Laser pumped light source and method for laser ignited plasma

Abstract

In a laser-pumped plasma light source (100) having a gas-filled chamber (1), the device for plasma ignition is a solid-state laser system (7) which generates two pulsed laser beams (8, 9) focused into the chamber (1): a first beam (8) is generated in free-running mode, a second beam (9) is generated in Q-switched mode. The solid-state laser system (7) comprises a single active element (10), a radiation source (11) for pumping the active element (10) and an optical resonator (12, 13) providing a plurality of passages of the laser beams through the active element (10). One optical resonator (13) is equipped with a Q-switch (14) mounted on the path of the second beam (9) so that the Q-switch overlaps only a part (15b) of the cross section of the laser beam.

Description

TECHNICAL FIELD

[0001] The present invention relates to a broadband light source with continuous optical discharge (COD) and a method of starting a plasma ignition maintained by continuous laser radiation. BACKGROUND

[0002] One of the challenges related to the creation of high-brightness COD-based light sources relates to the reliable starting ignition of the COD.

[0003] For example, from patent US 9368337 published on June 14, 2016, in a laser-pumped plasma light source, two needle electrodes located on the axis of the transparent chamber are used to start the plasma ignition, between which a short-lived arc discharge is generated. A CW laser beam is focused in the gap between the two electrodes in the center of the cavity. The light source is characterized by high brightness and ease of use. The latter is mainly due to the fact that the quartz chamber or bulb with two electrodes containing a gas, in particular high-pressure Xe (10 atm or more, i.e. 1 MPa or more), is a commercially available product.

[0004] However, the relatively cold electrodes located in the vicinity of the high-temperature plasma region perturb the convective gas flow in the chamber, thus weakening the spatial and energy stability of the laser-pumped plasma light source. In addition, the presence of electrodes in the vicinity of the radiation plasma region is characterized by limiting the "dead" spatial angle of the plasma radiation exit. Moreover, sputtering of the electrode material can lead to a decrease in the transparency of the bulb walls and, accordingly, to the degradation of the light source over time.

[0005] The high-brightness broadband light source from patent US 9357627 published on May 31, 2016, largely overcomes this drawback. In its embodiment, after the COD ignition, the laser beam focusing region and, accordingly, the radiation plasma region move from the gap between the ignition electrodes towards the chamber walls. By choosing the relative positions of the laser beam, the chamber axis and the radiation plasma region, high spatial and power stability of the broadband laser-pumped plasma light source is provided.

[0006] However, the need to move the radiation plasma region complicates the light source design and operation. In addition, this makes it more difficult to use sharp focusing of the laser beam, which can limit the achievement of high brightness of the light source. The drawbacks of the chamber containing the electrodes also include complex technology for sealing the metal / glass joint and a complex chamber shape that creates a stress concentration, which leads to a lower strength of the chamber when operating at high gas pressure.

[0007] The electrodeless laser-pumped plasma light source known from patent application JPS61193358 published on August 27, 1986, does not have the above-mentioned drawbacks, in which the laser is used to start the plasma ignition and COD maintenance.

[0008] However, the threshold power of laser radiation required for plasma ignition is usually about ten to several hundred kW or higher, while the intensity of laser radiation sufficient for maintaining the COD is usually only several tens of watts. Therefore, using the same laser with high output power for both plasma ignition and COD maintenance leads to a shortened lifetime of the light source (when the entire laser power is used for COD maintenance), or is redundant, expensive, and therefore impractical if only a small fraction of the total laser power is used to maintain the COD.

[0009] Patent US10057973 published on August 21, 2018 proposes to overcome this challenge by using a single CW laser with power less than 250 W and wavelength less than 1.1 pm. It is suggested to provide for COD ignition and maintenance by sharp focusing of the CW laser beam, the cross dimension of the focal region being less than 1-15 microns (i.e. 1-15 pm), the length of the focal region being 6 microns or less (i.e. 6 pm or less).

[0010] However, this solution is not universal, as the requirements for laser focusing are very high and do not guarantee high functional reliability of the proposed light source. In addition, the laser power of about 250 W provided to the light source can be too high for various applications.

[0011] The light source known from patent FR2554302 published on May 3, 1985 overcomes these drawbacks, in which a focused pulsed laser beam used for initial plasma ignition or optical breakdown serves as a means of plasma ignition, and a CW laser is used for COD maintenance. The above method eliminates the problem of the lifetime of the laser-pumped plasma light source.

[0012] However, both plasma ignition and ensuring high brightness of the laser-pumped plasma light source require sharp focusing of the laser beam. Therefore, extremely precise adjustment of the focal regions of pulsed and CW lasers is required. This leads to the complexity of laser ignition and poor reliability, making stable COD ignition in a high-brightness light source a problem.

[0013] The light source from patent US10244613 published on May 25, 2017 partially overcomes these drawbacks. In embodiments of the invention, the beams of one or more ignition lasers and one or more CW lasers used for COD maintenance are introduced into optical fibers used to transfer radiation of said lasers into a condensing or focusing optical system. In said device, if the wavelengths of said lasers are similar, superposition of the focal regions of pulsed and CW lasers is achieved.

[0014] However, if the wavelengths of pulsed and CW lasers are different, their focal regions will diverge due to chromatic aberration. In addition, the transmission of high-power laser pulses (hundreds of kW) for reliable COD ignition through optical fibers can lead to fiber damage, which determines the drawbacks of this solution.

[0015] The closest technical solution (occasionally referred to herein as a prototype) is the light source in patent RU2732999 (also US10770282B2) published on September 28, 2020, where the plasma ignition device is a solid-state laser system that can generate two laser beams focused in the chamber. The laser beam generated in the Q-switch mode is designed to optically break the gas. At the same time, the laser beam generated in the free-running mode cannot be optically broken itself, this laser beam is designed to generate plasma after optical breakdown, the volume and density of which are sufficient to maintain its stability by a continuous laser. In other words, the joint action of two laser beams generated in the giant pulse mode and free-running leads to the formation of plasma, the burning of which is picked up by a continuous laser. Thus, a reliable electrodeless ignition of a laser-pumped plasma light source is provided. This allows to create a high-brightness broadband light source with high spatial and energy stability and collection of plasma radiation at large spatial angles.

[0016] However, the presence of two active elements complicates the solid-state laser system and light source, reducing the reliability and convenience of its operation. When using a common resonator of the pulsed laser system, there are quite strict requirements for the parallelism of the ends of the two active elements made in the form of rods. Another problem in obtaining two parallel beams is the difference in thermal effects in different rods. In addition, the use of two pulsed laser beams spatially separated complicates their introduction into the chamber together with the continuous laser beam, requiring the use of a dichroic mirror. The latter, in turn, leads to the need to use polarized laser radiation passing through the dichroic mirror to reduce reflection losses and imposes restrictions on the wavelength selection of the ignition and continuous laser radiation, making it difficult to use pulsed and CW lasers with similar radiation wavelengths. SUMMARY

[0017] The technical problem solved by the invention relates to creating a highly reliable laser ignition for continuous optical discharge and developing a high-brightness, high-stability laser-pumped plasma light source on this basis.

[0018] The technical result of the invention consists in ensuring the simplification of the light source design, increasing its reliability and ease of use, and creating an electrodeless high-brightness broadband light source with high spatial and power stability on this basis.

[0019] This object can be achieved by a laser-pumped plasma light source, which includes a chamber at least partially filled with an optically transparent gas, a radiation plasma region maintained in the chamber by a focused beam of a continuous wave (CW) laser, and a device for plasma ignition.

[0020] The light source is characterized in that the device for plasma ignition is a solid-state laser system, which generates two pulsed laser beams and focuses them into the chamber; one of the two pulsed laser beams is generated in a free-running mode, and the other is generated in a Q-switched mode.

[0021] The solid-state laser system comprises a single active element (i.e. only one active element), a radiation source for pumping the active element, and an optical resonator, which provides multiple passes (i.e. repeated passes) of the intracavity laser beam through the active element; the optical resonator is equipped with a Q-switch mounted in the path of the intracavity laser beam, so that the Q-switch overlaps only a part of the cross-section of the intracavity laser beam.

[0022] In embodiments of the invention, the Q-switch is a saturable absorber made of a Cr 4+ :YAG crystal.

[0023] In embodiments of the invention, the Q-switch overlaps a small part of the cross-section of the intracavity laser beam, not more than 30% of its area.

[0024] In embodiments of the invention, the focusing optical element focuses the pulsed laser beam and the CW laser beam into the chamber, and the CW laser beam on the focusing optical element does not intersect the pulsed laser beam on the focusing optical element.

[0025] In embodiments of the invention, the deflection mirror is mounted in the beam path of the CW laser outside the beam path of the pulsed laser beam.

[0026] In embodiments of the invention, the deflection mirror is mounted in the beam path of the pulsed laser outside the beam path of the CW laser.

[0027] In embodiments of the invention, the output power of the CW laser sufficient for igniting and maintaining the radiation plasma does not exceed 30 W.

[0028] In embodiments of the invention, the axis of the CW laser focusing beam is directed vertically upwards or close to the vertical direction, at an angle of not more than 10 degrees to the vertical.

[0029] In embodiments of the invention, the density of the gas particles in the chamber is less than 90·10 19 cm -3 , which corresponds to a gas pressure of 33 atm (i.e. 3.3 MPa) at room temperature, and the temperature of the chamber surface is not lower than 600 K.

[0030] In embodiments of the invention, the temperature of the chamber surface does not exceed 900 K, and the density of the gas particles is not less than 45·10 19 cm -3 , which corresponds to a gas pressure of 16.5 atm (i.e. 1.6 MPa) at room temperature.

[0031] In a preferred embodiment of the application, the radiating plasma is characterized by a high spectral brightness (greater than 50 MW / (mm 2 • nm • sr) and a relative instability of the brightness sigma (less than 1 %).

[0032] In a preferred embodiment of the application, the gas belongs to a group of noble gases. The group includes the elements Xenon, Krypton, Argon, Neon, but the gas can also be a mixture of these elements.

[0033] In another aspect, the application relates to a method for plasma ignition in a laser-pumped plasma light source, comprising: directing a focused beam of continuous wave (CW) laser light into a gas-filled chamber, at least a portion of which is optically transparent, plasma ignition and stable maintenance of the radiating plasma by using the focused beam of CW laser light.

[0034] The method is characterized in that the plasma is ignited by a solid-state laser system having a single active element, which system generates two parallel pulsed laser beams focused into the chamber; one of the two pulsed laser beams is generated in free-running mode, the other pulsed laser beam is generated in Q-switched mode by a Q-switch mounted in the optical cavity, overlapping only a portion of the cross section of the intracavity laser beam. In other words, the first beam is generated in free-running mode, the second beam is generated in Q-switched mode.

[0035] In an embodiment of the application, the focused optical element focuses the pulsed laser beam and the CW laser beam into the region for maintaining the radiating plasma, the CW laser beam on the focused optical element does not intersect the pulsed laser beam on the focused optical element.

[0036] When the light source is adopted in the proposed form, thanks to the use of a laser system having only one active element for plasma ignition, the number of elements of the laser system is reduced, the design is simplified as much as possible, the reliability of the device for laser plasma ignition and of the radiating source is increased. Compared to the prototype, the problems related to the combination of the foci of the two pulsed laser beams due to the different prismaticity of the two active elements and the possible separation of the two active elements due to thermal effects are eliminated.

[0037] At the same time, since the two pulsed laser beams are generated by one active element, the maximum convergence in space of the two pulsed laser beams allows not to use a dichroic mirror to inject the pulsed and continuous laser beams into the chamber, thus eliminating the need to use polarized laser radiation that passes through the dichroic mirror.

[0038] In addition, the use of a passive Q-switch in the form of a saturable absorber in the form of a Cr 4+ : YAG crystal (in particular a flat plate) provides for the automatic operation of the two-beam laser system.

[0039] These also simplify the design of the radiation source, increasing its reliability and ease of operation.

[0040] Reliable ignition of the continuous optical discharge is achieved due to the following factors. The optical breakdown is provided by the laser beam generated in the Q-switched mode. However, ignition of the COD with only one laser beam is problematic. One of the reasons is the difficulty of combining the focal region of the continuous laser with the optical breakdown region, the size of which is usually very small, no more than about 50 pm. Even if the focal regions of the pulsed and continuous laser beams are combined, ignition of the COD with only one laser beam is still difficult to achieve. This is because the optical breakdown produced by the laser radiation is explosive. The explosive process, especially the shock wave, can cause the optical discharge sustained by the low-power (usually no more than 300 W) continuous laser to be extinguished. According to the present invention, this problem is solved by the fact that the laser beam generated in the free-running mode and not capable of optical breakdown itself provides plasma ignition after the optical breakdown by the laser beam generated in the Q-switched mode. The parameters of the laser beam generated in the free-running mode are chosen so that the optical discharge sustained by it does not have explosive phenomena itself, while resisting disturbances caused by the previous optical breakdown. At the same time, after the end of the ignition laser pulse, the volume and density of the plasma provided by the laser beam generated in the free-running mode are sufficient to achieve reliable stable maintenance by a continuous laser with a lower power (up to 30 W).

[0041] Thus, reliable electrodeless ignition of the continuous optical discharge is achieved. The elimination of the electrodes reduces disturbances of the convective gas flow in the vicinity of the emitting plasma region, simplifies the chamber, allows its design to be optimized to reduce the turbulence of the convective gas flow and minimize optical aberrations, especially when the plasma radiation is output through the transparent part of the cavity, and increases the spatial angle of plasma radiation collection.

[0042] According to the present invention, continuous generation of plasma radiation with high spectral brightness (more than 50 MW / (mm 2 ·nm·sr) and relative brightness instability (less than 0.1%) is achieved by the fact that the density of gas particles in the chamber should be as low as possible, and the temperature of the surfaces in the chamber should be as high as possible during operation, while ensuring that the gas pressure in the chamber is about 50 atm. There is more.

[0043] All this makes it possible to create the highest brightness broadband light source with a large spatial angle of plasma radiation collection, characterized by the highest spatial and energy stability.

[0044] The specific purposes, features and advantages of the present invention, as well as the invention itself, will be easier to understand from the subsequent description of the implementation options of the present invention, as shown in the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0045] The technical nature and operating principle of the proposed device are explained by means of the attached drawings, in which:

[0046] Figure 1 is a schematic view of a light source with a pulsed solid-state laser system for plasma ignition and a cross-sectional view of a plasma laser ignition device in the region of its optical resonator,

[0047] Figure 2 is a characteristic waveform diagram of the radiation intensity of a solid-state laser system for plasma ignition.

[0048] In the drawings, the matching elements of the device have the same reference numerals.

[0049] These drawings do not encompass and do not limit the range of all options for implementing the technical solution, but are only illustrative examples of specific cases of implementing the technical solution. DETAILED DESCRIPTION

[0050] The present description is intended to explain the embodiments of the present application and is not intended to limit the scope of the present application.

[0051] According to an example of the present application ( Figure 1 ), the laser pump light source 100 comprises a chamber 1 filled with a high-pressure gas. At least a portion of the chamber 1 is optically transparent. Figure 1 A variant of the chamber made of an optically transparent material, for example fused silica, is shown. In the chamber 1, there is a region of a radiation plasma 2 maintained in the chamber by a focused beam 3 of a continuous laser 4, i.e. a CW laser.

[0052] At least one beam of plasma radiation 5 directed towards an optical radiation collection system 6 and used for further use exits from the chamber 1. The optical radiation collection system 6 can include an axially symmetric ellipsoidal mirror ( Figure 1 ), forming a beam of plasma radiation, which is transmitted, for example, by a fiber or a mirror system to an optical system using broadband plasma radiation.

[0053] The light source further comprises a plasma ignition device, the ignition device using a solid state laser system 7 having the function of generating two laser beams 8, 9 which are focused into the chamber 1. One of the two laser beams 8 (first beam) is generated in free running mode, the other laser beam 9 (second beam) is generated in Q-switched mode. In this case, the solid state laser system comprises only one active element 10 (i.e. a single active element), a radiation source 11 (e.g. a compact pulsed xenon lamp for pumping the active element) and an optical resonator with mirrors 12, 13 and a Q-switch 14. The optical resonator provides for multiple passes (i.e. repeated passages) of the laser radiation flux 15 through the active element 10. Since the resonator with mirrors 12, 13 can be regarded as a cavity between the mirrors, the flux 15 can be referred to as "intracavity laser beam". In this case, the Q-switch 14 is mounted in the path of the intracavity laser radiation flux 15 such that it covers only a fraction of the aperture of the laser radiation flux 15.

[0054] A part of the intracavity laser radiation flux 15a is free of the Q-switch 14 in its path, which leads to the generation of the laser beam 8 in free running mode.

[0055] Another part of the intracavity laser radiation flux 15b has the Q-switch 14 in its path, which leads to the generation of the laser beam 9 in Q-switched mode.

[0056] From the cross section A-A of the solid state laser system 7 in Figure 1 It can be seen that in embodiments of the invention the apertures of the laser radiation fluxes 15a and 15b and the respective apertures of the laser radiation beams 8, 9 coming out of the resonator have the form of circular segments with different cross sections.

[0057] The laser beam 9 generated in Q-switched mode and the laser beam 8 generated in free running mode are focused into the chamber 1, i.e. into the chamber region designed for maintaining the emitting plasma.

[0058] The laser beam 9 generated in Q-switched mode is designed for initial plasma ignition or optical breakdown in the chamber 1. The laser beam 8 generated in free running mode is designed to ignite the plasma after the optical breakdown by the laser beam 8 generated in Q-switched mode. In this case, the Q-switch 14 preferably covers only a small fraction 15b of the aperture of the laser radiation flux 15, preferably not more than 30% of its area (at mirror 13), in order to ensure an optimum energy ratio of the laser beams 8 and 9 from the point of view of plasma ignition.

[0059] The laser beam 8 generated in free-running mode is designed to ignite the plasma after optical breakdown of the laser beam 9 generated in Q-switching mode. In this embodiment of the invention, the wavelength of the pulsed laser radiation generated in the solid-state laser system 7 is λ1 = 1.064 μm.

[0060] Q-switch 14 can be passive, made of orthotropic material, and represent a saturated absorber, for example, chromium-doped: Cr 4+ The form of YAG (aluminum garnet) crystals. The Q-switch 14 can be made in the form of a plate, but is not limited to this option. In other embodiments of the invention, the Q-switch 14 can be active.

[0061] In a preferred embodiment of the invention, all laser beams are focused onto the chamber region for maintaining the emitting plasma 2 (i.e., the radiating plasma) by a single focusing optical element 16 (e.g., in the form of a condenser lens 16, but not limited to this option).

[0062] exist Figure 1 In the illustrated embodiment of the device, a rotating mirror 18 mounted outside the paths of the pulsed laser beams 8 and 9 is used to guide the continuous laser beam 17 into the chamber. In these embodiments of the invention, another rotating mirror mounted directly outside the beam path may also be used to guide the pulsed laser beams 8 and 9 into the chamber. The continuous laser beam 17 guided to the focusing optics 16 does not intersect with the beams 8 and 9 of the solid-state laser system, which are also guided to the focusing optics 16.

[0063] In this embodiment of the invention, known limitations associated with the use of dichroic mirrors are mitigated.

[0064] The plasma ignition method in a laser-pumped plasma source is implemented as follows. For example... Figure 1 As shown, for example, a total internal reflection rotating mirror 18 is used to guide the focused beam 3 of the continuous laser 4 into a chamber 1 containing high-pressure gas. Xenon or other inert gases and mixtures thereof (including metal vapors such as mercury) and various gas mixtures (including halogen-containing gas mixtures) are used as efficient plasma-forming media.

[0065] The plasma is ignited by a solid-state laser system 7 with an active element 10. After the radiation source 11, designed for pulse pumping of the active element 10, is turned on, two parallel laser beams 8 and 9 are generated, focused into the chamber, and directed into the region designed to sustain the emission plasma 2. In this case, a pulsed laser beam 8 is generated in free-running mode and another pulsed laser beam 9 is generated in Q-switched mode by using a Q-switch 14 mounted in a resonator with mirrors 12 and 13, providing a laser radiation flux 15 through multiple channels of the active element 10. The Q-switch 14 is mounted in the path of the laser radiation flux 15 within the cavity such that it covers only a portion of the aperture of the laser radiation flux 15b. In this case, the pulsed laser beam 9 generated in Q-switched mode is used to provide optical breakdown, and the pulsed laser beam 8 generated in free-running mode is used to ignite the plasma, with a volume and density sufficient to sustain a stable plasma through the focused beam 3 of the continuous laser 4.

[0066] In the fixed mode, high-brightness broadband radiation is output from the region of the emitting plasma 2 of continuous optical discharge through at least one useful plasma radiation, which is emitted through the optically transparent part of the chamber 1 and used for further purposes.

[0067] In the example of this invention, the indoor air pressure Xe at room temperature is 30 atm (i.e., 30 MPa); the wavelength of the continuous laser is λ. CW =0.808μm, power ranges from 30 watts to 100 watts.

[0068] The characteristic time dependence of the laser radiation power generated by a solid-state laser system is as follows: Figure 2 As shown. In this example, the laser beam generated in peak free-running mode has an energy of approximately 150 mJ, a duration of approximately 100 microseconds, and a radiation wavelength of λ1 = 1.064 μm. The laser beam generated in passive Q-switching mode has a time delay, characterized by a laser pulse energy of 3 mJ and a duration of 20 ns. The characteristic size of the optically broken-down plasma is 50–100 μm.

[0069] Optical breakdown mode does not provide reliable ignition for continuous optical discharge. Therefore, after optical breakdown, the plasma is ignited by a laser beam generated in free-running mode, with a volume (up to 1 mm). 3 ) and density (over 10) 18 cm -3 This is sufficient to maintain a stable plasma using a focused beam of continuous laser light. Preferably, such as... Figure 2As shown, the laser beam emission pulses produced in free-running mode end no earlier than 50 microseconds after the end of the laser beam emission pulses produced in Q-switched mode. The time of about 50 microseconds ensures the decay of the disturbances from the optical breakdown and the evolution of the plasma size and density to values sufficient for the maintenance of a stable plasma by the focused beam of relatively low-power continuous laser.

[0070] Other embodiments of the invention aim to further improve the laser-pumped plasma light source.

[0071] Since the radiation power of the solid-state laser system 7 when producing giant pulses does not allow the use of optical fiber to transmit its radiation, in the embodiments of the invention only the continuous laser has an optical fiber radiation output (not shown).

[0072] At the same time, the output of the continuous laser radiation 4 is preferably carried out in an optical fiber (not shown). At the output end of the optical fiber, the expanded laser beam is directed to a collimator (not shown), after which the expanded parallel beam of continuous laser is directed to the focusing optical element 16, for example in the form of an aspherical collection lens. The focusing optical element 16 provides sharp focusing of the beam 3 of the continuous laser 4, which is necessary to ensure high brightness of the light source.

[0073] Due to the laser ignition of the plasma and the absence of an ignition electrode, in the preferred version of the invention, the useful plasma radiation beam 5 is output from the chamber along all azimuths, as Figure 1 shown. This means that in the azimuthal plane passing through the area of the emitting plasma 2 perpendicular to the axis of the beam 3 of the continuous laser, the useful plasma radiation exits along all azimuths from 0 to 360 degrees. In the preferred embodiment of the invention, the flat opening angle of the useful plasma radiation beam 5 (in Figure 1 the plane of the drawing) is at least 90°. This means that the output of the useful plasma radiation beam 5 from the chamber 1 to the radiation collection system 6 is carried out with a spatial angle of at least 9 sr or more than 70% of the total solid angle.

[0074] In this embodiment, the axis of the focused beam 3 of the continuous laser is directed vertically upwards, i.e. against the force of gravity or close to vertical, within the range of ± 10°. Preferably, the chamber 1 is axisymmetric, and the axis of the focused beam 3 of the continuous laser is aligned with the axis of symmetry of the chamber. When performed in the proposed form, the maximum stability of the radiation power of the laser-pumped light source is achieved.

[0075] According to the present application, in the mode of maintaining the radiation plasma, the temperature of the inner surface of the chamber is in the range of 600 to 900 K or higher, if the higher temperature does not have a significant negative impact on the strength of the chamber and its transparency. The positive effect achieved by the present application is due to the fact that for a given amount of gas in a chamber of a given volume, the gas pressure increases with the temperature of the chamber. Since the temperature of the emission plasma is practically fixed (about 15000 K, and it is difficult to try to increase this temperature, since it is accompanied only by an increase in the volume of the plasma), and the pressure in the plasma is equal to the pressure in the chamber, the density of the emission plasma increases with the increase in the pressure in the chamber, which means an increase in the temperature of the chamber walls. The increase in the density of the emission plasma leads to an increase in the volume luminosity of the emission plasma, and thus to an increase in the brightness of the light source in a wide optical range, where the emission plasma is practically transparent.

[0076] At a given temperature of the chamber, the brightness can also be increased by increasing the gas pressure. However, in this case, the density of the gas and the refraction associated with this density will increase, both in the region of the emission plasma and in the periphery, which will lead to a significant instability (fluctuations) of the brightness of the light source.

[0077] In order for the relative brightness instability to be sufficiently small (σ≤0.1%), the density of the gas particles in the chamber is selected to be lower than the experimentally determined upper limit of 90·10 19 cm -3 , which corresponds to a gas pressure of 33.5 atm (i.e. 3.3 MPa) at the temperature of the chamber. At the same time, in order to obtain a spectral brightness of the light source close to the maximum that can be achieved at a given temperature (more than 50 MW / (mm 2 · nm· sr), the gas pressure and the corresponding density of the emission plasma must be sufficiently high to ensure an optimal gas pressure of about 50 bar or more in steady-state operation. For this purpose, the density of the gas particles in the chamber is selected to be higher than the experimentally determined lower limit of 46·10 19 cm -3 , which corresponds to a gas pressure of at least 17 atm (i.e. 1.7 MPa) at the temperature of the chamber.

[0078] Therefore, in order to ensure a high spectral brightness and a lower relative brightness instability, the density of the gas particles should be as low as possible, the temperature of the inner surface of the chamber should be as high as possible during operation, while ensuring a gas pressure in the chamber of about 50 bar or more.

[0079] According to the present application, it is preferable to use inert xenon as a gas, which ensures safe operation and long life of the light source. In addition, compared to the emission plasma of other inert gases, the Xe plasma is characterized by the highest light output in a wide spectral range, including the ultraviolet, visible and near-infrared regions.

[0080] Preferably, high-efficiency near-infrared diode lasers are used as continuous lasers 4. The choice of the preferred wavelength is selected from the two wavelengths of 976 nm and 808 nm, for which high-efficiency diode lasers are available due to the following factors. Strong Xe absorption lines are located in the vicinity of the laser wavelength of 976 nm, with lower states increasing with temperature. At 808 nm, these lines are further from the absorption line, so that, at a given laser power, a sufficient absorption is achieved at a higher plasma density and temperature than in the case of 976 nm, sufficient to sustain the optical discharge.

[0081] Therefore, in a preferred embodiment of the application, the gas filling the chamber is xenon and the wavelength of the continuous laser is 808 nm.

[0082] In other variants, high-efficiency solid-state or fiber lasers can be used as continuous lasers. In this case, the wavelengths of the radiation from the continuous laser and the solid-state laser system can be close or coincident.

[0083] While ensuring a high degree of stability of the output parameters, the application also makes it possible to achieve maximum brightness of the broadband laser pumping light source, in particular by optimizing the shape and size of the electrodeless chamber. Therefore, in a preferred embodiment of the application, the outer and inner surfaces of the chamber or of its transparent portion have the shape of concentric spheres, and the region 2 of the emitting plasma is located at the center of these concentric spheres, as shown in Figure 1 In this version of the application, aberrations that distort the light ray course in the useful plasma radiation beam 5 are eliminated, increasing its brightness.

[0084] To ensure the output of plasma radiation in a wide spectral range from the ultraviolet to the near infrared, the optically transparent portion of the chamber is preferably made of the following materials: crystalline magnesium fluoride (MgF2), crystalline calcium fluoride (CaF2), crystalline sapphire or colorless sapphire (AI2O3), fused or crystalline quartz.

[0085] Overall, the application can ensure high reliability of the ignition of the plasma maintained by laser radiation and, on this basis, create an electrodeless high-brightness broadband light source with the highest spatial and energy stability and capable of collecting plasma radiation at spatial angles greater than 9 sr.

[0086] When the light source in the proposed form is used, the design is simplified as much as possible, the reliability of the device as a whole for the plasma laser ignition and the radiation source is increased. In comparison with the prototype, the problem of merging the focal points of two pulsed laser beams and separating two active elements of the laser system during operation is eliminated. The restrictions associated with the use of dichroic mirrors, which are usually used to inject several laser beams into the chamber, are eliminated. Overall, when the light source is performed in the proposed form, reliable ignition of the COD is achieved. This makes it possible to create an electrodeless high-brightness broadband laser pumping light source characterized by the highest spatial and energy stability while optimizing the conditions for maintaining the COD.

[0087] Industrial applicability

[0088] The high-brightness, high-stability laser pumping light source manufactured according to the present application can be used in various projection systems for spectral chemical analysis, spectral microanalysis of biological objects in biology and medicine, microcapillary liquid chromatography analysis, optical lithography process inspection, spectrophotometric measurements and other purposes.

Claims

1. A laser-pumped plasma source, comprising: A gas-filled chamber (1), at least a portion of which is optically transparent; a region (2) radiating plasma, which is maintained within the chamber by a focused beam (3) of a continuous-wave (CW) laser (4); and a device for plasma ignition, the laser-pumped plasma source being characterized by: The device used for plasma ignition is a solid-state laser system (7), which generates two pulsed laser beams (8) and (9) focused into the chamber; one of the two pulsed laser beams (8) is generated in free-running mode and the other pulsed laser beam (9) is generated in Q-switching mode. The solid-state laser system includes a single active element (10), a radiation source (13) for pumping the active element (10), and optical cavities (12, 13) providing an intracavity laser beam (15) through multiple channels of the active element; the optical cavities are equipped with Q switches (14) mounted on the path of the intracavity laser beam such that the Q switches overlap only a portion (15b) of the cross-section of the intracavity laser beam.

2. The light source according to claim 1, wherein the Q switch (14) is made of chromium-doped yttrium aluminum garnet crystal Cr 4+ : A saturable absorber made of YAG.

3. The light source according to claim 1, wherein the Q switch (14) overlaps with a small portion (15b) of the cross section of the intracavity laser beam (15), not exceeding 30% of its area.

4. The light source according to claim 1, wherein the pulsed laser beams (8), (9) and the beam (17) of the CW laser are focused into the chamber by a focusing optical element (16), and the beam (17) of the CW laser pointing to the focusing optical element does not intersect with the pulsed laser beams (8), (9) pointing to the focusing optical element.

5. The light source according to claim 1, wherein the deflector (18) is mounted on the path of the beam of the CW laser (4) outside the path of the pulsed laser beams (8), (9).

6. The light source according to claim 1, wherein the deflector (18) is mounted on the path of the pulsed laser beam outside the beam path of the CW laser.

7. The light source according to claim 1, wherein the output power of the CW laser (4) sufficient to ignite and sustain the radiative plasma does not exceed 30 watts.

8. The light source according to claim 1, wherein the axis of the focused beam (3) of the CW laser is vertically upward or nearly vertical, and the angle with the vertical direction does not exceed 10 degrees.

9. The light source according to claim 1, wherein the density of the gas particles in the chamber is less than 90.

10. 19 cm -3 This corresponds to an air pressure of 33 atm at room temperature, and the temperature of the inner surface of the chamber is not less than 600K.

10. The light source according to claim 1, wherein the temperature of the inner surface of the chamber does not exceed 900K, and the density of the gas particles is not less than 45.

10. 19 cm -3 This corresponds to an air pressure of 16.5 atm at room temperature.

11. The light source according to claim 1, wherein the radiating plasma is characterized by a strength greater than 50 MW / (mm²). 2 High spectral brightness (·nm·sr) and low relative instability of brightness σ less than 1%.

12. The light source according to claim 1, wherein the gas in the chamber belongs to a group of inert gases, including xenon, krypton, argon, neon, or mixtures thereof.

13. A method for plasma ignition in a laser-pumped plasma source, comprising: The focused beam (3) of a continuous wave (CW) laser (4) is guided into a gas chamber (1), at least a portion of which is optically transparent, for plasma ignition and stable maintenance of radiant plasma (2) via the focused beam of the CW laser. The invention is characterized in that... The plasma is ignited by a solid-state laser system (7) with a single active element (10), which generates two parallel pulsed laser beams (8), (9) focused into the chamber; one of the two pulsed laser beams (8) is generated in free-running mode, and the other pulsed laser beam (9) is generated in Q-switching mode by a Q-switch installed in the optical cavity, which overlaps only with a portion (15b) of the cross section of the intracavity laser beam (15).

14. The method according to claim 13, wherein the pulsed laser beam and the beam of the CW laser are focused by a focusing optics element (16) into a region (2) for maintaining the radiative plasma, and the beam (17) of the CW laser pointing to the focusing optics element does not intersect with the pulsed laser beams (8) and (9) pointing to the focusing optics element (16).

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