Flight focus-based backward air laser enhancement method

Abstract

The invention discloses a backward air laser enhancement method based on a flight focus. Firstly, the pulse width of a flight focus is determined according to the requirement for synchronous transmission of back air laser, and the speed of the flight focus is made to be the back light speed. Then, the required laser pulse chirp amount is calculated according to the target speed, and parameters of a dispersion regulation and control module are set; in an experiment, a laser pulse is modulated into a negative chirp by using a grating pair, and then a flight focus is formed through a dispersion focusing system. Enhanced backward air laser output is achieved by optimizing motion parameters of a flight focus. Dynamic movement of the focus is achieved through chirp regulation and dispersion focusing, the focus of the pump laser moves back to back and is consistent with the transmission direction of the back air laser, and therefore the gain length is remarkably prolonged, output energy is improved, limitation of the traveling wave excitation effect is effectively overcome, and the service life of the laser is prolonged. And a new technical approach is provided for realizing long-distance backward air laser output.

Description

Technical Field

[0001] This invention belongs to the field of laser technology and plasma physics, and relates to a method for enhancing back-to-air lasers based on flying focus technology. Specifically, it relates to a method that uses a combination of chirped pulses and a dispersive focusing system to achieve dynamic focus movement, so that it is transmitted synchronously with the back-to-air laser. This method can significantly extend the gain length of the back-to-air laser and thus significantly enhance the intensity of the air laser. It is particularly suitable for applications such as long-range atmospheric detection and high-gain laser amplification. Background Technology

[0002] In the fields of laser technology and plasma physics, air laser technology has attracted widespread attention due to its enormous application potential in areas such as long-range atmospheric environment detection, high-energy laser amplification, and high-precision laser processing. Back-air laser technology, in particular, utilizes the population inversion of excited-state molecules and atoms in plasma to generate laser light, theoretically achieving extremely high gain and energy output. However, in practical applications, traditional back-air laser technology faces numerous challenges, the most prominent being the short gain length and low output energy caused by the traveling-wave excitation effect. In traditional back-air laser systems, the pump light and laser propagate in opposite directions, resulting in a short laser gain length, low energy transfer efficiency in the plasma, and low output energy. These problems severely restrict the application of back-air laser technology in long-range atmospheric environment detection.

[0003] The emergence of flying-focusing technology offers a new approach to solving this problem. Flying-focusing technology combines chirped laser pulses with a dispersive focusing system to achieve dynamic movement of the laser focus. For example... Figure 1 As shown, an ultrashort laser pulse is passed through a diffraction grating pair and a variable spacing control system. Due to dispersion, the components of different frequencies in the pulse propagate at different speeds, resulting in temporal stretching and forming a long pulse with a continuously varying frequency over time. If the long-pulse broadband laser... ω 1> ω 2. This pulse is a negative chirped pulse. When this negative chirped pulse is focused by a chromatic aberration lens, due to the chromatic aberration effect, the focal point will become a focal line, with a length... L ≈ f 0Δλ / λ0. Wavefront of a pulsed laser. ω 1. Focus on the right end of the focal line, after wave ω 2. Focusing is done at the left end of the focal line, i.e., the focal region of the lens in the figure. Since the laser has a certain pulse width T, lasers of different frequencies arrive at the lens at different times. Therefore, lasers of different frequencies will take different time intervals to focus at different positions, making the focal point appear to have a certain "flight speed": v / c = (1 ± cT / L). -1When the initial laser chirp is negative, the flight velocity of the focal point is (-∞,0)∪(c,+∞). Although the flying focal point technology has been applied in other fields such as laser wake field acceleration and terahertz wave generation, its application in back-to-air laser enhancement remains a blank. Summary of the Invention

[0004] To address the technical problems in the background art, this invention proposes a back-to-air laser enhancement method based on flying-focus technology. Specifically, dynamic focus movement is achieved through chirp modulation and dispersive focusing, causing the pump laser focus to shift backward and align with the transmission direction and velocity of the back-to-air laser, thereby significantly extending the gain length, increasing output energy, and overcoming the traveling wave excitation effect. This method improves laser amplification efficiency and provides a new technical approach for realizing long-distance back-to-air lasers.

[0005] Based on the above technical concept, the purpose of this invention is to provide a back-air laser enhancement method based on flight focus. This method achieves dynamic movement of the laser focus by precisely controlling the spatiotemporal characteristics of the pulsed laser, thereby realizing precise spatiotemporal control of the excitation process. It solves the problems of short working distance, low excitation efficiency, unstable output and lack of dynamic control capability in existing back-air laser technology, and achieves a significant improvement in back-air laser intensity and effective gain length.

[0006] The technical solution of the present invention is as follows: A back-to-air laser enhancement method based on a flying focus is disclosed. The method utilizes a device comprising a pulsed laser source, a dispersion modulation module, and a dispersion focusing system. First, a laser pulse with a specific spectral distribution is generated by the pulsed laser source, and the laser pulse is temporally modulated by the dispersion modulation module to introduce controllable chirp characteristics. Then, the modulated laser pulse is introduced into the dispersion focusing system, utilizing its wavelength-space mapping characteristics to form a flying focus moving along the optical axis in the gas medium. This focus propagates back-to-air at a preset speed over an extended working distance, with its peak intensity consistently exceeding the gas excitation threshold, thereby generating a continuous and efficient air laser gain effect along the focus trajectory. Finally, by optimizing the motion parameters of the flying focus, significant enhancement and effective collection of the back-to-air laser signal are achieved.

[0007] In the above method, the pulsed laser source can output laser pulses with a certain bandwidth and a single pulse energy of not less than 0.1 mJ.

[0008] In the above method, the dispersion control module includes at least one optical element with strong dispersion capability for continuous adjustment of group velocity dispersion. Furthermore, the dispersion control module is a diffraction grating pair, the distance between which is designed based on the required chirp amount to ensure that the laser pulse obtains the required negative chirp after passing through the grating pair.

[0009] In the above method, the dispersive focusing optical system uses diffractive optical elements or metasurfaces, and based on the wavelength-space mapping principle, focuses light components of different wavelengths at different positions on the optical axis to form an extended focusing region.

[0010] In the above method, the motion characteristics of the flying focus are achieved by adjusting the chirp parameters of the laser pulse, including chirp amount, chirp direction, and chirp linearity, thereby controlling the propagation speed, direction, and acceleration of the focus. The propagation speed of the flying focus can be continuously adjusted within a range from rest to several times the speed of light, including motion modes that are the same as or opposite to the propagation direction of the laser pulse.

[0011] In the above method, the flight focus forms a dynamically moving high-intensity region in the medium, which can continuously maintain a power density higher than the excitation threshold of the medium, thereby achieving efficient population inversion.

[0012] In the above method, the output intensity of the back-to-air laser can be increased by more than an order of magnitude compared with the traditional fixed-focus method, and the effective working distance is expected to reach 50-100 times the conventional Rayleigh length.

[0013] In the above method, the back-to-air laser has good beam quality and stable spatial mode.

[0014] Furthermore, the present invention also includes a signal detection system, which sets up a signal collection optical path in the opposite direction of laser incident to comprehensively characterize the characteristics of lasers facing away from the air.

[0015] The advantages and beneficial effects of this invention are: The back-air laser enhancement method based on flight focus described in this invention overcomes the physical limitations of traditional optical focusing through an innovative temporal and terrestrial control mechanism. This method not only significantly improves the conversion efficiency and output intensity of back-air lasers but also greatly extends their effective working distance while maintaining excellent beam quality. Compared with traditional solutions such as external DC electric fields and multi-wavelength confocal lasers, this invention eliminates the need for external control fields or multiple laser paths, achieving performance enhancement solely through controlling the characteristics of the laser pulse itself. It boasts significant advantages such as a simple system structure, abundant control parameters, and wide applicability. This technology provides a novel solution for atmospheric remote sensing, environmental monitoring, and spectral analysis, possessing significant application value and broad development prospects. Attached Figure Description

[0016] Figure 1 A schematic diagram of an experimental setup for back-to-air laser enhancement based on flight focus is shown, including a laser source system, a dispersion control module, a dispersion focusing system, and a signal detection system.

[0017] Figure 2 The diagram shows a comparison between ordinary focusing and flight focusing. The left diagram illustrates the propagation of the laser beam against the air and the pump laser in opposite directions under ordinary focusing. The right diagram illustrates the shift of the focal position of a negatively chirped beam backward over time after being focused by a dispersive focusing system.

[0018] Figure 3 The relationship between the calculated flight focus movement speed and the pulse width under a negative chirped pulse is shown.

[0019] Figure 4 The figure shows the simulation results of light intensity versus time when the focal point of the flight moves backward at one time the speed of light. A line is marked in the figure with a - c The dashed trajectory of speed movement.

[0020] Figure 5 The effect of air laser enhancement caused by increased gain length is shown.

[0021] In the diagram: 1. Laser; 2. Grating pair; 3. Chromatic lens; 4. Mirror; 5. Filament; 6. Focusing lens; 7. Filter; 8. Fiber optic connector; 9. Detector; 10. Chirped pulse. Detailed Implementation

[0022] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can more clearly understand the present invention.

[0023] This invention proposes a back-to-air laser enhancement method based on a flight focus. The core of this method lies in forming a flight focus with an adjustable trajectory in the air by precisely controlling the spatiotemporal characteristics of the laser pulse, thereby significantly enhancing the intensity and effective range of the back-to-air laser. Figure 1 The diagram shows a device according to a specific embodiment of the present invention, including a laser light source system, a dispersion control module, a dispersion focusing system, and a signal detection system.

[0024] The specific implementation steps of the back-air laser enhancement method based on flight focus are given below: Step 1: Prepare the laser source system. Select a femtosecond laser 1 with sufficient bandwidth as the excitation source. This laser 1 should be able to output laser pulses with pulse widths on the order of femtoseconds and a spectral width of not less than 5 nm. The center wavelength of laser 1 can be selected in the visible to near-infrared range according to actual needs, with a typical value of 800 nm. The single-pulse energy of the laser should be not less than 0.1 mJ to ensure sufficient nonlinear effects are generated in air.

[0025] Step 2: Introduce the dispersion control module, such as... Figure 1As shown, the dispersion modulation module consists of a pair of diffraction gratings and a variable spacing control system. The variable spacing control system includes a high-precision electric displacement stage and a computer control unit connected to it. The system is implemented by mounting the diffraction gratings on independent displacement stages, and controlling the relative movement of the stages via computer to achieve micrometer-level precise control and continuous adjustment of the distance between the gratings. Continuous adjustment of group velocity dispersion is achieved by precisely controlling the grating spacing, introducing the desired linear or nonlinear chirp characteristics. During modulation, precise control of the chirp amount and direction is required to ensure the formation of a flight focus with a specific motion trajectory.

[0026] Step 3: Configure the dispersive focusing system, and refer to... Figure 2 As shown in the right figure, utilizing its wavelength-space mapping characteristics, a flying focal point moving along the optical axis is formed in the gas medium. This focal point propagates backward at a preset speed over an extended working distance, and its peak intensity is always higher than the gas excitation threshold, thus generating a continuous and efficient air laser gain effect along the focal trajectory. The dispersive focusing system of this invention uses a chromatic aberration lens 3 or a metasurface, a diffractive optical element with strong dispersive characteristics, as the focusing device. The aperture of the chromatic aberration lens 3 should match the diameter of the laser beam, typically 25 mm, to ensure sufficient light throughput and energy utilization. A reflector 4 is added after the chromatic aberration lens 3 to facilitate the acquisition of the backward laser signal.

[0027] Step 4: Optimize the flight focus parameters. Characterize the motion velocity of the flight focus by adjusting the parameters of the dispersion control module to control its motion characteristics. Achieve significant enhancement and effective collection of the laser signal facing away from the air. Specifically, when the initial laser chirp is negative, the flight velocity of the focus is (-∞,0)∪(c,+∞) (e.g., ...). Figure 3 (As shown). The dashed line represents the singularity position generated by a negative chirped pulse with a pulse width of T = 14.87 ps, at which point all colors focus simultaneously, creating a line focus. The special point of the triangle in the figure is located at the position of a negative chirped pulse with a pulse width of T = 31 ps, at which point the flight focus velocity is - c It can transmit synchronously with the back-air laser, which can significantly enhance the gain length of the back-air laser and amplify the gain of the back-air laser radiation.

[0028] Step 5: Configure the signal detection system. A signal collection optical path is set up in the opposite direction of laser incidence, including a focusing lens 6, a filter 7, an optical fiber connector 8, and a detector 9. The focusing lens 6 is used to collect back-radiated signals at large angles. The filter 7 is used to filter out stray light and background noise. The detector 9 includes a spectrometer, energy meter, CCD camera, high-speed oscilloscope, and photodetector, used to comprehensively characterize the properties of the back-to-air laser.

[0029] The characterization method of the back-air laser enhancement method based on flight focus is given below: The beam distribution of the back-to-air laser was measured using a CCD camera to analyze its beam quality and divergence angle. The spectral characteristics of the back radiation, including the center wavelength, bandwidth, and spectral line shape, were recorded using a high-resolution spectrometer. The pulse energy and average power of the back laser were measured using an energy meter to calculate the energy conversion efficiency. The temporal characteristics of the back signal were recorded using a high-speed oscilloscope and a photodetector to analyze its temporal relationship with the motion of the flight focus.

[0030] The spatiotemporal evolution characteristics of the flight focus were measured using a pump-probe plasma diffraction method. The specific configuration was as follows: a frequency-doubled femtosecond laser beam with a center wavelength of 400 nm was used as the probe beam, illuminating the interaction region perpendicular to the flight focus propagation direction. The probe beam pulse width was 50 fs, the repetition frequency was the same as the pump laser, and the single-pulse energy was 100 μJ. A precision optical delay line was incorporated into the probe beam path, with a delay adjustment range of 0-500 ps and a resolution of 10 fs, to precisely control the time delay between the pump and probe beams.

[0031] When the probe light passes through the plasma channel generated by the focal point of flight, the refractive index change caused by the plasma modulates the wavefront phase of the probe light, producing a diffraction effect. The diffraction pattern carrying information about the spatial distribution of plasma density is collected by an imaging lens and recorded by a scientific-grade CCD camera.

[0032] During measurement, pump light parameters are fixed, and the pump-probe time delay is continuously varied using a delay line. Multiple diffraction images are acquired at each delay position and averaged. By analyzing the spatial evolution characteristics of the diffraction pattern, the spatiotemporal dynamics of the flight focus propagating along the optical axis are reconstructed. In particular, the propagation velocity of the flight focus can be calculated by extracting the relationship between the spatial position of the diffraction fringe initiation point and the time delay. Example

[0033] The implementation of the present invention will be further described below through specific embodiments. A method for back-to-air laser enhancement based on flight focus includes the following steps: First, the laser source system was configured, using a Ti:sapphire femtosecond laser amplifier as the excitation source. This Ti:sapphire femtosecond laser amplifier has an output center wavelength of 800 nm, a spectral width of 9.2 nm, a pulse duration of 50 fs, a single pulse energy of 3 mJ, and a repetition rate of 500 Hz. The laser beam diameter is 70 mm, and the divergence angle is less than 0.5 mrad.

[0034] Subsequently, a dispersion modulation module was constructed. This module employs a structure combining a pair of diffraction gratings and a variable spacing control system. The variable spacing control system includes a high-precision electric displacement stage and a computer control unit. In practice, the two diffraction gratings are mounted on independent displacement stages. The relative movement between the stages is controlled by the computer to achieve micron-level precise control and continuous adjustment of the grating spacing, thereby chirping the laser pulse and introducing linear negative chirp. The pulse waveform and spectral phase are monitored in real time during modulation to ensure modulation accuracy.

[0035] Next, a dispersive focusing system was configured. A chromatic aberration lens 3, based on diffractive optics principles, was selected, with a focal length of 800 nm and a wavelength of 511 mm. The dispersive lens has an aperture of 80 mm and a radial groove density of [missing information]. G = r / λ 0 f The lens has an anti-reflective coating and a transmittance greater than 99% within its operating wavelength range. It can form a focusing area approximately 4.5 mm in length.

[0036] Optimize the flight focus parameters. By adjusting the spacing of the diffraction grating pairs 2, a negative chirped pulse with a specific pulse width is generated, controlling the flight focus to move in the opposite direction of laser propagation at 1 times the speed of light.

[0037] Configure a signal detection system. A signal collection device is positioned 180 degrees from the laser incident direction. The collection optical path includes a focusing lens 6, a set of bandpass filters 7, an optical fiber connector 8, and multiple detectors 9. Detectors 9 include a spectrometer, an energy meter, a CCD camera, a high-speed oscilloscope, and a photodetector. The spectrometer has a resolution of 0.1 nm and a detection range of 200-1100 nm; the energy measurement range is 1 nJ-10 mJ; and the high-speed photodetector has a response time of less than 100 ps.

[0038] The system was started for the experiment. First, the entire optical path was calibrated to ensure precise alignment of all components. Then, the laser system was activated. The backlight signal was observed using a high-speed photodetector, while the flight focus parameters were fine-tuned to optimize the output.

[0039] When the pulse width of the flight focus is 31 ps, the flight focus will move backward at twice the speed of light, such as Figure 4 The figure shows the simulation results of light intensity versus time at the focal point. A line is marked with − c The dashed trajectory of the velocity movement. The expression for the peak light intensity on the focal axis of flight is: (1) in I For light intensity, ε 0 is the vacuum permittivity. c The speed of light in a vacuum. Ea The amplitude is a constant. ω 0 represents the initial spot size at the diffraction lens. ω The pulse position in the axial direction z The size of the light spot at the point of origin. Simulation results show that if the light intensity reaches the ionization threshold of air, the laser electric field directly ionizes gas atoms or molecules, generating free electrons, thus fulfilling the prerequisite for generating optical filaments and air lasers. The plasma will move with the focal point at a - c It spreads at a speed.

[0040] Based on the ideal small-signal gain model of laser intensity, we have: (2) in I seed For seed light intensity, g This is the gain coefficient. L For a back-to-air laser under traveling-wave excitation, which propagates in the opposite direction to the pump laser, its gain length is determined by the product of the speed of light and the gain lifetime, and is only on the order of millimeters. The velocity is... c The gain length of the laser beam against the air at the flight focus is determined by the length of the flight focus. For example... Figure 5 As shown, if the gain length increases by a factor of 10, I 1 represents the normalized laser intensity without a flight focus. I 2 represents the normalized laser intensity at the flight focus. Simulation results show that its intensity is increased by approximately 2 × 10⁻⁶ compared to the case without a flight focus. 4 times.

[0041] Finally, it should be noted that the embodiments and specific examples of this invention are merely for the purpose of helping to further understand this invention and are not intended to limit the scope of protection of this invention. Those skilled in the art should understand that various substitutions and modifications are possible without departing from the spirit and scope of this invention and the appended claims. Therefore, this invention should not be limited to the content disclosed in the preliminary embodiments, and the scope of protection claimed by this invention shall be determined by the scope defined in the claims.

Claims

1. A back-to-air laser enhancement method based on flight focus, characterized in that, include: Step 1: Generate laser pulses with a specific spectral distribution using a pulsed laser source; Step 2: Modulate the laser pulse into negative chirp using the dispersion control module; Step 3: Focus using a dispersive focusing system and measure the speed of the flight focus movement using a signal detection system; Step 4: Adjust the parameters of the laser system and dispersive element to make the flight focus move synchronously with the air laser. Step 5: Achieve enhanced back-to-air laser output.

2. The method according to claim 1, characterized in that, The pulsed laser source is capable of outputting laser pulses with a certain bandwidth, and the energy of a single pulse is not less than 0.1 mJ.

3. The method according to claim 1, characterized in that, The dispersion control module includes at least one optical element with strong dispersion capability, used to achieve continuous adjustment of group velocity dispersion.

4. The method according to claim 3, characterized in that, The dispersion control module is a diffraction grating pair. The distance between the grating pairs is designed based on the required chirp to ensure that the laser pulse obtains the required negative chirp after passing through the grating pairs.

5. The method according to claim 1, characterized in that, The dispersive focusing system is a diffractive optical element or a metasurface. Its design is based on the wavelength-space mapping principle, which sequentially focuses the components of different wavelengths in the laser pulse at different positions on the optical axis to form an extended focal line.

6. The method according to claim 5, characterized in that, The diffractive optical element is a chromatic aberration lens, and the aperture of the chromatic aberration lens should match the diameter of the laser beam to ensure sufficient light flux and energy utilization.

7. The method according to claim 1, characterized in that, The signal detection system uses a frequency-doubled probe beam, and controls its time delay with the pump beam through a precision delay line. It illuminates the beam in a vertical direction, analyzes the spatial evolution characteristics of the diffraction pattern, and reconstructs the spatiotemporal dynamics of the flight focus propagating along the optical axis.

8. The method according to claim 7, characterized in that, The signal detection system includes a signal collection optical path consisting of a focusing lens, a filter, an optical fiber head, and a detector, which is set in the opposite direction of laser incident to comprehensively characterize the characteristics of lasers facing away from the air.

9. The method according to claim 1, characterized in that, The flying focus achieves precise control of the focus position and velocity through chirp modulation and dispersive focusing, so that the focus of the pump laser moves backward and is consistent with the transmission direction and velocity of the laser in the air, thereby significantly extending the gain length, effectively overcoming the limitation of traveling wave excitation effect, and thus significantly improving laser amplification efficiency and output energy.

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