A device for detecting a remote electromagnetic field based on neutral nitrogen molecule back laser

Abstract

The application discloses a device for remote electromagnetic field detection based on neutral nitrogen molecular back laser, which comprises the following parts: a femtosecond laser light source, a first beam splitter arranged right above the femtosecond laser light source, a quarter-wave plate arranged on one side of the first beam splitter, a first mirror arranged on one side of the quarter-wave plate, a dichroic mirror arranged above the first mirror, a first focusing lens arranged on one side of the dichroic mirror, a vacuum cavity arranged on one side of the first focusing lens, a second dichroic mirror arranged on one side of the vacuum cavity, a second focusing lens arranged on one side of the second dichroic mirror, an interference filter arranged on one side of the second focusing lens and a spectrometer arranged on one side of the interference filter, and a light barrier arranged right above the second dichroic mirror; a plasma is arranged in the vacuum cavity, and an additional direct-current electric field is arranged on the plasma. According to the application, remote single-end detection is realized, and the sensitivity of detection can be significantly improved due to the high-intensity characteristics of the back laser.

Description

Technical Field

[0001] This invention relates to the technical field of electromagnetic field detection, and in particular to a device for remote electromagnetic field detection based on a back-facing laser beam from neutral nitrogen molecules. Background Technology

[0002] Femtosecond laser pulse filamentation involves numerous nonlinear optical effects, broadening the research field of ultrafast nonlinear optics. Several effects induced by femtosecond lasers in air plasma, such as broadband terahertz radiation generation, laser pulse compression, and induced high-voltage discharge, have attracted widespread attention in the scientific community. Air lasers, as a unique phenomenon in femtosecond laser filamentation, have become a cutting-edge and hot topic in ultrafast optics over the past decade. Utilizing the main components of air and their derivatives as the laser gain medium, air lasers can form virtual lasers in the atmosphere at long distances, potentially providing a novel light source for long-range optical detection. This new light source has the potential to bring breakthrough improvements to atmospheric pollution detection, long-range electromagnetic radiation measurement, battlefield trace gas detection, and precision measurement.

[0003] In their 2019 research paper, "Terahertz control of air lasing" (PRA.99.053802(2019)), M. Clerici et al. first achieved modulation of the forward signal of nitrogen molecular ion air laser using strong terahertz radiation. In the experiment, they combined a single-cycle terahertz pulse with a 790nm ultra-intense near-infrared pulse with a pulse width of 47 fs in a plasma. They observed that the coherent radiation of nitrogen ions at wavelengths of 391nm and 428nm could be controlled by the amplitude of the terahertz field, providing a fundamental basis for electromagnetic field detection using air lasers. However, this research is based on the forward air laser effect of nitrogen ion radiation and can only detect forward coherent optical signals, making it unsuitable for single-end detection and thus limiting its practical value for long-range electromagnetic field detection. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a device for remote electromagnetic field detection based on a neutral nitrogen molecule back-laser, achieving remote single-end detection. Furthermore, relying on the high intensity of the back-laser, the detection sensitivity can be significantly improved. To achieve the above-mentioned objectives and other advantages of the present invention, a device for remote electromagnetic field detection based on a neutral nitrogen molecule back-laser is provided, comprising:

[0005] The system comprises a femtosecond laser source, a first beam splitter positioned directly above the femtosecond laser source, a quarter-wave plate positioned to one side of the first beam splitter, a first reflecting mirror positioned to one side of the quarter-wave plate, a dichroic mirror positioned above the first reflecting mirror, a first focusing lens positioned to one side of the dichroic mirror, a vacuum cavity positioned to one side of the first focusing lens, a second dichroic mirror positioned to one side of the vacuum cavity, a second focusing lens positioned to one side of the second dichroic mirror, an interference filter positioned to one side of the second focusing lens, and a spectrometer positioned to one side of the interference filter. A light-blocking plate is positioned directly above the second dichroic mirror.

[0006] The vacuum chamber contains plasma, and an external DC electric field is applied to the plasma.

[0007] Preferably, a second reflector is provided on the other side of the first beam splitter, a third reflector is provided on one side of the second reflector, a BBO crystal is provided below the third reflector, a half-wave plate is provided below the BBO crystal, and a laser beam splitter is provided below the half-wave plate.

[0008] Preferably, a second focusing lens, an interference filter, and a spectrometer are sequentially arranged on the side of the laser beam splitter away from the dichroic mirror.

[0009] Preferably, the first beam splitter, the laser beam splitter, and the second dichroic mirror are all tilted, and the first beam splitter and the laser beam splitter are tilted in opposite directions, while the first beam splitter and the second dichroic mirror are tilted in the same direction.

[0010] Preferably, the first reflector, the second reflector and the third reflector are all tilted, the tilting directions of the second reflector and the third reflector are opposite, and the tilting direction of the second reflector is the same as that of the first reflector.

[0011] Preferably, the pulsed beam emitted from the femtosecond laser source is split into reflected light and transmitted light by the first beam splitter. The reflected light is then converted into circularly polarized pump light after passing through a quarter-wave plate. The circularly polarized pump light is reflected by the first mirror and then combined onto a dichroic mirror.

[0012] Compared with the prior art, the beneficial effects of this invention are:

[0013] (1) Using back-to-air lasers, it is possible to remotely measure the electric field distribution in areas with strong electric fields that are not suitable for workers to enter, and it is conceivable to detect unknown electric fields in the atmosphere.

[0014] (2) By using a 400nm probe light with rotatable polarization direction, the electric field direction in a plane perpendicular to the direction of pump light propagation can be accurately measured.

[0015] (3) By monitoring the change in the amplitude of the air laser signal, the magnitude of the applied electric field can be measured.

[0016] (4) Since back-to-air lasers are coherent light sources, this scheme can greatly improve the sensitivity of electric field measurement compared to traditional remote sensing technology that targets fluorescence signals. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the device for remote electromagnetic field detection based on a neutral nitrogen molecule back-facing laser according to the present invention.

[0018] Figure 2 The image shows the spectra of forward (a) and backward (b) air laser signals measured by the device based on the back-facing laser of neutral nitrogen molecules for remote electromagnetic field detection according to the present invention.

[0019] Figure 3 The figure shows the effect of the change in electric field intensity measured by applying electric fields in different directions on the laser signal facing away from the plasma at a distance of 3m from the plasma, according to the present invention. Detailed Implementation

[0020] 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Reference Figure 1-3A device for detecting remote electromagnetic fields based on back-facing laser beams from neutral nitrogen molecules includes: a femtosecond laser source 1, a first beam splitter 2 positioned directly above the femtosecond laser source 1, a quarter-wave plate 3 positioned to one side of the first beam splitter 2, a first reflecting mirror 4 positioned to one side of the quarter-wave plate 3, a dichroic mirror 5 positioned above the first reflecting mirror 4, a first focusing lens 6 positioned to one side of the dichroic mirror 5, a vacuum cavity 7 positioned to one side of the first focusing lens 6, a second dichroic mirror 10 positioned to one side of the vacuum cavity 7, a second focusing lens 11 positioned to one side of the second dichroic mirror 10, and a second focusing lens 11 positioned to one side of the second dichroic mirror 10. An interference filter 12 is located on one side of the focusing lens 11, and a spectrometer 13 is located on one side of the interference filter 12. A light-blocking plate 14 is located directly above the second dichroic mirror 10. A plasma 8 is located inside the vacuum cavity 7, and an external DC electric field 9 is located on the plasma 8. The pulse beam emitted from the femtosecond laser source 1 is split into reflected light and transmitted light by the first beam splitter 2. The reflected light is converted into circularly polarized pump light after passing through the quarter-wave plate 3. The circularly polarized pump light is reflected by the first reflecting mirror 4 to the dichroic mirror 5 for beam combining. The pump light is focused into the vacuum cavity 7 by the lens 6 and forms plasma 8. A second reflecting mirror 15 is disposed on the other side of the first beam splitter 2. A third reflecting mirror 16 is disposed on one side of the second reflecting mirror 15. A BBO crystal 17 is disposed below the third reflecting mirror 16. A half-wave plate 18 is disposed below the BBO crystal 17. A laser beam splitter 19 is disposed below the half-wave plate 18. A second focusing lens 11, an interference filter 12, and a spectrometer 13 are disposed sequentially on the side of the laser beam splitter 19 away from the dichroic mirror 5. A DC electric field 9 is applied to the plasma on the parallel plate electrode installed in the gas chamber. The residual pump light is reflected by the dichroic mirror 10 to the optical baffle 14. The stimulated emission signal generated at the plasma is focused by the lens 11 onto the spectrometer 13, and the background light signal such as stray light is filtered out by the filter 12 to be forward and backward. Another transmitted beam is reflected by mirrors 15 and 16 into the BBO crystal 17 to generate frequency-doubled light. The frequency-doubled light passes through a half-wave plate 18 to change its polarization direction, and then passes through a beam splitter 19 before being combined on a dichroic mirror 5. This combined beam then acts on the plasma along with the pump light, allowing the experimenter to detect coherent 337.2 nm radiation in either the back or front direction, thereby achieving the measurement of the magnitude and direction of the applied DC electric field.

[0022] The device comprises a pump assembly and a detector assembly. The pump assembly uses a high-intensity 800nm ​​circularly polarized femtosecond pulse to pump nitrogen gas, generating forward and backward 337.2nm lasing signals. Detection employs a linearly polarized 400nm probe beam with continuously variable polarization. By rotating the polarization direction of the 400nm pulse, the modulation of the backward-propagating air laser is observed, thus enabling simultaneous measurement of both the intensity and direction of the electric field. The pump assembly includes, in sequence, a femtosecond laser source, an 800nm ​​quarter-wave plate, a 75cm focusing lens, a vacuum cavity, an applied electric field, a reflective 800nm ​​and a transmissive 337nm dichroic mirror, a 337nm interference filter, and a spectrometer. The linearly polarized 800nm ​​pulse passes through the quarter-wave plate to generate circularly polarized light, which is then focused by the 75cm lens into a gas chamber filled with pure nitrogen, forming plasma and generating a 337.2nm coherent radiation signal. After filtering, the 337.2nm signal is coupled into the spectrometer in both the forward and backward directions. At this point, by adding an external DC electric field, the modulation of the air laser signal by the electric field can be observed.

[0023] The detection assembly comprises a barium borate (BBO) crystal and a 400nm half-wave plate. An 800nm ​​pulsed light is generated as 400nm linearly polarized light after passing through the BBO crystal. The polarization direction of the 400nm detection light is then changed by passing through the half-wave plate. In this invention, the 400nm field polarized along different directions alters the orientation of electrons in the plasma. This causes some electrons to undergo directional motion, thus the electric field in different directions will enhance or weaken the electron energy, thereby modulating the coherent radiation signal back to 337.2nm.

[0024] Furthermore, the first beam splitter 2, the laser beam splitter 19, and the second dichroic mirror 10 are all tilted, and the first beam splitter 2 and the laser beam splitter 19 are tilted in opposite directions, while the first beam splitter 2 and the second dichroic mirror 10 are tilted in the same direction.

[0025] Furthermore, the first reflector 4, the second reflector 15, and the third reflector 16 are all tilted, with the second reflector 15 and the third reflector 16 tilting in opposite directions, and the second reflector 15 tilting in the same direction as the first reflector 4.

[0026] Example 1

[0027] This embodiment uses a commercial titanium sapphire laser light source with a center wavelength of 796nm.

[0028] The electric field detected in this embodiment is an externally introduced DC electric field, the magnitude and direction of which can be freely controlled. Thus, with the known electric field and direction, we can verify the accuracy of this implementation scheme.

[0029] A horizontally polarized femtosecond pulsed laser with a center wavelength of 796 nm, a pulse width of 35 fs, an energy of 12 mJ, and a repetition rate of 1 kHz, output from a Ti:sapphire laser 1, is split into a pump beam and a probe beam by a beam splitter 2 with a reflectivity of 90% and a transmittance of 10%. In the pump beam path, the linearly polarized pump beam is converted into circularly polarized light after passing through a zero-order 800 nm quarter-wave plate. This high-energy circularly polarized pump beam is then reflected by a zero-degree-angle 800 nm mirror 4 onto a dichroic mirror 5 (reflecting 800 nm and transmitting 300-400 nm), which also reflects at a zero-degree angle. The advantage of zero-degree-angle reflection is that it does not change the polarization state of the incident light, thus obtaining circularly polarized light with an ellipticity close to 1. A quartz lens 6 with a focal length of 75 cm then focuses the high-energy circularly polarized light reflected by the dichroic mirror 5 into a sealed gas chamber 7 filled with 1 bar of pure nitrogen, thereby forming a slender plasma filament 8 with a length close to 6 cm. The plasma will emit stimulated emission light signals with a wavelength of 337.2 nm in both the forward and backward directions. Subsequently, we used signal receiving system 11-13 to detect the 337.2 nm signals generated in both directions. The experimental results are shown in [Figure number missing]. Figure 2 In the signal receiving system, a 10cm quartz lens 11 couples the generated 337.2nm signal into the fiber optic spectrometer (Avantes, Avaspec-3648) 13. An interference filter 12 with a center wavelength of 340nm and a bandwidth of 10nm filters out the influence of background light on the target signal detection. Furthermore, in the forward signal detection, a dichroic mirror 10 (reflecting 800nm, transmitting 337nm) incident at a 45° angle is added to reflect the remaining high-energy circularly polarized light to a light-blocking plate to prevent high-energy pump light from damaging the subsequent signal receiving system. After observing stimulated emission signals based on nitrogen molecules in both the forward and backward directions, a DC electric field 9 is applied externally, and the effect of this electric field on the 337.2nm signal is observed. By changing the magnitude and direction of the electric field, the test results obtained are as follows... Figure 3As shown. For the probe optical path, the transmitted 800nm ​​probe light is first reflected by two 800nm ​​mirrors onto a 100μm thick, 29.2° cut-angle type I barium metaborate (BBO) crystal 17. This crystal generates vertically polarized linear light of 400nm through frequency doubling. Then, a zero-order 400nm half-wave plate is added to the generated 400nm optical path. The purpose of this half-wave plate is to change the polarization direction of the incident 400nm probe light. Next, the rotatably polarized 400nm probe light is reflected by a 1:1 400nm beam splitter 19 (operating wavelength 300-400nm) to a dichroic mirror 5, and then propagates collinearly with the pump light, acting together in the plasma. At this point, by rotating the polarization direction of the 400nm probe light, the electron current in the plasma will be redirected to different azimuth angles, which will affect the intensity of the back-facing 337.2nm lasing. By rotating the polarization of a 400nm linearly polarized pulse by one revolution and measuring the dependence of the back signal on the polarization direction, the direction of the electric field can be measured.

[0030] Neutral nitrogen molecular air lasers are used as a means of probing the electric field because, firstly, neutral nitrogen molecules can generate stimulated emission signals that propagate in the opposite direction to the pump light propagation direction, which can be applied to practical single-end measurements. Secondly, under the action of a high-intensity laser field, the kinetic energy of electrons in the plasma can exceed 16 eV. In the presence of high-energy electrons, the collisional excitation energy between electrons and nitrogen molecules effectively transfers nitrogen molecules from the ground state to the third excited state, thereby achieving population inversion between the D3Πu state and the C3Πg state. This inelastic collision process can be represented as follows:

[0031] N 2(X 1Σg+)+e=N 2(D 3Πu+)+e

[0032] Here, the applied DC electric field can modulate the energy of electrons in the plasma, thereby affecting the formation of population inversion and ultimately modulating the air laser signal. The applied 400nm laser field breaks the spatial symmetry of the electron current in the circularly polarized 800nm ​​field within the plane perpendicular to the laser propagation direction (xy plane). In this case, some electrons will form directional motions within the xy plane. The energy of these directional electrons will be enhanced or weakened under the influence of the applied DC electric field, thus strengthening or weakening the air laser signal. Therefore, by measuring the change in the polarization state of the 337.2nm nitrogen molecule laser signal with the 400nm light field, the direction of the electric field in the xy plane can be determined. The magnitude of the electric field can be determined by the amplitude of the enhancement or weakening of the air laser signal.

[0033] Figure 1This is a diagram of the experimental setup for detecting a remote DC electric field using nitrogen molecules facing away from the air with a laser.

[0034] Figure 2 These are the spectra of the forward a and backward b air laser signals measured by this invention;

[0035] Figure 3 This invention relates to the effect of changes in electric field intensity measured in different directions on the laser signal facing away from the plasma at a distance of 3m (enhancement a and weakening b).

[0036] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention, and applications, modifications and variations thereof will be apparent to those skilled in the art.

[0037] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A device for remote electromagnetic field detection based on a back-facing laser beam from neutral nitrogen molecules, characterized in that, include: A femtosecond laser source (1), a first beam splitter (2) positioned directly above the femtosecond laser source (1), a quarter-wave plate (3) positioned on one side of the first beam splitter (2), a first reflecting mirror (4) positioned on one side of the quarter-wave plate (3), a dichroic mirror (5) positioned above the first reflecting mirror (4), a first focusing lens (6) positioned on one side of the dichroic mirror (5), a vacuum cavity (7) positioned on one side of the first focusing lens (6), a second dichroic mirror (10) positioned on one side of the vacuum cavity (7), a second focusing lens (11) positioned on one side of the second dichroic mirror (10), an interference filter (12) positioned on one side of the second focusing lens (11), and a spectrometer (13) positioned on one side of the interference filter (12). A light-blocking plate (14) is positioned directly above the second dichroic mirror (10). A plasma (8) is provided inside the vacuum chamber (7), and an external DC electric field (9) is provided on the plasma (8).

2. The device for remote electromagnetic field detection based on neutral nitrogen molecule back-facing laser as described in claim 1, characterized in that, A second reflector (15) is provided on the other side of the first beam splitter (2), a third reflector (16) is provided on one side of the second reflector (15), a BBO crystal (17) is provided below the third reflector (16), a half-wave plate (18) is provided below the BBO crystal (17), and a laser beam splitter (19) is provided below the half-wave plate (18).

3. The device for remote electromagnetic field detection based on a neutral nitrogen molecule back-facing laser as described in claim 2, characterized in that, A second focusing lens (11), an interference filter (12), and a spectrometer (13) are sequentially arranged on the side of the laser beam splitter (19) away from the dichroic mirror (5).

4. The device for remote electromagnetic field detection based on a neutral nitrogen molecule back-facing laser as described in claim 3, characterized in that, The first beam splitter (2), the laser beam splitter (19) and the second dichroic mirror (10) are all tilted, and the first beam splitter (2) and the laser beam splitter (19) are tilted in opposite directions, while the first beam splitter (2) and the second dichroic mirror (10) are tilted in the same direction.

5. The device for remote electromagnetic field detection based on neutral nitrogen molecule back-facing laser as described in claim 4, characterized in that, The first reflector (4), the second reflector (15) and the third reflector (16) are all tilted. The tilting directions of the second reflector (15) and the third reflector (16) are opposite, and the tilting direction of the second reflector (15) is the same as that of the first reflector (4).

6. The device for remote electromagnetic field detection based on a neutral nitrogen molecule back-facing laser as described in claim 1, characterized in that, The pulsed beam emitted from the femtosecond laser source (1) is split into reflected light and transmitted light by the first beam splitter (2). The reflected light is then converted into circularly polarized pump light after passing through a quarter-wave plate (3). The circularly polarized pump light is then reflected by the first reflector (4) onto the dichroic mirror (5) for beam combining.

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