CN110911947B - Pulse width compression device and method based on electronegative gas plasma - Google Patents
Pulse width compression device and method based on electronegative gas plasma Download PDFInfo
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- CN110911947B CN110911947B CN201911117919.8A CN201911117919A CN110911947B CN 110911947 B CN110911947 B CN 110911947B CN 201911117919 A CN201911117919 A CN 201911117919A CN 110911947 B CN110911947 B CN 110911947B
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- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
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Abstract
The invention relates to a pulse width modulation device and method based on electronegative gas plasma, comprising the following steps: the device comprises a pump laser, an electronegative gas plasma generating unit and a first pulse light path adjusting unit; the electronegative gas plasma generating unit is used for ionizing electronegative gas to generate electronegative plasma, and oscillating the electronegative plasma to obtain a periodic density modulation structure; the first pulse light path adjusting unit is used for converting the pumping laser generated by the pumping laser into circular polarization laser, and then the circular polarization laser is incident to the electronegative gas plasma generating unit, so that the circular polarization laser and the periodic density modulation structure generate stimulated Brillouin scattering, and output pulses after pulse width modulation are generated. The pulse width modulation device and the pulse width modulation method provided by the embodiment realize a stimulated Brillouin scattering scheme based on photoelectric synchronization, reduce the energy consumption of tens of lasers required for preparing the periodic density modulation structure, and realize the miniaturization of the whole equipment while solving the energy consumption.
Description
Technical Field
The invention relates to the technical field of photoelectricity, in particular to a pulse width modulation device and method based on electronegative gas plasma.
Background
In the prior art, when pulse width modulation is performed by adopting a method of combining energy based on stimulated brillouin scattering of plasma, target gas is synchronously heated through multiple paths of laser to obtain plasma with density period modulation. For example, the lawrence lipvermore national laboratory prepares a plasma with periodic density modulation by using a 60-way preheated laser, and pulse width modulates the pump laser pulse with the acquired plasma.
When the method is adopted for pulse width modulation, dozens of laser generators are required to act on target gas at the same time, the comprehensive modulation of the output laser beams of all the lasers is complex in operation, high in difficulty, high in energy consumption and difficult to miniaturize equipment.
At present, no device capable of completing pulse width modulation on the basis of electronegative gas plasma is available.
Disclosure of Invention
The embodiment of the invention provides a pulse width modulation device and method based on electronegative gas plasma, provides a brand-new pulse width modulation method based on electronegative gas plasma, and can greatly overcome the defects caused by synchronous heating of target gas by dozens of lasers.
The embodiment of the invention provides a pulse width modulation device based on electronegative gas plasma, which comprises: the device comprises a pump laser, an electronegative gas plasma generating unit and a first pulse light path adjusting unit; the electronegative gas plasma generating unit is used for ionizing electronegative gas to generate electronegative plasma, and oscillating the electronegative plasma to obtain the periodic density modulation structure. The first pulse light path adjusting unit is used for converting the pumping laser generated by the pumping laser into circular polarization laser, and then the circular polarization laser is incident to the electronegative gas plasma generating unit, so that the circular polarization laser and the periodic density modulation structure generate stimulated Brillouin scattering, and output pulses after pulse width modulation are generated.
Further, the electronegative gas plasma generating unit comprises a radio frequency impedance matching unit, an alternating power supply upper electrode, an alternating power supply lower electrode and a vacuum chamber; wherein:
the radio frequency impedance matching unit is connected with the function generator and the upper electrode of the alternating power supply and is used for generating periodically modulated alternating current between the upper electrode of the alternating power supply and the lower electrode of the alternating power supply; the alternating power supply upper electrode and the alternating power supply lower electrode are both of plate-shaped structures and are positioned in the vacuum chamber; the hole is arranged on the alternating power supply electrode, and the circularly polarized laser is incident into the vacuum chamber through the hole; the vacuum chamber is used for storing electronegative gas.
Further, a power amplifier is arranged between the radio frequency impedance matching unit and the function generator.
Further, the first pulse optical path adjusting unit comprises a first polarization beam splitter, a first quarter glass, a first focusing lens and a first quartz window piece which are sequentially connected on the optical path, wherein:
the first polarization spectroscope deflects the pump laser by 90 degrees so as to realize injection and output based on polarization of an optical path at the same time; the function generator is connected with the pump laser and is used for uniformly allocating the pump laser and the radio frequency impedance matching unit.
Further, the output pulse is transmitted along the opposite direction of the transmission direction of the circular polarized laser, and is converted into the linear polarized pulse laser after being subjected to pulse width modulation after sequentially passing through the hole, the first quartz window sheet, the first focusing lens, the first quarter glass sheet and the first polarization beam splitter.
Further, the pulse width modulation device based on electronegative gas plasma provided by the embodiment of the invention further comprises a seed laser and a second pulse optical path adjusting unit, wherein the second pulse optical path adjusting unit is used for adjusting the seed pulse laser output by the seed laser, so that the adjusted seed pulse laser and the circularly polarized laser coincide in the periodic density modulation structure.
Further, the second pulse optical path adjusting unit comprises a second polarization beam splitter, a second quarter glass, a second focusing lens and a second quartz window sheet which are sequentially connected on the optical path, wherein:
the second polarizing beam splitter is used for deflecting the seed pulse laser by 90 degrees; the function generator is connected with the seed laser and is used for uniformly allocating the seed laser and the pump laser.
Further, function generator is connected with the seed laser for carry out unified allotment to seed laser and pump laser, include: the frequency difference of the output laser frequency of the seed laser and the pump laser is equal to the phonon frequency generated by the electrostriction of the periodic density modulation structure.
Furthermore, the pump laser and the seed laser are tunable lasers with frequency broadband.
Further, the pump laser can be a titanium sapphire laser with the central wavelength of 800 nm; the focal length of the first focusing lens is slightly larger than half of the distance between the alternating power supply upper electrode and the alternating power supply lower electrode.
According to the pulse width modulation device and method based on the electronegative gas plasma, provided by the embodiment of the invention, the electronegative gas plasma generating unit is arranged to oscillate the electronegative plasma to obtain the periodic density modulation structure, so that the periodic density modulation structure is utilized to perform pulse width modulation on the entered laser pulse based on the photoelectric synchronous stimulated Brillouin scattering scheme, the energy consumption of tens of paths of lasers for preparing the periodic density modulation structure is effectively reduced, and the miniaturization of the whole equipment is realized while the energy consumption is solved.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.
FIG. 1 is a pulse width modulation device based on electronegative gas plasma according to an embodiment of the invention;
FIG. 2 is another pulse width modulation device based on electronegative gas plasmas, according to an embodiment of the invention;
FIG. 3 is a schematic diagram of another embodiment of an electronegative gas plasma-based pulse width modulation apparatus according to the invention;
FIG. 4 is a schematic diagram of a connection relationship of a function generator in an electronegative gas plasma-based pulse width modulation device according to an embodiment of the invention;
in the figure: 1. a pump laser; 2. a first pulse optical path adjusting unit; 3. an electronegative gas plasma generating unit; 4. a seed laser; 201. a first polarizing beam splitter; 202. a first quarter slide; 203. a first focusing lens; 204. a first quartz window piece; 205. a second polarizing beam splitter; 206. a second quarter slide; 207. a second focusing lens; 208. a second quartz window piece; 301. a radio frequency impedance matching unit; 302. an alternating power supply upper electrode; 303. an alternating power supply lower electrode; 304. a vacuum chamber; 305. an alternating power supply ground electrode; 306. a gas input port; 307. a gas outlet; 308. the hole is positioned on the upper electrode of the alternating power supply; 309. and the hole is positioned on the lower electrode of the alternating power supply.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The main components of the electronegative gas plasma are ions with positive and negative charges, and under the pointing action of an alternating electric field of a radio frequency electric field, the positive and negative charge ions can feel reverse coulomb force, so that the ions are separated in space. This separate motion can be described by the equations of motion of the two charged ions:
wherein, subscript + -represents the positive and negative of the charge of the ion, m is the ion mass, x is the ion displacement, omega is the resonance term introduced during the separation of the charged ion, E0cos (ω t) is the alternating electric field. At this time, since the plasma is generated by the alternating radio frequency electric field, the ionization degree is low, and the ions mainly exist in the form of single positive charge or single negative charge with the charge amount of e, the above formula 1 is further processed by using positive chargesNegative ion displacement difference X ═ X+-x-An equivalent equation of motion under reduction can be obtained:
wherein the content of the first and second substances,
μ＝m+m-/(m++m-)，ωRfor the resonant frequency, μ is the reduced mass, where equation 2 is a typical Lorentz equation.
When the alternating frequency of the alternating radio frequency electric field is consistent with the resonance frequency, a resonance phenomenon can be generated, and at the moment, the density periodic modulation phenomenon can occur on the positive and negative charged ions along the direction of the alternating radio frequency electric field, so that a space plasma grating structure (namely a periodic density modulation structure) is formed.
In the Raman Effect (Raman Effect), a beam of light (i.e., a pump laser) is incident, and on its scattering spectrum, there are not only the laser light of the original frequency ω but also the stokes light of the frequency ω - Ω and the anti-stokes light of Ω + ω, and Ω (the amount of frequency shift) can be various normal modes, that is, can be transited between energy levels not allowed by the usual dipole transition.
The obtained space plasma grating structure can realize laser coherent beam combination based on plasma, and the physical mechanism is a Stimulated Brillouin Scattering (SBS) method established in the fields of optical engineering and the like. The Stimulated Brillouin Scattering (SBS) is mainly caused by that the incident light is very high in power, ultrasonic waves are excited in a substance by an electromagnetic stretching effect generated by light waves, and the incident light is scattered by the ultrasonic waves. The basic principle of the SBS method is that the frequency of the pump light is assumed to be omegapThe frequency of Stokes scattered light is ωsThe phonon frequency generated by electrostriction of the periodic density modulation structure is delta omega, and when the frequency meets the condition that delta omega is omegap-ωsSBS causes photon energy to be transferred from the pump light to the scattered light. While the scattering itself is due to fluctuations in the internal density (refractive index) of the plasma, when the pump light intensity is highAfter a certain increase, the backscattered stokes light gets the maximum gain relative to the other modes in the spontaneously scattered light, and therefore this mode will be stimulated to amplify, which is the SBS effect. The SBS relies on acousto-optic coupling, and positive feedback excitation of a sound field is beneficial to amplification of the SBS process, so the stokes light becomes a main process therein (negative feedback of the anti-stokes light process weakens the output of the anti-stokes light), thereby obtaining scattered stokes light with compressed pulse width and improved peak power, and the scattered stokes light is an output pulse obtained after modulation in the embodiment of the invention.
Based on the discussion of the above principle, as shown in fig. 1, in an embodiment of the present invention, there is provided a pulse width modulation device based on electronegative gas plasma, including but not limited to: a pump laser 1, an electronegative gas plasma generating unit 2 and a first pulse optical path adjusting unit 3. The electronegative gas plasma generating unit 2 is used for ionizing electronegative gas to generate electronegative plasma, and oscillating the electronegative plasma to obtain a periodic density modulation structure; the first pulse light path adjusting unit 3 is configured to convert the pump laser generated by the pump laser 1 into a circularly polarized laser, and then inject the circularly polarized laser into the electronegative gas plasma generating unit 2, so that the circularly polarized laser and the periodic density modulation structure generate stimulated brillouin scattering, and an output pulse after pulse width modulation is generated.
The pump laser 1 is used for generating linear pump laser, and a function generator may be connected to the pump laser 1, and a trigger signal generated by the function generator is used for controlling the linear pump laser (such as pulse frequency, wavelength, etc.) output by the pump laser according to actual needs.
A sealed electronegative gas storage cavity is arranged in the electronegative gas plasma generating unit 2, wherein the electronegative gas can be methane (CH)4) The gas may be other gases, and the embodiment of the present invention is not particularly limited. Specifically, the electronegative gas plasma generating unit 2 can be connected with a high-voltage alternating power supply, and is sealed to be negative by the alternating power supplyAnd a periodic alternating radio frequency electric field is generated in the electric gas storage cavity, and the periodic alternating radio frequency electric field acts on the electronegative gas to ionize the electronegative gas. In conjunction with the description of the principle in the above embodiment, when the alternating frequency of the alternating radio frequency electric field coincides with the resonance frequency at the time of positive and negative ion separation, a resonance phenomenon is generated, so that the periodic density modulation structure composed of plasma is generated in the electronegative gas plasma generating unit 2.
It should be noted that, since the plasma formed by the electronegative gas is different from the normal plasma, the inside of the plasma is negatively charged, mainly ions rather than electrons, which results in a periodic modulation characteristic in density. The non-uniform relaxation time scale in the plasma is generally within nanosecond or above, and for femtosecond-level middle and far infrared laser or picosecond-level terahertz wave, the transient plasma structure can be regarded as a crystal material. In other words, for ultrafast optical processes, the periodic modulation of the transient density inside the plasma can be approximated as the absence of movement of the inner particles, with only electrostrictive effects due to the applied electric field.
Further, the linearly polarized pumping pulse laser generated by the pumping laser 1 controlled by the function generator may be converted into a circularly polarized laser after being subjected to polarization selection, filtering, and spot adjustment, and then the circularly polarized laser is incident into the electronegative gas plasma generating unit 2, specifically, the circularly polarized laser (pulse laser) is subjected to a periodic density modulation structure (equivalent to a spatial plasma grating structure), and the frequency of the circularly polarized laser and the phonon frequency generated by electrostriction of the periodic density modulation structure are adjusted to satisfy the condition of stimulated brillouin scattering, and the generated stokes light scattering pulse laser is the output pulse referred to in the embodiments of the present invention.
According to the pulse width modulation device based on the electronegative gas plasma, provided by the embodiment of the invention, the electronegative gas plasma generation unit is arranged to oscillate the electronegative plasma to obtain the periodic density modulation structure, so that the periodic density modulation structure is utilized to perform pulse width modulation on the entered laser pulse based on the photoelectric synchronous stimulated Brillouin scattering scheme, the energy consumption of tens of paths of laser used for preparing the periodic density modulation structure is effectively reduced, and the miniaturization of the whole equipment is realized while the energy consumption is solved.
Based on the content of the above embodiments, as an alternative embodiment, the electronegative gas plasma generating unit 2 includes but is not limited to: a radio frequency impedance matching unit 301, an alternating power supply upper electrode 302, an alternating power supply lower electrode 303 and a vacuum chamber 304; the radio frequency impedance matching unit 301 is connected with a function generator (not shown in the figure) and the alternating power supply upper electrode 302, and is used for generating periodically modulated alternating current between the alternating power supply upper electrode 302 and the alternating power supply lower electrode 303; the alternating power supply upper electrode 302 and the alternating power supply lower electrode 303 are both of plate-shaped structures and are positioned in the vacuum chamber 304; the alternating power supply upper electrode 302 is provided with a hole 308, and the circularly polarized laser is incident into the vacuum chamber 304 through the hole 308; the vacuum chamber 304 is for holding an electronegative gas.
As shown in fig. 2, the function generator is connected to the rf impedance matching unit 301, and is configured to provide a periodic trigger signal, that is, the function generator controls the rf impedance matching unit 301 to input an alternating rf electric field between the ac power supply upper electrode 302 and the ac power supply lower electrode 303. The alternating radio frequency electric field is used for ionizing electronegative gas stored in the vacuum chamber 304 and oscillating the ionized electronegative plasma to form a periodic density modulation structure.
Further, a gas input port 306 and a gas output port 307 may be disposed on the vacuum chamber 304, and each of the gas input port 306 and the gas output port 307 is provided with a gas-tight structure for charging and discharging electronegative gas to the vacuum chamber 304 according to actual needs.
Further, the electronegative gas plasma generating unit 2 according to the embodiment of the invention is provided, wherein the alternating power supply upper electrode 302 is close to the top surface of the vacuum chamber 304, the alternating power supply lower electrode 303 is close to the bottom surface of the vacuum chamber 304, and the alternating power supply lower electrode 303 is grounded through the alternating power supply ground electrode 305.
Further, a hole 308 is formed in the alternating power supply upper electrode 302, so that the adjusted circular polarized laser enters the region formed by the alternating power supply upper electrode 302 and the alternating power supply lower electrode 303 through the alternating power supply upper electrode 302. The shape of the hole 308 is not particularly limited in the embodiments of the present invention, and may be a circle, and the diameter of the hole is slightly larger than the diameter of the spot when the circularly polarized laser passes through the hole.
The pulse width modulation device based on the electronegative gas plasma provided by the embodiment of the invention provides the electronegative gas plasma generating unit 2 formed by simple components to obtain the periodic density modulation structure, so that the energy consumption of dozens of paths of lasers for preparing the periodic density modulation structure is effectively reduced, and the miniaturization of the whole equipment is realized.
Based on the content of the above embodiments, as an alternative embodiment, a power amplifier is further disposed between the radio frequency impedance matching unit 301 and the function generator.
Since a high-voltage environment is required for constructing an alternating radio-frequency electric field in the electronegative gas plasma 2, a power amplifier is arranged between the radio-frequency impedance matching unit 301 and the function generator in the embodiment of the invention to provide a periodically-varying modulated high voltage, reduce impedance loss and improve transmission efficiency.
The function generator may be configured to generate an alternating current signal, amplify the alternating current signal by the power amplifier, and obtain a modulated high voltage by the radio frequency impedance matching unit 301, so that the electronegative gas in the vacuum chamber 304 is ionized to obtain a plasma, and the plasma is continuously oscillated to obtain a periodic density modulation structure. Specifically, the amplified alternating voltage signal is output by the radio frequency impedance matching unit 301, so that a periodically modulated radio frequency alternating electric field, that is, a driving electric field on the right side of the medium sign in formula (2), is formed between the alternating power supply upper electrode 302 and the alternating power supply lower electrode 303. Under the periodic driving action of the electric field, positive and negative charged ions can realize resonance with the radio frequency alternating electric field, so that the ions are separated in space step by step to form dipole moment. Through analysis of simulation results, a spatial density periodic modulation structure is formed between the two electrodes after approximately tens of radio frequency periods.
Based on the content of the above embodiments, as an alternative embodiment, the first pulse optical path adjusting unit 2 includes but is not limited to: a first polarization beam splitter 201, a first quarter glass 202, a first focusing lens 203 and a first quartz window 204 which are connected in sequence on an optical path; the first polarization beam splitter 201 may be set to be 45 ° with the pump laser, so as to deflect the optical path by 90 °, and to implement injection and output based on polarization of the optical path at the same time; the function generator is connected with the pump laser 1 and is used for uniformly allocating the pump laser 1 and the radio frequency impedance matching unit 301.
As shown in fig. 2, the pump laser 1 is triggered by the function generator and then performs trigger signal selection, so as to obtain the polarized pulse laser of kHz, which lags behind the pump signal of MHz in time sequence, and may generate the polarized pulse laser of kHz after receiving the pump signal of MHz and oscillating corresponding to 100 electrical cycles.
The first polarization beam splitter 201 has polarization-dependent characteristics, and can be used for emitting incident polarized pulse laser into two mutually orthogonal laser beams, and because the first polarization beam splitter 201 is mainly used for deflecting an optical path by 90 °, injection and output based on polarization of the optical path are simultaneously realized. Therefore, one of the acquired laser beams after polarization selection is perpendicular to the pump laser beam. The generated polarized pulse laser is reflected after being subjected to polarization selection by a first polarization beam splitter 201, and is changed into circular polarized laser after passing through a quarter glass sheet 202; the light spot of the original polarization laser is large, passes through the first quartz window plate 204 after being focused by the first focusing lens 203, and generates stimulated Brillouin scattering through the hole 308 on the upper electrode of the alternating power supply and the sound field of the periodic density modulation structure, so as to generate output pulse after pulse width modulation.
Based on the content of the above embodiment, as an alternative embodiment, the output pulse propagates along the opposite direction of the propagation direction of the incoming circularly polarized laser, and after passing through the hole 308, the first quartz window 204, the first focusing lens 203, the first quarter glass 202 and the first polarizing beam splitter 201 in sequence, is converted into the pulse width modulated linearly polarized pulsed laser.
Output pulses (high-power scattered light) generated by the stimulated brillouin scattering are transmitted in the reverse direction of the transmission direction of the circularly polarized laser, sequentially pass through the hole 308 of the upper electrode of the alternating power supply and the first quartz window plate 204, and are converted into parallel light by the first focusing lens 203. In the present embodiment, an approximation process is employed, that is, an output pulse after the stimulated brillouin scattering occurs is processed as a point light source, that is, from the point light source, through the first focusing lens 203 in accordance with a divergence angle similar to the original path. At this time, since the point light source is positioned at the focal point of the first focusing lens 203, the lens focal length coincides with the output pulse propagation length, and thus the output pulse can be converted into parallel light. The parallel circularly polarized light is converted into linearly polarized light perpendicular to the original incident direction by the first quarter-glass 202. At this time, the linearly polarized pulse laser beam perpendicular to the polarization direction of the incident pump light is output from the transmission polarization beam splitter 201, and thus an output signal after pulse width compression with frequency shift (shift amount corresponding to the frequency of the sound field) can be obtained.
According to the pulse width modulation device based on the electronegative gas plasma, provided by the embodiment of the invention, the possibility that high-power scattered light enters the pump laser again to damage the laser is avoided by arranging the transmission polarization spectroscope.
Based on the content of the foregoing embodiment, the pulse width modulation apparatus based on electronegative gas plasma according to the embodiment of the present invention further includes a seed laser and a second pulse optical path adjusting unit, where the second pulse optical path adjusting unit is configured to adjust seed pulse laser output by the seed laser, so that the adjusted seed pulse laser and circular polarization laser coincide in a periodic density modulation structure.
As shown in fig. 3, the stimulated brillouin scattering process at lower energy can be achieved by adding a seed laser in the embodiment of the present invention. In the electronegative gas plasma-based pulse width modulation device not including the seed laser provided in the foregoing embodiments, the generation of the output pulse can be realized only by requiring the interaction of the incident pump laser in the periodic density modulation structure, but if the seed laser is provided in advance, the energy threshold of the stimulated brillouin scattering can be reduced, the generation difficulty of the process can be reduced, and thus the use threshold of the technique can be reduced.
The seed laser 4 provided in the embodiment of the present invention is configured to generate seed pulse laser, and the specific structure of the second pulse optical path adjusting unit is similar to the specific structure of the first pulse optical path adjusting unit in the above embodiment, and is mainly configured to adjust the passing seed pulse laser, at this time, in order to avoid mutual interference between the seed pulse laser and the pump pulse laser, a hole 309 corresponding to the hole 308 of the alternating power supply upper electrode is disposed on the alternating power supply lower electrode 303. The seed laser 4 may be the same as or different from the pump laser 1 in type, that is, the seed pulse laser and the pump pulse laser may also be the same as or different from each other, but in this embodiment, it is required that the seed pulse laser entering the periodic density modulation structure coincides with the circularly polarized laser.
According to the pulse width modulation device based on the electronegative gas plasma, provided by the embodiment of the invention, the seed laser 4 is independently arranged to provide the seed pulse laser, so that the difficulty in generation of stimulated Brillouin scattering is reduced, and the output pulse is more quickly obtained.
Based on the content of the foregoing embodiments, as an alternative embodiment, the second pulse optical path adjusting unit includes but is not limited to: a second polarization beam splitter 205, a second quarter glass 206, a second focusing lens 207 and a second quartz window 208 which are connected in sequence on the optical path; the second pbs 205 is mainly used to deflect the seed pulse laser by 90 ° to avoid the seed light returning. The function generator is connected with the seed laser 4 and is used for uniformly allocating the seed laser 4 and the pumping laser 1.
Specifically, the pump laser 1 outputs linearly polarized pulse laser to enter the periodic density modulation structure through the first pulse optical path adjusting unit 3; the seed pulse laser emitted by the seed laser 4 sequentially passes through the second polarization beam splitter 205, the second quarter glass 206, the second focusing lens 207 and the second quartz window 208 and enters the periodic density modulation structure. The function generator simultaneously carries out comprehensive adjustment on the seed laser and the pump laser, so that the seed pulse laser entering the periodic density modulation structure is contacted with the circular polarization pulse laser to realize the interaction of stimulated Brillouin scattering.
Based on the content of the foregoing embodiments, as an optional embodiment, the function generator is connected to the seed laser 4, and is configured to uniformly allocate the seed laser 4 and the pump laser 1, including: the frequency difference between the output laser frequencies of the seed laser 4 and the pump laser 1 is made equal to the phonon frequency generated by the electrostriction of the periodic density modulation structure.
Based on the contents described in the above embodiments, it can be seen that: the basic principle of the SBS method is that the frequency of the pump light is assumed to be omegapThe frequency of Stokes scattered light is ωsThe phonon frequency generated by electrostriction of the periodic density modulation structure is delta omega, and when the frequency meets the condition that delta omega is omegap-ωsSBS causes photon energy to be transferred from the pump light to the scattered light. Therefore, in the embodiment of the present invention, the same function generator is used to allocate the seed laser 4 and the pump laser 1, so that the frequency of the two laser beams finally entering the periodic density modulation structure satisfies that the frequency difference is equal to the phonon frequency generated by electrostriction of the periodic density modulation structure, so as to realize temporal and spatial coincidence, and the sound field caused by density modulation in the plasma is used to realize stimulated brillouin scattering of the pump laser, so as to achieve the characteristic of pulse compression of the pump laser.
The embodiment of the invention also provides a pulse width modulation method based on electronegative gas plasma, which comprises the following steps:
firstly, ionizing electronegative gas to generate electronegative plasma, and oscillating the generated electronegative plasma to obtain a periodic density modulation structure;
then, the pump laser is converted into the circularly polarized laser, and then is incident to the periodic density modulation structure, and the stimulated brillouin scattering occurs in the periodic density modulation structure, so that output pulses after pulse width modulation are generated.
As shown in fig. 4, in the embodiment of the present invention, a method for controlling pulse width modulation by a function generator is provided, and specifically, on one hand, a power amplifier is controlled by the function generator, and then the power amplifier is connected to a radio frequency impedance matching unit, and a periodic density modulation structure is generated in a plasma generating unit 2 by using the radio frequency and off-duty resonance principle; on the other hand, the output of the pump laser 1 is controlled by the same function generator, so that the output pump pulse laser light is used as a pump light source of stimulated brillouin scattering after passing through the first pulse light path adjusting unit 3; on the other hand, the same function generator is used as the output of the seed laser 4, so that the output seed laser is input to the periodic density modulation structure after passing through the second pulse light path adjustment unit and coincides with the adjusted pumping circle laser in space and time to be used as the seed laser of the stimulated brillouin scattering, the threshold value of the stimulated brillouin scattering is reduced, and the efficiency of pulse width modulation is effectively improved.
Based on the content of the above embodiments, as an alternative embodiment, the pump laser 1 and the seed laser 4 are both frequency broadband tunable lasers.
Specifically, in the embodiment of the present invention, if the selected pump laser 1 and the seed laser 4 are tunable lasers with a tunable frequency bandwidth, a pulse width compression function with a tunable output frequency can be realized, that is, the pulse width of the pulse can be selectively tuned.
Based on the contents of the above embodiments, as an alternative embodiment, the pump laser 1 is a titanium sapphire laser with a center wavelength of 800 nm; the focal length of the first focusing lens 203 is slightly longer than half of the distance between the alternating power supply upper electrode and the alternating power supply lower electrode.
Specifically, the pump laser is triggered by the function generator and then signal selection is performed, so that pulse laser with the kHz magnitude can be obtained. The frequency of the pump laser can be selected from a titanium sapphire laser with a central wavelength of 800 nanometers (the frequency of the laser can be selected and not limited, and other lasers with certain pulse width can be adopted).
Further, the focal length of the first focusing lens 203 depends on the length of the whole vacuum chamber 304, and in order to ensure that the first quartz window piece 204 is not damaged due to the smaller laser pulse spot, the first focusing lens 203 is usually disposed closer to the first quartz window piece 204, and the focal length of the first focusing lens 203 should be slightly larger than half of the distance between the upper alternating power supply electrode 302 and the lower alternating power supply electrode 303 in the vacuum chamber 304.
Finally, it is to be noted that: change of dielectric density caused by acoustic wave when pump laser passes through periodic density modulation structure
Resulting in a change of the refractive index properties of the periodic density modulation structure and ultimately a non-linear polarization response P inside the periodic density modulation structureNLΔ E. Substituting the expression of delta and introducing electrostriction coefficient
The nonlinear polarization term introduced by the periodic density modulation structure of the density periodic modulation can be expressed as:
meanwhile, the driving equation of the acoustic wave field after receiving the electrostrictive force of the pumping optical wave field can be expressed as density fluctuation Δ ρ
Substituting the formula 3 into the pumping light field, and combining the formula 4 to obtain the coupling equation of the electronegative gas molecular plasma and the pumping light. The simulation of the experimental result of the pulse width modulation can be realized by solving the coupling equation.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A pulsed pulse width modulation device based on electronegative gas plasmas, comprising: the device comprises a pump laser, an electronegative gas plasma generating unit and a first pulse light path adjusting unit;
the electronegative gas plasma generating unit is used for ionizing electronegative gas to generate electronegative plasma, and oscillating the electronegative plasma to obtain a periodic density modulation structure;
the first pulse light path adjusting unit is used for converting the pump laser generated by the pump laser into circular polarization laser, and then the circular polarization laser is incident to the electronegative gas plasma generating unit, so that the circular polarization laser and the periodic density modulation structure generate stimulated Brillouin scattering, and output pulses after pulse width modulation are generated.
2. The electronegative gas plasma-based pulse width modulation device of claim 1, wherein the electronegative gas plasma generating unit comprises a radio frequency impedance matching unit, an alternating power supply upper electrode, an alternating power supply lower electrode, and a vacuum chamber;
the radio frequency impedance matching unit is connected with the function generator and the upper electrode of the alternating power supply and is used for generating periodically modulated alternating current between the upper electrode of the alternating power supply and the lower electrode of the alternating power supply;
the alternating power supply upper electrode and the alternating power supply lower electrode are both of plate-shaped structures and are positioned in the vacuum chamber; the alternating power supply electrode is provided with a hole, and the circularly polarized laser is incident into the vacuum chamber through the hole;
the vacuum chamber is used for storing the electronegative gas.
3. The electronegative gas plasma-based pulse width modulation device of claim 2, wherein a power amplifier is further disposed between the radio frequency impedance matching unit and the function generator.
4. The electronegative gas plasma-based pulse width modulation device according to claim 2, wherein the first pulse optical path adjustment unit comprises a first polarizing beam splitter, a first quarter glass, a first focusing lens, and a first quartz window, which are connected in sequence on the optical path;
the first polarization beam splitter deflects the optical path by 90 degrees so as to realize injection and output based on polarization of the optical path simultaneously;
the function generator is connected with the pump laser and used for uniformly allocating the pump laser and the radio frequency impedance matching unit.
5. The electronegative gas plasma-based pulse width modulation device according to claim 4, wherein the output pulse propagates in a direction opposite to a propagation direction of the circularly polarized laser, and is converted into the pulse width modulated linearly polarized pulse laser after passing through the aperture, the first quartz window, the first focusing lens, the first quarter glass, and the first polarization beam splitter in sequence.
6. The electronegative gas plasma-based pulse width modulation device according to claim 4, further comprising a seed laser and a second pulse optical path adjusting unit, wherein the second pulse optical path adjusting unit is configured to adjust the seed pulse laser output by the seed laser such that the adjusted seed pulse laser coincides with the circularly polarized laser in the periodic density modulation structure.
7. The electronegative gas plasma-based pulse width modulation device according to claim 6, wherein the second pulse optical path adjustment unit comprises a second polarizing beam splitter, a second quarter glass, a second focusing lens, and a second quartz window sequentially connected in the optical path;
the second polarization beam splitter is used for deflecting the seed pulse laser by 90 degrees;
and the function generator is connected with the seed laser and is used for uniformly allocating the seed laser and the pumping laser.
8. The electronegative gas plasma-based pulse width modulation device of claim 7, wherein the function generator is connected to the seed laser for uniform deployment of the seed laser and the pump laser, comprising:
and enabling the output laser frequency difference of the seed laser and the pump laser to be equal to the phonon frequency generated by electrostriction of the periodic density modulation structure.
9. The electronegative gas plasma-based pulse width modulation device of claim 6, wherein the pump laser and the seed laser are frequency broadband tunable lasers.
10. A pulse width modulation method based on electronegative gas plasma is characterized by comprising the following steps:
ionizing electronegative gas to generate electronegative plasma, and oscillating the electronegative plasma to obtain a periodic density modulation structure;
and after the pumping laser is converted into the circular polarization laser, the circular polarization laser is incident to the periodic density modulation structure and generates stimulated Brillouin scattering, and then output pulse after pulse width modulation is generated.
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