the gas medium.

[0059] The calculation of wavenumber difference specifically includes:

[0060] Define the rotational energy level difference of molecules in a gaseous medium as The unit is The center wavelength of the monochromatic pump light is The unit is The center wavelength of the monochromatic signal light is The unit is ;

[0061] A correlation model is constructed between the center wavelength of the monochromatic pump light and the center wavelength of the monochromatic signal light, and the calculation formula is as follows:

[0062]

[0063] The wavelength difference between the monochromatic pump light and the monochromatic signal light is calculated using the correlation model. The formula is as follows:

[0064] .

[0065] Furthermore, when the gas medium is atmosphere or pure nitrogen, the wavenumber difference between the monochromatic pump light and the monochromatic signal light is taken as 75. Or 91 Because gas molecules have discrete rotational energy level structures, the rotational population of nitrogen molecules at room temperature is highest for the energy level pairs corresponding to the S8 and S10 transitions, forming a thermal population advantage. When monochromatic pump light and monochromatic signal light act on the gas medium simultaneously, the two beams undergo stimulated Raman scattering, generating a periodic electromagnetic driving force with a frequency equal to the difference in their wavenumbers.

[0066] If the wavenumber difference is equal to the molecular rotational energy level difference (within the bandwidth of the atomic polarization field), then the driving force resonates with the molecular rotational energy level transition, and the energy transfer efficiency from the light field to the molecular rotational degree of freedom reaches its maximum. After the molecule is coherently driven, it forms an atomic polarization field, which acts as an "intermediary" to directionally transfer the energy of the monochromatic pump light to the monochromatic signal light and the higher-order Stokes sideband, thereby achieving spectral broadening.

[0067] In addition, the deviation between the wavenumber difference of the monochromatic pump light and the monochromatic signal light and the rotational energy level difference does not exceed the bandwidth of the atomic polarization field of the gas excited by the monochromatic pump light and the signal.

[0068] Example 2

[0069] As one embodiment, this embodiment discloses two methods for forming polychromatic light, one of which is:

[0070] The monochromatic pump light and the monochromatic signal light are combined using a spatial beam combiner. This method achieves spatial overlap of the two beams in free space by utilizing the beam combiner, and is suitable for direct injection into the final stage optical path.

[0071] Spatial beam combining elements, such as polarizing beam combining prisms and dichroic mirrors, utilize the polarization characteristics or wavelength differences of light to combine two beams onto the same optical axis. Furthermore, spatial beam combining elements are typically small in size and can be flexibly installed at appropriate locations in the final stage optical path of a laser device. For existing large-scale high-power laser devices, using spatial beam combining requires no modification to the pre-stage seed source, amplification link, or frequency conversion system; functional upgrades can be achieved simply by adding beam combining elements and introducing a monochromatic signal optical path in the final stage transmission optical path. This characteristic makes it significantly convenient and economical for engineering retrofitting scenarios.

[0072] Specifically, select As a monochromatic pump light, the gas medium is chosen to be atmospheric gas at normal temperature and pressure, mainly utilizing the S8 rotational energy level of nitrogen molecules (wavenumber difference of 75). Then, the monochromatic signal light selection is:

[0073]

[0074] like Figure 2As shown, the monochromatic pump light and monochromatic signal light are spatially combined by a beam combiner. The two laser beams pass through the atmospheric region synchronously, exciting stimulated rotational Raman scattering, generating multi-order Stokes lasers, forming quasi-continuous broadband light, such as... Figure 4 As shown.

[0075] Another way to form polychromatic light is to use an optical fiber combiner to combine the monochromatic pump light and the monochromatic signal light in the initial stage of pulse generation, i.e., the seed source stage of the optical fiber, to form polychromatic light.

[0076] After the polychromatic light undergoes laser amplification and frequency conversion, it is injected into the transmission path. This method achieves dual-beam beam combining at the fiber seed source stage, ensuring spatial overlap and time synchronization from the source. After amplification and frequency doubling, the beam naturally remains in an overlapping state.

[0077] Fiber optic combining devices can be such as fused biconical taper fiber combiners; and fiber optic combiners combine monochromatic pump light and monochromatic signal light into the same fiber core for transmission. Under the strong constraint of the fiber waveguide, the spatial modes of the two beams completely overlap, eliminating the pointing deviation problems caused by mechanical vibration, thermal drift, etc., that occur in free-space combining. At the same time, the two seed beams undergo exactly the same transmission optical path in the fiber, naturally ensuring time synchronization. This "hardware-level" synchronization and overlap is far superior to the accuracy and stability achievable by free-space adjustment.

[0078] Specifically, select As a monochromatic pump light, the gas medium is chosen to be atmospheric gas at normal temperature and pressure, mainly utilizing the S8 rotational energy level of nitrogen molecules (wavenumber difference of 75). Since the stimulated rotation Raman scattering process occurs in the third harmonic band, the monochromatic signal light is selected as follows:

[0079]

[0080] like Figure 3 As shown, two fiber seed sources, a monochromatic pump light and a monochromatic signal light, are combined using a fiber combiner. The two seed beams undergo time-synchronized laser amplification and frequency conversion, ultimately forming two ultraviolet laser beams with center wavelengths of 351 nm and 351.926 nm. These beams jointly excite stimulated rotational Raman scattering, generating a multi-order Stokes laser and forming a quasi-continuous broadband light, as shown in the diagram. Figure 5 As shown.

[0081] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., 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 laser spectrum broadening based on stimulated rotational Raman scattering, characterized in that, Specifically, it includes: It provides a monochromatic pump light and a monochromatic signal light; Monochromatic pump light and monochromatic signal light are spatially overlapped and time-synchronized to form polychromatic light; According to the set distance, polychromatic light is injected into the transmission path containing the gas medium for transmission, and stimulated rotational Raman scattering is performed to finally output a broadband laser with broadened spectrum. The wavenumber difference between the monochromatic pump light and the monochromatic signal light matches the molecular rotational energy level difference in the gas medium.

2. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 1, characterized in that: The wavenumber difference between the monochromatic pump light and the monochromatic signal light matches the molecular rotational energy level difference in the gas medium, specifically including: Define the center wavelength of the monochromatic pump light as The center wavelength of the monochromatic signal light is Furthermore, when the monochromatic pump light undergoes stimulated rotational Raman scattering alone, the wavenumber difference corresponding to the molecular rotational energy levels in the gas medium is: ; Construct a correlation model between the center wavelength of the monochromatic pump light and the center wavelength of the monochromatic signal light; The wavelength difference between the monochromatic pump light and the monochromatic signal light is calculated based on the correlation model; If the calculated wavelength difference is within the preset fluctuation range of the actual wavelength difference, then the wavenumber difference value is determined to match the molecular rotational energy level difference of the gas medium. If the calculated wavelength difference is not within the preset fluctuation range of the actual wavelength difference, the wavenumber difference value needs to be adjusted.

3. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 2, characterized in that: The correlation model between the center wavelength of the monochromatic pump light and the center wavelength of the monochromatic signal light is constructed, and the calculation formula is as follows: 。 4. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 3, characterized in that: The wavelength difference between the single-color pump light and the single-color signal light is calculated according to the correlation model The calculation formula is: 。 5. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 1, characterized in that: The deviation between the wavenumber difference of the monochromatic pump light and the monochromatic signal light and the rotational energy level difference does not exceed the bandwidth of the atomic polarization field of the gas medium excited by the monochromatic pump light and the signal.

6. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 1, characterized in that: The monochromatic pump light and the monochromatic signal light are combined using a spatial beam combiner.

7. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 1, characterized in that: A fiber optic combiner is used to combine the monochromatic pump light and the monochromatic signal light, which are in the fiber seed source stage, to form polychromatic light. After undergoing laser amplification and frequency conversion, the polychromatic light is injected into the transmission path.

8. The method for laser spectral broadening based on stimulated rotational Raman scattering according to claim 1, characterized in that: The gaseous medium is atmosphere, pure nitrogen, hydrogen, deuterium, methane, or carbon dioxide.

9. The method of laser spectrum broadening based on stimulated rotational Raman scattering according to claim 8, characterized in that: When the gas medium is atmosphere or pure nitrogen, the wavenumber difference between the monochromatic pump light and the monochromatic signal light is taken as 75. Or 91 .

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