Device and method for measuring intensity of focused femtosecond laser under low pressure condition

Abstract

The application discloses a device and method for measuring femtosecond laser focusing light intensity under low air pressure conditions, comprising a near-infrared femtosecond laser (1), a first mirror (2), a second mirror (3), a first beam splitter (4), a first pinhole (5), a focusing lens (6) and a gas cavity (7) arranged in sequence along the transmission direction of the light path; further comprising a lateral fluorescence collection module and a forward transmission laser energy detection module connected with the gas cavity (7). The application simultaneously acquires lateral fluorescence spectrum and forward transmission laser energy, can distinguish the contribution of 'local tunneling ionization probability drop' and'macroscopic plasma defocusing' to the fluorescence signal, and solves the defect that the traditional method cannot identify the defocusing effect when only measuring fluorescence.

Description

Technical Field

[0001] This invention relates to the field of laser signal technology, and in particular to a device and method for measuring the intensity of a femtosecond laser focusing under low air pressure conditions. Background Technology

[0002] Femtosecond laser (pulse width 10⁻¹) 5 (s-level) due to ultra-high peak power (up to 10¹) 4 ~10¹ 8 With its W / cm² intensity and ultrashort interaction time, the focused light intensity has become a core tool in fields such as strong-field physics, attosecond science, laser inertial confinement fusion, atmospheric remote sensing, aerospace material testing, and vacuum environment microfabrication. The focused light intensity is a key parameter in the interaction between femtosecond lasers and matter, directly determining the occurrence and efficiency of physical processes such as ionization, higher harmonics, and plasma generation. Accurate measurement of the focused light intensity under low atmospheric pressure is a prerequisite for experimental design, parameter calibration, and quantitative analysis of results.

[0003] Over the past few decades, scientists have developed various techniques to study plasma products in filament structures, such as laser-induced breakdown spectroscopy (LIBS), microwave scattering analysis, and electrical diagnostics. Among these, fluorescence spectroscopy, as a direct and easy-to-implement method, has been widely used in excited-state dynamics studies. However, existing techniques have the following limitations:

[0004] There is a complex coupling between the macroscopic propagation characteristics of femtosecond laser pulses in low-pressure nitrogen and the microscopic excited-state dynamics: the emission intensity of 391 nm fluorescence is determined by the local ionization probability, molecular density and subsequent electron-ion quenching rate; while the 380 nm fluorescence exhibits abnormal decay under higher pressure, which is due to the synergistic effect of microscopic collisional quenching effect and macroscopic equivalent plasma volume contraction caused by plasma defocusing.

[0005] Existing measurement devices often only collect lateral fluorescence and lack synchronous measurement of forward transmitted laser (which reflects the degree of plasma defocusing). This makes it impossible to distinguish whether the decrease in light intensity is due to a decrease in ionization probability or shrinkage of the effective working volume, resulting in a large error in the measurement of focused light intensity under low pressure conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a device and method for measuring the focused light intensity of a femtosecond laser under low pressure conditions. The device is simple, highly sensitive, damage-resistant, and highly reliable. It can effectively solve the problem of not being able to distinguish the defocusing effect when directly measuring the focused light intensity under low pressure conditions, and realize the indirect and accurate measurement of the focused light intensity of a femtosecond laser under low pressure.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A femtosecond laser low-pressure focusing intensity measurement device based on comparative analysis of different fluorescence spectra of nitrogen gas and ionized nitrogen ions, combined with forward transmission laser energy monitoring, includes: a near-infrared femtosecond laser, a first reflecting mirror, a second reflecting mirror, a first beam splitter, a first pinhole, a focusing lens, and a gas cavity arranged sequentially along the optical path transmission direction; it also includes a lateral fluorescence acquisition module and a forward transmission laser energy detection module connected to the gas cavity.

[0009] The first beam splitter transmits the main beam as pump light and reflects a portion of the beam to an energy monitoring meter to monitor the incident pulse energy in real time; the first pinhole filters stray light and improves beam quality; the focusing lens focuses the beam into the gas cavity to excite 391 nm and 380 nm fluorescence; the lateral fluorescence acquisition module includes a lateral fluorescence collection lens and a spectrometer; the forward transmission laser energy detection module includes a second pinhole, a third reflecting mirror, a backward laser energy collection lens, and an integrating sphere laser power detector, used to simultaneously measure transmittance to quantitatively characterize the macroscopic effective volume change caused by plasma defocusing.

[0010] On the other hand, the present invention provides a measurement method based on the above-mentioned device, which obtains data under different air pressures and focal lengths by simultaneously acquiring lateral fluorescence intensity at 391 nm and 380 nm and forward transmission laser energy; by utilizing the changes in fluorescence intensity and transmittance, combined with a pre-calibrated nonlinear propagation model, the influence of local ionization probability and macroscopic plasma volume contraction on fluorescence yield is decoupled and inverted to obtain the actual focused light intensity.

[0011] Due to the adoption of the above technical solutions, the present invention has the following beneficial effects:

[0012] 1. Dual-channel synchronous measurement solves the problem of mechanism confusion: Simultaneously acquiring lateral fluorescence spectral lines and forward transmission laser energy can distinguish the contribution of "local ionization probability decrease" and "macroscopic plasma volume contraction" to the fluorescence signal, thus solving the defect of traditional methods that cannot identify defocusing effects when only fluorescence is measured.

[0013] 2. Clear physical mechanism and high measurement accuracy: Based on the evolution of 391 nm and 380 nm fluorescence in different pressure ranges, the degree of defocus is quantified by forward transmission measurement, and the quenching difference is corrected by the dual fluorescence ratio, thereby calibrating the focused light intensity.

[0014] 3. Simple device, high reliability and strong resistance to damage: No complex interference or autocorrelation measurement is required. The indirect measurement of focused light intensity under low pressure conditions can be achieved using a conventional spectrometer and an integrating sphere power detector. Moreover, the integrating sphere detector has extremely strong resistance to laser damage and is suitable for high-energy femtosecond laser environments. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the structure of the femtosecond laser low-pressure focusing light intensity measurement device provided in an embodiment of the present invention.

[0017] Figure 2 This is a schematic diagram of typical experimental results of an embodiment of the present invention (curves showing the change in fluorescence intensity at 391 nm and 380 nm under different atmospheric pressures). Detailed Implementation

[0018] To make the technical means, creative features, objectives and effects of the present invention easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0019] like Figure 1 As shown in the figure, this embodiment provides a device for measuring the intensity of a femtosecond laser focusing under low air pressure conditions. The device includes a near-infrared femtosecond laser (1), a first mirror (2), a second mirror (3), a first beam splitter (4), a first pinhole (5), a focusing lens (6), a gas cavity (7), a lateral fluorescence collecting lens (8), a spectrometer (9), a second pinhole (10), a third mirror (11), a rearward laser energy collecting lens (12), and an integrating sphere laser power detector (13).

[0020] The near-infrared femtosecond laser (1) emits a linearly polarized femtosecond laser with a pulse width of approximately 35 fs and a repetition frequency of 1 kHz at 800 nm. The laser first passes through the first reflector (2) and the second reflector (3) to adjust the height and direction of the optical path before reaching the first beam splitter (4).

[0021] The function of the first beam splitter (4) is to transmit the main beam as pump light for subsequent focusing. By monitoring the energy meter reading, it is ensured that the incident pulse energy remains stable (fluctuation <2%) when the focal length of the focusing lens (6) or the gas pressure in the gas cavity (7) is changed.

[0022] The pump light then passes through the first aperture (5) to filter out stray light at the edge of the beam and improve beam quality.

[0023] The pump light after passing through the first small hole (5) is focused by the focusing lens (6) (focal length can be selected from 10 cm, 25 cm, or 40 cm, depending on the required focusing intensity range) onto the center of the gas chamber (7). The gas chamber (7) is a sealed stainless steel chamber equipped with an inlet valve and a vacuum pump. The internal gas pressure can be precisely adjusted within the range of 1 to 50 mbar. The gas inside the chamber can be high-purity nitrogen (99.999% purity) or high-purity argon gas.

[0024] In the gas cavity (7), the high peak power femtosecond laser interacts with nitrogen gas: firstly, N2⁺ and electrons are generated through multiphoton ionization or tunneling ionization, and some N2⁺ and neutral N2 are formed into N4⁺ through three-body collision. After N4⁺ recombines with electrons, it enters the N2(C³Πᵤ) state and then emits 380 nm fluorescence; at the same time, the direct excitation radiation of N2⁺ generates 391 nm fluorescence.

[0025] Lateral fluorescence: collected by a lateral fluorescence collecting lens (8) (focal length 50 mm, f / 2 quartz lens) perpendicular to the laser propagation direction, coupled through a 4f system into the 100 μm slit of the spectrometer (9) (grating spectrometer, such as Andor SR-500i), with an integration time of 0.2 seconds, accumulating one acquisition, and recording the fluorescence intensity at 391 nm and 380 nm.

[0026] Forward transmission laser: The remaining pump laser (partially defocused and absorbed by the plasma) after passing through the gas cavity (7) propagates in the original direction. It first passes through the second small aperture (10) (1.5 mm in diameter) for filtering the laser. Then, the light path is deflected by the third reflecting mirror (11), and it is focused by the backward laser energy collection lens (12) (focal length 30 mm, ultraviolet fused silica). Finally, it enters the integrating sphere entrance of the integrating sphere laser power detector (13) (such as Ophir IS6-D). The integrating sphere detector records the single pulse energy or average power of the transmitted laser, and calculates the transmittance T = I_out / I_in by combining it with the incident energy.

[0027] By changing the gas pressure inside the gas chamber (7) (e.g., 1 mbar, 5 mbar, 10 mbar, 20 mbar, 50 mbar), the above measurements were repeated to obtain the lateral fluorescence intensity (I) at different gas pressures. 391 , I 380 ) and forward transmittance T.

[0028] Data analysis method: Use T to obtain V_eff; calculate I 380_corr = I 380 ×(V_ref / V_eff); Calculate R = I 391 / I 380_corr; By calibrating the curve R → P_ion → I_focus.

[0029] The typical operating procedure is as follows:

[0030] 1. Turn on the near-femtosecond laser (1) and preheat for 30 minutes until the output is stable;

[0031] 2. Adjust the reflectors (2, 3) so that the pump light is aligned with the center of the first beam splitter (4);

[0032] 3. Adjust the first beam splitter (4) to make the direction of the transmitted main beam correct, and at the same time observe the reading of the energy meter and record the incident energy;

[0033] 4. Adjust the position of the first small hole (5) to make the transmitted light spot symmetrical and without diffraction rings;

[0034] 5. Install the focusing lens (6) with the required focal length and adjust its position so that the focal point is located at the center of the gas cavity (7);

[0035] 6. Evacuate the gas chamber (7) to <0.1 mbar, and then fill it with high-purity nitrogen to the set pressure (e.g., 1 mbar).

[0036] 7. Simultaneously turn on the spectrometer (9) and the integrating sphere laser power detector (13) to record the lateral fluorescence spectrum and forward transmission energy;

[0037] 8. Change the air pressure (e.g., 5, 10, 20, 50 mbar) and repeat step 7;

[0038] 9. Change the focal length of the focusing lens (6) and repeat steps 5-8;

[0039] 10. Data Processing: Calculate transmittance T, obtain V_eff from the pre-calibrated T-V_eff curve, and calculate I. 380_corr = I 380_corr ×(V_ref / V_eff); Calculate the eigenvalue R = I 391 / I 380_corr Plot the curves of R and T as a function of air pressure;

[0040] 11. The actual focused light intensity I_focus is obtained by looking up or fitting the pre-calibrated R-I_focus relationship curve (obtained through low-pressure reference experiments or numerical simulations).

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A device for measuring the intensity of a femtosecond laser focusing beam under low-pressure conditions, characterized in that, include: The near-infrared femtosecond laser (1), first reflector (2), second reflector (3), first beam splitter (4), first pinhole (5), focusing lens (6), and gas cavity (7) are arranged sequentially along the optical path transmission direction; it also includes a lateral fluorescence acquisition module and a forward transmission laser energy detection module connected to the gas cavity (7); The near-infrared femtosecond laser (1) is used to emit near 800nm ​​infrared polarized femtosecond laser; The first beam splitter (4) is used to transmit the main beam as pump light and reflect part of the beam to the energy monitoring meter to monitor the incident pulse energy in real time. The first small hole (5) is disposed between the first beam splitter (4) and the focusing lens (6) to filter stray light and optimize the quality of the laser spot; The focusing lens (6) is used to focus the pump light into the gas cavity (7), where it interacts with low-pressure nitrogen to produce nitrogen ion fluorescence of 391 nm (N2⁺ B²Σ⁺ᵤ → X²Σ⁺ᵍ) and nitrogen molecular fluorescence of 380 nm (N2 C³Πᵤ → B³Πᵍ); The lateral fluorescence acquisition module includes a lateral fluorescence collection lens (8) and a spectrometer (9). The lateral fluorescence collection lens (8) is disposed on the side of the gas cavity (7) and is used to couple the generated fluorescence to the spectrometer (9) to record the fluorescence intensity. The forward transmission laser energy detection module includes a second small hole (10), a third reflector (11), a backward laser energy collection lens (12), and an integrating sphere laser power detector (13) arranged sequentially along the transmission optical path. It is used to collect the remaining forward pump laser (carrying plasma defocusing information) passing through the gas cavity (7) to measure the laser transmittance and quantitatively characterize the macroscopic effective volume change caused by plasma defocusing.

2. The device for measuring the intensity of a femtosecond laser focusing beam under low-pressure conditions according to claim 1, characterized in that, The near-infrared femtosecond laser (1) emits a laser wavelength of 800 nm, a pulse width on the order of 35 fs, and a repetition frequency of 1 kHz.

3. The device for measuring the intensity of a femtosecond laser focusing beam under low-pressure conditions according to claim 1, characterized in that, The first aperture (5) is used to filter out stray light and improve beam quality; the second aperture (10) has a diameter of 1.5 mm and is used for angular spectrum filtering.

4. The device for measuring the intensity of a femtosecond laser focusing beam under low-pressure conditions according to claim 1, characterized in that, The focusing lens (6) is an interchangeable lens with a focal length of 10 cm, 25 cm or 40 cm.

5. The device for measuring the intensity of a femtosecond laser focusing beam under low-pressure conditions according to claim 1, characterized in that, The gas chamber (7) is a sealed chamber equipped with an inlet valve and a vacuum pump. The internal gas pressure can be precisely adjusted within the range of 1~50 mbar. The chamber is filled with high-purity nitrogen (99.999% purity) and high-purity argon.

6. The device for measuring the intensity of a femtosecond laser focusing beam under low-pressure conditions according to claim 1, characterized in that, The lateral fluorescence collecting lens (8) is a quartz lens, which is coupled to the spectrometer (9) through a 4f system. The slit width of the spectrometer (9) is 100 μm and the integration time is 0.2 seconds.

7. The device for measuring the intensity of a femtosecond laser focusing under low-pressure conditions according to claim 1, characterized in that, The backward laser energy harvesting lens (12) is an ultraviolet fused silica lens, and the integrating sphere laser power detector (13) is used to record single pulse energy or average power.

8. A method for operating a femtosecond laser focusing light intensity measuring device under low-pressure conditions according to any one of claims 1-7, characterized in that, Includes the following steps: S1: Turn on the near-infrared femtosecond laser (1) to output femtosecond laser, which reaches the first beam splitter (4) after the optical path is adjusted by the reflector group. S2: The beam is split by the first beam splitter (4), and the transmitted main beam is used as the pump light. At the same time, the energy monitoring meter ensures that the incident pulse energy remains stable and consistent when the focal length of the focusing lens or the gas pressure of the gas cavity is changed. S3: The pump light passes through the first small hole (5) to filter stray light and improve beam quality; S4: The pump light is focused by the focusing lens (6) to the center of the gas cavity (7), and is ionized and coupled with the low-pressure nitrogen gas in the gas cavity to excite 391 nm nitrogen ion fluorescence and 380 nm nitrogen molecule fluorescence. S5: Use the lateral fluorescence acquisition module to collect lateral fluorescence, input it into the spectrometer (9) to record the fluorescence intensity I at 391 nm. 391 and 380 nm fluorescence intensity I 380 ; S6: The forward transmission laser energy detection module is used to collect the forward transmission laser passing through the gas cavity (7), and the transmission laser energy is recorded and the transmittance T is calculated by the integrating sphere laser power detector (13); S7: Change the nitrogen pressure in the gas chamber (7) and / or the focal length of the focusing lens (6), repeat steps S4~S6, and obtain the 391 nm fluorescence intensity I under different conditions. 391 380 nm fluorescence intensity I 380 And transmittance T; S8: Based on fluorescence intensity I 391 I 380 The relationship between the transmittance T and the actual focused light intensity is obtained by combining the focused light intensity inversion model calibrated in advance through experiments or numerical simulations. The inversion model is constructed based on the following physical relationships: 391nm fluorescence intensity I 391 The local ionization probability P_ion, nitrogen molecule density ρ_nt, and electron collision quenching rate k_q are related. 391 ·ρ_e is related, and electron quenching dominates in the low-to-medium pressure region (1–50 mbar), therefore I 391 It cannot be directly regarded as a monotonic function of P_ion; 380nm fluorescence intensity I 380 In addition to being affected by ionization probability and quenching, it also strongly depends on the effective plasma volume V_eff (formed through three-body bonding to N4). + (Macroscopic integral effect of intermediates) Forward transmittance T directly reflects the intensity of plasma defocusing, and the degree of decrease in T has a quantitative relationship with the saturation and contraction of V_eff; The inversion model decouples I_focus in the following way: (a) Use the T value to find the effective volume V_eff under the current conditions from the pre-calibrated T-V_eff relationship curve; (b) The measured I 380 Multiplying by the normalization factor V_ref / V_eff yields the corrected intermediate quantity I. 380_corr= I 380 ×(V_ref / V_eff), where V_ref is the effective volume at a reference pressure (e.g., 1 mbar); (c) Using I 391 / I 380_corr Subtracting the difference between electronic quenching and neutral quenching from the ratio, we obtain an eigenvalue R that is proportional to the local ionization probability P_ion; (d) Calculate the actual I_focus based on the pre-calibrated functional relationship between P_ion and the focused light intensity I_focus (e.g., by solving the tunneling ionization model or a benchmark experiment with known light intensity).

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