Laser pumped plasma light source and plasma ignition method

Abstract

The light source includes an inflated chamber having a radiation plasma region maintained by a focused beam of a CW laser. The plasma ignition device is a pulsed laser system that generates first and second laser beams focused inside the chamber. The first laser beam provides optical breakdown, and after the optical breakdown, the second laser beam ignites the plasma, whose volume and density are sufficient to maintain a stable plasma by the CW laser after the end of the second laser pulse. Preferably, the first laser beam is generated in a Q-switching mode, and the second laser beam is generated in a free-running mode. The technical results include ensuring high reliability of plasma ignition, creating on this basis an electrode-free high-brightness broadband light source with high spatial and power stability, and providing the ability to collect broadband plasma radiation within a spatial angle greater than 9 sr.

Description

Technical Field

[0001] The present invention relates to an electrodeless laser-pumped plasma light source for generating high-brightness light in the ultraviolet (UV), visible, and near-infrared (NIR) spectral bands, and a method for initiating plasma ignition. Background Art

[0002] Continuous optical discharge (COD) is a stable gas discharge sustained by laser radiation in a pre-generated relatively dense plasma. COD sustained by a focused beam of a continuous-wave (CW) laser is achieved in various gases, particularly in Xe at high pressures of 10 - 200 atm ("Continuous Optical Discharges at Very High Pressure" by Carlhoff et al., Physica 103C, 1981, pp. 439 - 447). Due to the high plasma temperature of approximately 20,000 K (Raizer, "Optical Discharges", Sov. Phys. Usp. 23(11), November 1980, pp. 789 - 806), the light source based on COD is the brightest continuous light source in a wide spectral range between approximately 0.1 μm and 1 μm. Compared with arc lamps, such laser-pumped plasma light sources not only have higher brightness but also longer lifetimes, making these light sources preferred for various applications.

[0003] One of the challenges associated with designing high-brightness laser-pumped plasma light sources involves generating an initial plasma that provides reliable ignition of the COD.

[0004] For example, as known from the patent U.S. 9368337 announced on June 14, 2016, in a laser-pumped plasma light source, two needle electrodes located on the axis of a transparent chamber are used to initiate plasma ignition, and a short-time arc discharge is generated between the two needle electrodes. The CW laser beam is focused in the center of the chamber, in the gap between the two electrodes. This light source is characterized by high brightness and ease of use. The ease of use is largely due to the fact that a quartz chamber or bulb with two electrodes containing gas (especially high-pressure Xe (10 atm or higher)) is a commercially available product.

[0005] However, the relatively cold electrodes located near the high-temperature plasma region can perturb the convective airflow in the chamber, thereby weakening the spatial and energy stability of the laser-pumped plasma light source. In addition, the presence of electrodes near the radiative plasma region is characterized by a "dead" space angle that limits the plasma radiative emission. Moreover, electrode material sputtering may cause a reduction in the transparency of the bulb wall and, correspondingly, lead to the degradation of the light source over time.

[0006] The high-brightness broadband light source known from the patent U.S.9357627 announced on May 31, 2016 largely overcomes this shortcoming. In its embodiment, after the COD is ignited, the laser beam focusing region and the corresponding radiation plasma region move from the gap between the ignition electrodes towards the chamber wall. By selecting 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.

[0007] 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 may limit the achievement of high brightness of the light source. The disadvantages of the chamber containing the electrodes also include the complex technology for sealing the metal / glass joints and the complex chamber shape that generates stress concentration, which results in lower chamber strength when operating at high gas pressure.

[0008] The electrodeless laser-pumped plasma light source in the patent application JPS61193358 published on August 27, 1986 does not have the above disadvantages, where the laser is used to initiate plasma ignition and COD maintenance.

[0009] However, the threshold power of the laser radiation required for plasma ignition is usually about ten to several hundred kilowatts or higher, while the laser radiation intensity sufficient for COD maintenance is usually only a few tens of watts. Therefore, using the same high-output power laser for both plasma ignition and COD maintenance either results in a shortened light source life (when all the laser power is used for COD maintenance) or is redundant and expensive. Thus, it is not practical if only a small part of the total laser power is used to maintain COD.

[0010] The patent U.S.10057973 announced on August 21, 2018 proposes to overcome this challenge by using a single CW laser with a power less than 250 watts and a wavelength less than 1.1 μm. This patent proposes to provide COD ignition and maintenance through sharp focusing of the CW laser beam, with the cross-sectional size of the focal region of the CW laser beam less than 1 - 15 microns and the length of the focal region 6 microns or less.

[0011] However, this solution is not universal because the requirements for laser focusing are very high and cannot guarantee high functional reliability of the proposed light source. In addition, the approximately 250 watts of laser power provided to the light source may be too high for various applications.

[0012] The light source known from the patent FR2554302 announced on May 3, 1985 overcomes these drawbacks. Among them, a focused pulsed laser beam for initial plasma ignition or optical breakdown is used as a plasma ignition device, and a CW laser is used to maintain COD. The above method eliminates the problem of the lifetime of the laser-pumped plasma light source.

[0013] However, both plasma ignition and ensuring high brightness of the laser-pumped plasma light source require clear focusing of the laser beam. Therefore, extremely precise adjustment of the focusing regions of the pulsed and CW lasers is required. This leads to complexity and poor reliability of laser ignition, making stable COD ignition in high-brightness light sources a problem.

[0014] The light source known from the patent U.S.10244613 announced on May 25, 2017 partially overcomes these drawbacks. In one embodiment of the invention, the beams of one or several ignition lasers for maintaining COD and the beams of one or several CW lasers are introduced into an optical fiber for transmitting the radiation of the lasers to a condenser or focusing optical system. In the said device, if the wavelengths of the lasers are similar, the focusing regions of the superimposed pulsed laser and CW laser are achieved.

[0015] However, if the wavelengths of the pulsed and CW lasers are different, their focusing regions will diverge due to chromatic aberration. In addition, transmitting high-power laser pulses (hundreds of kW) for reliable COD ignition through an optical fiber may cause damage to the optical fiber, which determines the drawbacks of this solution. Summary of the Invention

[0016] The technical problem to be solved by the present invention relates to creating a method and device for highly reliable laser ignition for continuous optical discharge, and on this basis, developing a highly bright and highly stable laser-pumped plasma light source.

[0017] The technical achievements of the present invention include ensuring high reliability of plasma ignition maintained by CW laser and creating an electrodeless high-brightness broadband light source with high spatial and power stability on this basis.

[0018] This object can be achieved by the proposed laser-pumped plasma light source, which includes: a high-pressure gas-filled chamber, at least a part of which is optically transparent; a radiation plasma region maintained in the chamber by a focused beam of a continuous-wave (CW) laser; at least one plasma radiation output beam (which can also be called a useful beam) leaving the chamber; and a plasma ignition device.

[0019] The light source is characterized in that the plasma ignition device is a pulsed laser system, which generates first and second laser beams focused in the chamber, wherein the first laser beam is arranged for optical breakdown of the gas, and the second laser beam is arranged for plasma ignition after the optical breakdown.

[0020] In one embodiment of the present invention, the first laser beam has a peak radiation power greater than 104 watts and a pulse length less than 0.1 microseconds.

[0021] In one embodiment of the present invention, compared with the first laser beam, the second laser beam has at least three times higher laser pulse energy and at least one order of magnitude lower laser peak power.

[0022] In a preferred embodiment of the present invention, the volume of the plasma ignited by the second laser beam exceeds the volume of the plasma generated by the first laser during optical breakdown by one order of magnitude or more.

[0023] In a preferred embodiment of the present invention, the volume and density of the plasma ignited by the second laser beam are sufficient to stably maintain the plasma through the focused beam of the CW laser.

[0024] In one embodiment of the present invention, the second laser beam provides a plasma size of up to about 1 mm (measured by the FWHM of the free electron density or the FWHM of the brightness profile of the luminous plasma region) and a plasma density of up to 10 18 cm -3 or higher (measured by free electrons per unit volume).

[0025] In one embodiment of the present invention, the output power of the CW laser does not exceed 300 watts.

[0026] In one embodiment of the present invention, the radiation pulse of the second laser beam ends no earlier than 50 μs after the end of the radiation pulse of the first laser beam.

[0027] In a preferred embodiment of the present invention, the focusing regions of the first and second laser beams at least partially overlap or are superimposed.

[0028] In a preferred embodiment of the present invention, the pulsed laser system includes two lasers having a common cavity mirror, and the first and second laser beams are parallel and introduced into the chamber through a common focusing optical system.

[0029] In a preferred embodiment of the present invention, the pulsed laser system is a solid-state laser system.

[0030] In a preferred embodiment of the present invention, the pulsed laser system generates the first laser beam in a Q-switching mode or a giant pulse generation mode.

[0031] In a preferred embodiment of the present invention, the pulsed laser system generates a second laser beam in a free-running mode.

[0032] In a preferred embodiment of the present invention, only the CW laser has a fiber output.

[0033] In an embodiment of the present invention, the wavelength of the CW laser is different from the wavelengths of the radiation of the first and second laser beams.

[0034] In a preferred embodiment of the present invention, the focused beam of the CW laser is vertically upward or nearly vertical.

[0035] In an embodiment of the present invention, the outer and inner surfaces of the transparent part of the chamber are shaped as concentric spheres or parts thereof, and the region of the radiation plasma is located at the center of the concentric spheres.

[0036] In a preferred embodiment of the present invention, the output beam of the plasma radiation leaves the chamber at all azimuth angles.

[0037] In an embodiment of the present invention, the output beam of the plasma radiation leaves the chamber at a solid angle of not less than 9 sr.

[0038] In an embodiment of the present invention, the laser-pumped plasma light source has three or more output beams of plasma radiation.

[0039] On the other hand, the present invention relates to a method for igniting a plasma in a laser-pumped plasma light source, comprising: guiding the focused beam of the CW laser into a chamber having a high-pressure gas, initiating plasma ignition, and stably maintaining the radiation plasma by the focused beam of the CW laser.

[0040] The method is characterized in that the plasma ignition is provided by a pulsed laser system that generates a first and a second laser beam focused inside the chamber, and the first laser beam is used to provide optical breakdown, and after the optical breakdown, the second laser beam is used to ignite a plasma having a volume and density sufficient to maintain a stable plasma by the focused beam of the CW laser.

[0041] In a preferred embodiment of the present invention, the pulsed laser system is a solid-state laser system that generates the first laser beam in a Q-switched mode and the second laser beam in a free-running mode.

[0042] Designing the light source in the proposed manner allows for reliable ignition of COD by selecting appropriate energies, durations, and pulse powers of the first and second laser beams, for the following reasons. The first laser beam provides reliable optical breakdown. However, COD ignition using only one laser beam is unstable and problematic. One reason is that it is difficult to overlap the CW laser focusing region with the optical breakdown region, which typically has a very small size and does not exceed a value of approximately 50 μm. Even when the focusing regions of the pulsed and CW laser beams overlap, COD ignition using only one laser beam remains challenging. This is because the optical breakdown generated by the laser radiation is explosive. The explosion process (especially the shock wave) can cause suppression of the optical discharge maintained by the CW laser with low power (usually not exceeding 300 watts). According to the present invention, this problem is solved by using a second pulsed laser beam to provide plasma ignition after optical breakdown. In this case, the pulsed optical discharge maintained by the second laser beam itself has no explosion phenomenon, and the plasma ignited by the second laser beam resists the interference caused by the optical breakdown. At the same time, the second laser beam ensures that the plasma volume and density are sufficient to maintain a reliable and stable plasma through the focused beam of the CW laser with a smaller output power. Thus, reliable COD ignition is achieved.

[0043] Advantages and features of the present invention will become more apparent from the following non - limiting description of exemplary embodiments given by way of example with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The essence of the present invention is explained with reference to the drawings, in which:

[0045] Figure 1 is a schematic diagram of a laser - pumped plasma light source with a pulsed laser system for plasma ignition according to the present invention;

[0046] Figure 2 is a diagram of the radiation power of a laser beam according to an embodiment of the present invention;

[0047] Figure 3 is a schematic diagram of a laser - pumped plasma light source with a solid - state laser system for plasma ignition;

[0048] Figure 4 is a schematic diagram of a light source with a three - channel output of useful plasma radiation;

[0049] Figure 5 is a diagram of the radiation power of a laser beam according to an embodiment of the present invention.

[0050] In the drawings, matching elements of the devices have the same reference numerals.

[0051] These drawings do not cover and do not limit the entire scope of options for implementing the technical solution, but are merely illustrative examples of specific cases of its implementation methods. Detailed implementation mode

[0052] This description aims to illustrate how to implement the present invention, rather than to show the scope of the present invention.

[0053] According to Figure 1 In the illustrated embodiment of the invention, the laser-pumped plasma light source includes a high-pressure gas-filled chamber 1, typically 10 atm or higher. At least a portion of chamber 1 is optically transparent. Figure 1 An embodiment is shown having a completely transparent chamber made of an optically transparent material (e.g., fused silica). Chamber 1 contains a radiative plasma region 2 maintained in the chamber by the focused beam 3 of CW laser 4. At least one output beam (or useful beam) of plasma radiation 5 directed to a condenser 6 and for subsequent use exits chamber 1. Condenser 6 forms a radiative beam 7 which is transmitted, for example, via an optical fiber and / or mirror system to one or more optical consumption systems 8 which utilize the broadband radiation emitted by the plasma.

[0054] The condenser is described in more detail in the patent U.S. 9357627 published on May 31, 2016, the entire content of which is incorporated herein by reference.

[0055] The light source further includes a plasma ignition device. The light source is characterized in that the plasma ignition device is a pulsed laser system 9 which generates a first laser beam 10 and a second laser beam 11 focused in chamber 1, i.e., focused into the region for maintaining the radiative plasma 2. The first laser beam 10 is used to initiate plasma ignition or optical breakdown in chamber 1. The second laser beam 11 is used for plasma ignition after the optical breakdown provided by the first laser beam 10.

[0056] By selecting appropriate energies, durations, and corresponding pulse powers of the two laser beams, the light source designed in the proposed manner enables reliable ignition of a continuous light discharge. This allows for the production of an electrodeless high-brightness broadband laser-pumped plasma light source characterized by the highest possible spatial and energy stability.

[0057] There is no electrode, which simplifies the design of the high-pressure chamber, improves the chamber strength and reliability, and in a preferred embodiment of the present invention, ensures that the output beam 5 of plasma radiation exits the chamber at a planar angle of 360° or all azimuth angles, see Figure 1。This means that in the azimuthal plane perpendicular to the axis of beam 3 of the CW laser and passing through the region of the radiation plasma 2, the output beam of the plasma radiation exits the chamber at all azimuthal angles from 0° to 360°. Additionally, in a preferred embodiment of the present invention, the angular spread of the output beam 5 of the plasma radiation (relative to Figure 1 the flat angle of the drawing in

[0058] is not less than 90°, and correspondingly, the plasma radiation is condensed by the condenser 6 at a solid angle of 9 sr or greater. Preferably, a low-cost near-infrared diode laser with a fiber output is used as the CW laser 4. In this case, at the exit of the optical fiber 12, the expanded laser beam is directed to a collimator 13 in the form of, for example, a condenser lens. After the collimator 13, the expanded parallel beam 14 of the CW laser is directed to a focusing optical element 15, which is, for example, in the form of an aspherical condenser lens. The focusing optical element 15 ensures a clear focus of the beam 3 of the CW laser 4 required to achieve a high brightness of the light source.

[0059] In an embodiment of the present invention, the pulsed laser system 9 includes a first laser 16 for generating a first laser beam 10 and a second laser 17 for generating a second laser beam 11, see Figure 1 。For example, an optical element in the form of a condenser lens can be used to focus the first and second laser beams, but this is not the only option.

[0060] In a preferred embodiment of the present invention, the focusing regions of the first and second laser beams at least partially overlap or coincide.

[0061] In Figure 2 the characteristic time dependence of the radiation power in the first and second laser beams 10, 11 and the beam of the CW laser 4 is schematically shown on a logarithmic scale.

[0062] Preferably, in order to ensure reliable initiation of plasma ignition or optical breakdown, the first laser beam 10 is characterized by a high (at least 10 4 W) pulsed radiation power. In this case, it is sufficient that the full width at half maximum of the laser pulse does not exceed 0.1 μs.

[0063] According to the present invention, compared with the first laser beam, the pulsed power of the second laser beam is many times lower, for example, 10 3The wattage, laser pulse length, and energy are many times higher. This allows, after exposure to the first laser beam, the use of a second laser beam to generate a plasma volume many times (an order of magnitude or more) larger than the plasma volume generated by the first laser beam. At the same time, the radiation power in the second laser beam is more than an order of magnitude higher than the CW laser power, see Figure 2 .

[0064] The second laser beam is used to generate a plasma whose volume and density are sufficient for stable plasma maintenance by the focused beam of the CW laser.

[0065] In an embodiment of the present invention, the generation of the second laser beam starts before the generation of the first laser beam and ends no earlier than 50 μs after the end of the first laser pulse, see Figure 2 . On the one hand, it makes it easier to synchronize the first and second laser beams, and on the other hand, it provides sufficient time for plasma evolution under the influence of the second laser beam. As a result, a large plasma volume of up to about 1 mm and a plasma density of up to 10 18 cm -3 are provided, which is sufficient for reliable stable plasma maintenance by the focused CW laser beam. 10 18 cm -3 The plasma density corresponds to a gas with a temperature of 18,000 K and has 10% ionization in the radiation plasma region at an initial gas pressure in the chamber of about 16 atm.

[0066] In one of its embodiments, the laser-pumped plasma light source operates as follows. The focused beam 3 of the CW laser 4 is directed into at least partially transparent high-pressure gas-filled chamber 1, see Figure 1 . Xenon, other noble gases, and their mixtures, including metal vapors (e.g., mercury) and / or various gas mixtures (including gas halides), can be included in the chamber as efficient plasma fuels. The focused second laser beam 11 of the second laser 17 is directed into the region for maintaining the radiation plasma 2. In an example of an embodiment of the present invention, the maximum radiation power in the second laser beam 11 can have a value of about 10 3 watts, while the laser pulse length can be about 10 -4 s. During the radiation pulse of the second laser beam 11, the first laser beam 10 is generated, and its focused region at least partially overlaps the focused region of the second laser beam. A short, less than 0.1 μs, high-power, about 10 4 watt or higher radiation pulse (whose energy is about a few mJ) of the first laser 16 is used to provide initial local gas ionization for optical breakdown in a small volume with a characteristic size of 50 to 100 μm. The energy and laser pulse length of the second laser beam 11 are many times higher than those of the first laser beam 10, and the second laser beam is used at a laser radiation power (about 103 maintain a glow discharge at a power (W or more) many times higher than the radiation power in the beam 3 of the CW laser. In the case where the glow discharge is maintained by the second laser beam 11 with a pulse length of about 100 μs or longer, the plasma volume increases due to its movement along the caustic line towards the laser beam 11 and its radial expansion. Thus, a plasma size of up to 1 mm can be achieved. Due to the sufficiently high (about 0.1 J / pulse or higher) radiation pulse energy of the second laser beam 11, in a larger plasma volume, an electron density level is provided that is sufficient to reliably maintain the radiation plasma by the focused beam 3 of the CW laser 4 with a relatively small power of no more than 300 watts. Thus, the plasma density provided by the second laser beam is higher than the threshold plasma density for continuous glow discharge of about 10 18 electrons / cm 3 or higher. In the stable mode, at least one output beam 5 of the plasma radiation outputs broadband radiation from the radiation plasma region 2, and this plasma radiation exits through the optically transparent part of the chamber 1 and is used for subsequent use.

[0067] Designing the light source as described above enables reliable ignition of continuous glow discharge without using ignition electrodes. This allows for significant improvement in the chamber design by simplifying its shape and eliminating mechanical stresses at the points where the metal seals are introduced into the chamber, increasing the reliability and lifespan of the light source. The design simplification allows for the use of a chamber shape that reduces the aberration of the output beam of the plasma radiation exiting the chamber, thereby increasing the light source brightness. Additionally, there is a possibility of using chamber materials with higher transparency in the UV spectral range. The electromagnetic noise during light source startup is reduced. Since the metallization of its optically transparent part is eliminated, the chamber lifespan is increased. Moreover, without electrodes, it is possible to significantly increase the spatial angle of the radiation output and improve the power of the output beam of the plasma radiation. At the same time, the elimination of the ignition electrodes significantly reduces the turbulence of convection inside the chamber, thereby significantly increasing the spatial and power stability of the laser-pumped plasma light source. Due to the possibility of optimizing the size of the electrodeless chamber, further improvement in stability is achieved. Generally, an increase in light source brightness and stability is achieved, the possibility of increasing its light output in the UV range is realized, reliability and lifespan are increased, the convenience of its operation is improved, and the operating cost is reduced.

[0068] The above possibilities are most easily realized in a light source where the pulsed laser system 9 is solid-state, see Figure 3。In this embodiment of the present invention, the pulsed laser system 9 includes two optically pumped solid-state lasers 16, 17. For example, flash lamps 18, 19 with reflectors can be used as the optical pumping sources. The lamps are turned on with an optimized delay relative to each other. Rods made of a transparent substrate can be used as the active elements 20, 21, for example, yttrium aluminum garnet (YAG) doped with metal ions (e.g., neodymium (Nd)). The first and second laser beams 10, 11 are preferably parallel and are introduced into the chamber 1 via a common focusing optical system 22 (e.g., in the form of an aspherical condenser lens). To ensure the parallelism of the laser beams 10, 11, the first and second solid-state lasers 16, 17 preferably have a common cavity mirror 23, 24. This provides an overlap of the focusing regions of the first and second laser beams 10, 11 required for plasma ignition.

[0069] In a preferred embodiment of the present invention, the pulsed laser system 9 generates the first laser beam 10 in a Q-switching mode or a giant pulse generation mode and generates the second laser beam 11 in a free-running mode. To achieve the Q-switching mode, the first laser is equipped with a Q-switch 25, for example, a passive Q-switch made of a phototropic material. In another embodiment of the present invention, an active Q-switch can be used.

[0070] During giant pulse generation, the excessive radiation power of the pulsed laser system 9 does not allow the use of an optical fiber to transmit its radiation because the optical fiber may be damaged. Therefore, in an embodiment of the present invention, only the CW laser is equipped with an optical fiber output, see Figure 1 , Figure 3 。

[0071] Preferably, the first and second lasers 10, 11 have the same radiation wavelength, for example, λ1 = λ2 = 1.064 μm, which is different from the wavelength of the CW laser λ CW ,for example, λ CW = 0.808 μm or 0.976 μm: λ CW ≠ λ1 = λ2. This allows the use of a dichroic mirror 26 to direct the expanded beam 14 of the CW laser to the chamber, see Figure 3 。

[0072] To facilitate optical alignment and improve the light source configuration, additional deflection mirrors 27 (see Figure 3 ) or several such mirrors can be used therein.

[0073] In an embodiment of the present invention, additional optical elements (not shown) can be installed in the path of the CW laser beam 14 or in the pulsed laser system 9 to cancel chromatic aberration and more precisely align the focusing regions of the CW and pulsed laser beams. In the pulsed laser system, in particular, additional optical elements, such as polarizers, filters, diaphragms, can be installed in the cavity formed by the mirrors 23, 24 to control the parameters of the first and second laser beams.

[0074] In a preferred embodiment of the present invention, the axis of the CW laser focused beam 3 is vertically upward, i.e., against gravity 28, see Figure 3 , or nearly vertical. The proposed design achieves the highest stability of the radiation power of the light source. This is because the region of the radiation plasma 2 usually moves slightly from the focus towards the focused beam 3 of the CW laser until the intensity of the focused beam 3 of the CW laser is still sufficient to maintain the cross-section of the focused laser beam of the radiation plasma region 2. When the focused beam 3 of the CW laser is guided from bottom to top, the radiation plasma region 2 containing the hottest plasma with the lowest mass density tends to float under the influence of buoyancy. The ascending region of the radiation plasma 2 terminates at the position closest to the focus, where the cross-section of the focused beam 3 of the CW laser is smaller and the laser radiation intensity is higher. On the one hand, this increases the plasma radiation brightness, and on the other hand, it balances the forces acting on the radiation plasma region, which ensures the high stability of the radiation power of the high-brightness laser-pumped plasma light source.

[0075] To achieve these positive effects, preferably, the chamber 1 must be axisymmetric, and the axis of the focused beam 3 of the CW laser must be aligned with the axis of symmetry of the chamber.

[0076] In addition to providing highly stable output parameters, the present invention also enables the possibility of achieving the highest brightness of a laser-pumped broadband light source, particularly by optimizing the shape and size of the electrodeless chamber. Accordingly, in a preferred embodiment of the present invention, the outer and inner surfaces of the chamber or its transparent part are shaped as concentric spheres, and the region of the radiation plasma 2 is located at the center of the concentric spheres, see Figure 3 . In this embodiment of the present invention, the aberration introduced by the chamber wall is eliminated, enabling a clearer focusing of the beam 3 of the CW laser and increasing the light source brightness. In addition, the aberration that distorts the optical path in the useful plasma radiation beam 5 is eliminated, increasing its brightness.

[0077] Another positive result of the invention is the possibility of minimizing the size of the chamber. This increases the sharp focusing of the CW laser beam 3 due to the fact that the focusing optical system 22 is moved closer to the region of the irradiated plasma 2. In addition, the closer the region of the irradiated plasma is to the wall of the chamber 1, in particular to the top wall of the chamber, the smaller the pulses obtained by the gas heated in the region of the irradiated plasma 2 under the effect of buoyancy. Therefore, the speed and turbulence of the gas convection are smaller, the smaller the distance from the plasma to the chamber wall. Therefore, the possibility of further increasing the brightness and stability of the laser pumped plasma light source designed according to the invention is provided.

[0078] To ensure plasma radiation output in a wide spectral range from ultraviolet to near infrared, the optically transparent parts of the chamber are preferably made of the following materials: crystalline magnesium fluoride (MgF2), crystalline calcium fluoride (CaF2), crystalline sapphire or colorless sapphire (Al2O3), fused quartz or crystalline quartz.

[0079] In an embodiment of the present invention, the chamber ensures that the output beam of the plasma radiation 5 leaves the chamber at a plane angle of 2π radians, but is not limited to Figure 1 , Figure 3 in the selection.

[0080] In another embodiment of the present invention, the light source may have at least three divergent output beams 5a, 5b, 5c of plasma radiation, such as Figure 4 As shown, Figure 4 A light source cross section in a horizontal plane through the region of the radiating plasma 2 is shown. Figure 4 The laser beam used for ignition and maintenance of COD is located below the plane of the figure. In various industrial applications, it is necessary to use several (especially three) plasma radiations from a single light source. In this embodiment of the invention, the chamber 1 of the laser-pumped plasma light source is installed in a housing 29, which is equipped with three concentrators 6a, 6b, 6c. The concentrators 6a, 6b, 6c form plasma radiation beams 7a, 7b, 7c that are transmitted, for example, via optical fibers to optical consumption systems 8a, 8b, 8c that use broadband plasma radiation. This allows one light source to be used for three or more optical consumption systems, resulting in compact dimensions of the system and the same parameters of broadband radiation in all optical channels.

[0081] According to the present invention, Figure 1 , Figure 3The method of plasma ignition in the laser-pumped plasma light source shown is as follows. The focused beam 3 of the CW laser 4 is guided into a high-pressure gas-filled chamber 1, typically at 10 atm or higher. Plasma ignition is provided by a pulsed laser system 9 that generates first and second laser beams 10, 11 focused in the chamber. The first laser beam 10 is set to provide optical breakdown, and after the optical breakdown, the second laser beam 11 is used to ignite the plasma, whose volume and density are sufficient to maintain a stable plasma through the focused beam 3 of the CW laser 4.

[0082] In a preferred embodiment of the present invention, a solid-state laser system is used, which generates the first laser beam 10 in Q-switching mode and the second laser beam 11 in free-running mode, as shown in Figure 3 . The pulsed laser system 9 preferably includes two solid-state lasers 16, 17, for example, Nd:YAG lasers with optical pump sources 18, 19 in the form of flash lamps. The first and second laser beams 10, 11 are preferably parallel and are introduced into the chamber 1 via a focusing optical system 22. To superimpose the focused regions of the first and second laser beams 10, 11, the solid-state lasers 16, 17 preferably have common mirrors 23, 24 of the cavity. The first laser 16 is equipped with a Q-switch 25.

[0083] In an example of an embodiment of the present invention, the Xe gas pressure in the chamber is 30 atm. The pulsed energy emitted by the first laser 16 in Q-switching mode is 3 mJ, the pulse duration is 20 ns, and the laser wavelength is λ1 = 1.064 μm. The optically broken-down plasma has a characteristic size of 50 to 100 μm. The optical breakdown mode does not provide reliable ignition of the optical discharge maintained by the focused beam 3 of the CW laser 4. Therefore, after the optical breakdown, the second laser beam is used to ignite the plasma, whose volume (up to 1 mm 3 ) and density (exceeding 10 18 cm -3 ) are sufficient to maintain a stable plasma through the focused beam 3 of the CW laser 4. In an example of an embodiment of the present invention, the energy of the second laser beam is 150 mJ, the pulse length is 100 μs, and the laser wavelength is λ2 = 1.064 μm.

[0084] Preferably, the radiation pulse of the second laser beam ends no earlier than 50 μs after the end of the radiation pulse of the first laser beam, as shown in Figure 2 . It takes at least 50 μs for the interference from the optical breakdown to decay and for the plasma size and density to develop to values sufficient to maintain a stable plasma through the focused beam of the CW laser.

[0085] The generation of the second laser beam can be started before the first laser pulse, as shown in Figure 2。At the same time, the present invention is not limited to these embodiments. As studies have shown, as Figure 5 shown, when the second laser beam has a delay of up to ten seconds or longer after the first laser beam is generated, COD ignition is also provided. When long-lived clusters or solid particles are generated due to this effect, the mechanism of this plasma ignition may be related to the influence of the giant pulse on the chamber wall.

[0086] Generally speaking, the present invention allows to ensure high reliability of laser ignition of laser-maintained plasmas and, on this basis, to create a high-brightness broadband light source with the highest spatial and power stability.

[0087] Industrial Applicability

[0088] The high-brightness and high-stability laser-pumped plasma light source designed according to the present invention can be used in various projection systems, for spectrochemical analysis, spectral microanalysis of biological objects in biology and medicine, microcapillary liquid chromatography, optical lithography process inspection, spectrophotometry and other applications.

Claims

1. A laser-pumped plasma light source, comprising: An inflatable chamber, at least a part of which is optically transparent; a radiation plasma region maintained in the chamber by a focused beam of a continuous-wave (CW) laser; An output beam of at least one plasma radiation exiting the chamber; a plasma ignition device, characterized in that, The plasma ignition device is a pulsed laser system that generates a first laser beam and a second laser beam focused in the chamber, and The first laser beam is arranged for optical breakdown of the gas, and The second laser beam is arranged for plasma ignition after optical breakdown.

2. The light source according to claim 1, wherein, The first laser beam has a peak radiation power greater than 10 4 watts and a pulse length less than 0.1 μs.

3. The light source according to claim 1, wherein, Compared with the first laser beam, the second laser beam has at least three times higher laser pulse energy and at least one order of magnitude lower laser peak power.

4. The light source according to claim 1, wherein, The volume of the plasma ignited multiple times by the second laser beam exceeds the volume of the plasma generated by the first laser during optical breakdown by one order of magnitude or more.

5. The light source according to claim 1, wherein, The volume and density of the plasma ignited by the second laser beam are sufficient for stable maintenance of the plasma by the focused beam of the CW laser.

6. The light source according to claim 1, wherein, The second laser beam provides a plasma size of up to about 1 mm and a plasma density of up to 10 18 cm -3 or higher.

7. The light source according to claim 1, wherein, The output power of the CW laser does not exceed 300 watts.

8. The light source according to claim 1, wherein, The radiation pulse of the second laser beam ends no earlier than 50 μs after the end of the radiation pulse of the first laser beam.

9. The light source according to claim 1, wherein, The focusing regions of the first and second laser beams at least partially overlap.

10. The light source according to claim 1, wherein, The pulsed laser system includes two lasers having a common cavity mirror, and wherein the first and second laser beams are parallel and introduced into the chamber through a common focusing optical system.

11. The light source according to claim 1, wherein, The pulsed laser system is a solid-state laser system.

12. The light source according to claim 1, wherein, The pulsed laser system generates the first laser beam in Q-switch mode or giant pulse generation mode.

13. The light source according to any one of claims 1 to 12, wherein, The pulsed laser system generates the second laser beam in free-running mode.

14. The light source according to any one of claims 1-12, wherein, Only the CW laser has a fiber output.

15. The light source according to any one of claims 1 to 12, wherein, The wavelength of the CW laser is different from the wavelengths of the radiation of the first and second laser beams.

16. The light source according to any one of claims 1-12, wherein, The axis of the focused beam of the CW laser is vertically upward or nearly vertical.

17. The light source according to any one of claims 1 - 12, wherein, The outer surface and the inner surface of the transparent part of the chamber are shaped as concentric spheres or parts thereof, and the region of the radiation plasma is located at the center of the concentric spheres.

18. The light source according to any one of claims 1-12, wherein, The output beam of the plasma radiation exits the chamber at all azimuth angles.

19. The light source according to any one of claims 1 - 12, wherein, The output beam of the plasma radiation exits the chamber at a solid angle of not less than 9 sr.

20. The light source according to any one of claims 1-12, having three or more output beams of plasma radiation.

21. A method for igniting a plasma in a laser-pumped plasma light source, comprising: Guiding the focused beam of the CW laser into a chamber with high-pressure gas, plasma ignition and stable maintenance of the radiation plasma by the focused beam of the CW laser, characterized in that, The plasma ignition is provided by a pulsed laser system that generates a first laser beam and a second laser beam focused in the chamber, and The first laser beam is used to provide optical breakdown, and after optical breakdown, the second laser beam is used to ignite the plasma, the volume and density of which are sufficient for stable plasma maintenance by the focused beam of the CW laser.

22. The method according to claim 21, wherein The pulsed laser system is a solid-state laser system that generates the first laser beam in Q-switch mode and the second laser beam in free-running mode.

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