EP4193384A1

EP4193384A1 - High-brightness laser-pumped plasma light source and method for reducing aberrations

Info

Publication number: EP4193384A1
Application number: EP21755458.3A
Authority: EP
Inventors: Robert Rafilevich Gayasov; Denis Aleksandrovich GLUSHKOV; Yurii Borisovich Kiryukhin; Vladimir Mikhailovich KRIVTSUN; Aleksandr Andreevich LASH; Dmitry Borisovich ABRAMENKO
Original assignee: Isteq Group Holding BV; Isteq BV
Current assignee: Isteq Group Holding BV; Isteq BV
Priority date: 2020-08-06
Filing date: 2021-08-04
Publication date: 2023-06-14
Also published as: CN116235277B; CN116235277A; WO2022029187A1; KR20230044314A; JP3246482U; JP2023536367A

Abstract

A laser-pumped plasma light source is provided. The plasma light source comprises a chamber filled with high-pressure gas, a means for plasma ignition, a region of radiating plasma sustained in the chamber by a focused beam of a continuous wave (CW) laser, and a beam of plasma radiation exiting the chamber. The chamber comprises a tube, a bottom and a cap. One end of the tube is hermetically connected to the bottom, while the other end of the tube is hermetically connected to the cap. The tube and the bottom are made from an optically transparent material. The bottom is arranged for introducing the focused beam of the CW laser into the chamber. The tube is configured to allow the beam of plasma radiation to exit from the chamber over at least 70% of azimuthal angles in a plane perpendicular to the beam of the CW laser and passing through said region of radiating plasma (2). The cap is equipped with a gas inlet.

Description

HIGH-BRIGHTNESS LASER-PUMPED PLASMA LIGHT SOURCE AND METHOD FOR

REDUCING ABERRATIONS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of US patent application 16/814317 filed on March 10, 2020, now U.S. Pat. 10770282 and continuation of US patent application 16/986424, filed on August 6, 2020, now U.S. Pat. 10964523, the entire contents and disclosures of which are incorporated herein by reference.

FIELD OF INVENTION

[0002] The present invention relates to high-brightness broadband light sources with continuous optical discharge.

BACKGROUND OF INVENTION

[0003] A stationary gas discharge sustained by laser radiation in pre-created relatively dense plasma is known as continuous optical discharge (COD). A COD, sustained by a focused beam of a continuous wave (CW) laser, is realized in various gases, in particular, in Xe at a high gas pressure of up to 200 atm (Carlhoff et al., “Continuous Optical Discharges at Very High Pressure,” Physica 103C, 1981, pp. 439-447). COD-based light sources with a plasma temperature of about 20,000 K (Raizer, “Optical Discharges,” Sov. Phys. Usp. 23(11), Nov. 1980, pp. 789-806) are among the highest brightness continuous light sources in a wide spectral range between about 0.1 pm and 1 pm.

[0004] In order to further increase brightness, pulsed lasers with a high pulse rate may be used as well, also in combination with CW laser with a power of not lower than the threshold value required to stably sustain the COD, for example, as described in RU Patent 2571433 published on 20.12.2015.

[0005] Compared to arc lamps, COD-based light sources not only have a higher brightness, but also a longer lifetime, making them preferable for a variety of applications.

[0006] In the broadband light source known from US Patent 9368337 published on 14.6.2016, COD plasma has an elongated shape along the laser beam axis, and plasma radiation is collected in the longitudinal direction, that provides for high brightness of the source.

[0007] However, a problem of laser radiation locking within the useful beam of plasma radiation occurs in case of longitudinal collection of plasma radiation.

[0008] This drawback is overcome in the broadband light source known from US Patent 9357627 published on 31.5.2016, wherein radiation is collected in the directions other than the direction of laser beam propagation. In this case, by choosing the relative position of the chamber, laser beam (directed upwards along the chamber axis) and radiating plasma region (close to the upper part of the chamber), a higher spatial and power stability of the broadband source is achieved.

[0009] However, the shape and design of the chamber as well as COD sustaining conditions may not be optimal to achieve the highest possible brightness of the light source, in particular, due to optical aberrations introduced into the path of radiating plasma rays by the transparent walls of the chamber.

[0010] This drawback is partially overcome in the laser-pumped light source known from US Patent 8525138 published on 9.3.2013, where optical aberrations introduced into the path of radiating plasma rays by the transparent walls of the chamber are eliminated by modifying the shape of the optical collector, for example, an elliptical mirror.

[0011] However, modification of reflector shape is difficult to realize in practice for the most of laser-pumped light sources.

[0012] These drawbacks are partially overcome in the light source known from US Patent 9232622 published on 5.1.2016, where the CW laser beam is focused in the chamber using a system of mirrors with a high numerical aperture (NA). Transparent wall of the chamber, through which the focused CW laser beam is introduced, has a variable thickness that eliminates optical aberrations in the system due to high pressure gas. This provides for sharp focusing of the CW laser beam, thus increasing brightness of the light source.

[0013] However, the light source has no provisions to eliminate optical aberrations introduced into the useful beam of plasma radiation when it passes through the transparent walls of the chamber, reducing brightness of the light source. Besides, disadvantages caused by electrodes used for starting plasma ignition are inherent in the light source. Another disadvantage of this design is the propagation through the transparent chamber walls of both plasma radiation and unabsorbed laser radiation passing through the plasma, which requires special measures to block it.

[0014] In general, laser-pumped light sources are characterized with some of the following disadvantages:

[0015] - optical aberrations produced by the chamber with high pressure gas, which reduce brightness of the light source,

[0016] - the need to block laser radiation in the beam of plasma radiation, [0017] - imperfect shape of the chamber, in particular, due to the use of igniting electrodes restricting solid angle of plasma radiation output and increasing convective gas flows between high-temperature plasma regions and surrounding gases with a lower temperature, and

[0018] - high turbulence of convective gas flows in the chamber, reducing spatial and power stability of the light source.

SUMMARY

[0019] In view of the above, there is a need for creation of high-brightness and highly stable laser-pumped light sources, which are, at least partially, free from one or more, preferably all, of the said disadvantages.

[0020] The technical problem and technical result of the invention refers to the creation of broadband light sources with the highest possible brightness and stability, also characterized by compensation of aberrations, both when laser radiation is incoming into the chamber and when broadband plasma radiation is outcoming from it.

[0021] Achievement of the purpose is possible by means of a laser-pumped plasma light source, containing: a chamber filled with high-pressure gas, a region of radiating plasma sustained in the chamber by a focused beam of a continuous wave (CW) laser; at least one beam of plasma radiation exiting the chamber, and a means for plasma ignition.

[0022] The laser-pumped light source is characterized in that the chamber comprises or consists of a tube, a bottom and a cap; one end of the tube is hermetically connected to the bottom, while the other end of the tube is hermetically connected to the cap; the tube and the bottom are made from an optically transparent material; the bottom is arranged for introducing the focused beam of the CW laser; the tube may be configured to allow the beam of plasma radiation to exit from the chamber over at least 70%, 80%, 90% or even 100% of the full azimuthal angular range. The azimuthal angle may refer to a plane (azimuth plane) that is perpendicular to the axis of the beam of the CW laser (e.g. corresponding to the tube’s longitudinal axis or axial direction, e.g. the symmetry axis) and passes through said region of radiating plasma. For example, the tube excluding its end parts may be arranged for exit of the beam of plasma radiation from the chamber in all azimuths (e.g. with respect to the azimuth plane). Optionally, the cap may be made from a metal and/or may be equipped with a gas inlet. [0023] In an embodiment, the means for plasma ignition is a pulsed laser system generating at least one pulsed laser beam focused in the chamber. The bottom of the chamber may be arranged for introducing the one or more, preferably each, pulsed laser beam into the chamber. [0024] In a preferred embodiment of the invention, a portion of the tube arranged for exit of the beam of plasma radiation has an axis of symmetry, a center of symmetry, a cylindrical shape of an internal surface, a barrel-like or a toroidal shape of an external surface, and said center of symmetry is located at the region of radiating plasma.

[0025] In a preferred embodiment of the invention, the beam of the CW laser is focused in the chamber by means of an optical system comprising the chamber bottom and a focusing optical element with a surface minimizing total aberrations of the optical system.

[0026] In a preferred embodiment of the invention, the focusing optical element is an aspherical lens.

[0027] In an embodiment of the invention, the focusing optical element, e.g. a lens, is fixed in a rim, which in turn is fixed on an end part of the tube at the end at which the bottom is provided.

[0028] In an embodiment of the invention, bottom of the chamber is in the form of a lens.

[0029] In an embodiment of the invention, a part or a detail of the cap is designed as a concave spherical mirror with a center in the region of the radiating plasma.

[0030] In the embodiment of the invention, the concave spherical mirror radius is not more than 5 mm.

[0031] In a preferred embodiment of the invention the focused beam of the CW laser is directed into the chamber vertically upwards.

[0032] In an embodiment of the invention, a part of the cap is made of a refractory material such as tungsten, molybdenum or alloys based thereof.

[0033] In a preferred embodiment of the invention, a radius of the internal surface of the tube is not more than 5 mm, preferably not more than 3 mm. The radius may in particular be a radius perpendicular to the symmetry axis of the tube, e.g. perpendicular to the tube’s longitudinal axis.

[0034] In an embodiment of the invention, the tube and/or the bottom of the chamber are made from a material belonging to a group of sapphire, leuco sapphire, fused quartz, crystalline quartz.

[0035] In a preferred embodiment of the invention, to seal the chamber, the end parts of the tube are used. The tube and the bottom of the chamber may be sealed with glass cement. The cap and tube of the chamber may be sealed using soldering. [0036] In an embodiment of the invention, the cap is equipped with a gas inlet arranged for filling the chamber with the gas or/and controlling a pressure and a composition of the gas in the chamber.

[0037] In an embodiment of the invention the cap of the chamber is equipped with a heater.

[0038] In a preferred embodiment of the invention the means for plasma ignition is a solid- state laser system generating a pulsed laser beam in Q-switching mode and a pulsed laser beam in free-running mode.

[0039] In an embodiment of the invention the chamber is located in an external bulb that is external to the chamber, in particular surrounds or encloses the chamber.

[0040] In an embodiment of the invention, the laser-pumped plasma light source further comprises an optical collector.

[0041] In the embodiment of the invention, the optical collector is multichannel, comprising at least three channels.

[0042] In another aspect, the invention relates to a method for reducing aberrations in a laser- pumped plasma light source, characterized by sustaining a radiating plasma in a gas-filled chamber by a focused beam of a continuous wave (CW) laser and exiting a beam of plasma radiation from the chamber.

[0043] The method may include the use of the chamber comprising or consisting of a tube, a bottom and a cap, while the cap is equipped with a gas inlet, the bottom is arranged for introducing the focused beam of the CW laser into the chamber, the tube is arranged for exit of the beam of plasma radiation. The tube may be arranged to allow the beam of plasma radiation to exit from the chamber over at least 70%, 80%, 90% or even 100% of azimuthal angles in a plane (azimuth plane) that is perpendicular to the axis of the beam of the CW laser (e.g. corresponding to the tube’s longitudinal axis or axial direction, e.g. the symmetry axis) and passes through said region of radiating plasma. The tube may in particular be arranged for exit of the beam of plasma radiation in all azimuths.

[0044] The method may further include reducing aberrations which distort a path of rays of beam of plasma radiation passing through a tube wall, by making use of a tube that has an axis of symmetry, and a center of symmetry, which may be located at the region of radiating plasma. Preferably, the tube may have a cylindrical shape of an internal surface and/or a barrel or a toroidal shape of an external surface. The axis of symmetry may correspond to a cylinder axis of the cylindrical shape. [0045] In a preferred embodiment of the invention, the beam of the CW laser is focused by means of an optical system including the bottom and an aspherical lens with a surface minimizing total aberrations of the optical system.

[0046] In a preferred embodiment of the invention, the beam of plasma radiation exits the chamber in a solid angle of not less than 9 sr.

[0047] If implemented according to the proposed embodiment, significant increase of spatial and power stability of the high-brightness laser-pumped plasma light source may be achieved due to suppressing the turbulence of convective gas flows in the chamber, which may be caused by a combination of the following provisions:

[0048] use of laser ignition eliminating the presence of relatively cold electrodes near the high temperature plasma region;

[0049] implementation of the possibility to optimize density, temperature and composition of gas and dimensions of the chamber;

[0050] temperature stabilization of the chamber;

[0051] use of the chamber geometry with vertical introduction of the laser beam; and

[0052] use of an external shell in some of the embodiments.

[0053] Ignition of continuous optical discharge (COD) without the use of igniting electrodes allows to significantly increase the solid angle of the output beam and the power in the useful beam of plasma radiation.

[0054] In the proposed design of the light source, the cap serves to suppress turbulence of convective gas flows in the chamber, and also blocks the laser radiation transmitted through the plasma, that provides both high output stability of light source and highly effective elimination of laser radiation in a beam of plasma broadband radiation.

[0055] Using a metal cap equipped with a gas inlet provides the possibility to optimize the gas pressure and temperature in the chamber to obtain maximum brightness and stability of the light source. Unlike known solutions, placing the gas inlet on the cap provides optimization of the light source design, since it provides unimpeded output of the beam of plasma radiation from the chamber in all azimuths. Making a part of the cap of the chamber in the form of a concave spherical mirror with its center in the region of radiating plasma allows an additional increase the brightness of the light source.

[0056] The proposed light source provides for sharp focusing of the laser beam in the radiating plasma region. The proposed shape of the chamber reduces aberrations introduced into the beam of plasma radiation when it exits the chamber. All of these, together with optimization of gas temperature and pressure, increase brightness of the light source.

[0057] The possibility to use chamber material, which allows for expanding the variety of applied gas compositions, in particular, metal-halide additives when used with sapphire, is also realized.

[0058] Thus, the invention provides for a possibility to increase brightness, power capacity, and quality of radiation of the laser-pumped light source, substantially improves its spatial and power stability, and expands options to control the plasma radiation spectrum.

[0059] It should be clear that the features of the different embodiments described above can be combined with each other unless noted to the contrary.

[0060] The above said and other purposes, advantages and features of the present invention will become more apparent from the following non-limiting description of embodiments thereof, given by way of example with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0061] Technical essence and operating principle of the proposed device are explained by the drawings, in which:

[0062] Fig. 1 - Schematic diagram of laser-pumped plasma light source in accordance with an embodiment,

[0063] Fig. 2A and Fig. 2B. - Illustration of reduction of the light source brightness due to aberrations caused by the tube wall (Fig. 2A) and of the mechanism of their suppression (Fig. 2B),

[0064] Fig. 3A and Fig. 3B - Schematic diagram of the focusing optical system (Fig. 3 A) and calculated laser power distribution in the focal spot (Fig. 3B),

[0065] Fig. 4, Fig. 5. - Schematic diagram of the light source in accordance with embodiments, [0066] Fig. 6. Schematic view of the light source with three-channel optical collector.

[0067] Fig. 7. Schematic diagram of the light source chamber, equipped with an external bulb.

[0068] In the drawings, the matching elements of the device have the same reference numbers.

[0069] These drawings do not cover and, moreover, do not limit the entire scope of embodiments of this technical solution, but are only illustrative examples of particular cases of implementation thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0070] This description is provided to illustrate how the invention may be implemented and is not to be taken in a limiting sense. Rather, the representation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted.

[0071] According to the example of the invention embodiment (Fig. 1), the laser-pumped light source comprises chamber 1 filled with high-pressure gas, typically higher than 10 atm. Chamber 1 contains radiating plasma region 2 sustained by focused beam 3 of CW laser 4. At least one beam 5 of plasma radiation, directed to an optical collector 6 and intended for subsequent use, exits chamber 1. The optical collector comprises, according to an embodiment of the invention, a parabolic mirror 6 that forms a beam of plasma radiation 7, which is transmitted, for example, via an optical fiber or a system of mirrors to optical consumer system 8, which uses broadband plasma radiation.

[0072] The light source is characterized in that the means for plasma ignition is a pulsed laser system 9 generating as least one pulsed laser beam 10 focused in chamber 1, namely into the region intended for sustaining radiating plasma 2.

[0073] According to the invention, chamber 1 comprises or consists of a tube 11, a bottom 12 and a cap 13. One end of tube 11 is tightly connected to bottom 12, and the other end of tube 11 is tightly connected to cap 13. Cap 13 is intended for filling the chamber with gas, for example, through a tube 14 sealed off after the filling. Tube 11 and bottom 12 of the chamber are made from optically transparent material.

[0074] The bottom is intended for introducing focused beam 3 of CW laser 4 into the chamber, as well as one or more, preferably each one of pulsed laser beams 10 used for plasma ignition.

[0075] Tube 11 of the chamber, made of optically transparent material, is intended to output beam 5 of plasma radiation from chamber 1.

[0076] If implemented according to the proposed embodiment, a possibility to optimize the design of the chamber and operating modes of the laser-pumped light source is realized in order to increase brightness as well as spatial and power stability. [0077] In beam 5 of plasma radiation, the path of rays non-perpendicular to the internal and/or external surface of the tube is distorted when they pass through the wall of tube 11. As a result of these aberrations, brightness of the light source may significantly decrease.

[0078] In order to increase brightness of the light source, in the preferred embodiments of the invention, the shape of tube 11 incorporates the function of reducing aberrations, which distort the path of rays of plasma radiation when they pass through the tube walls. Complete elimination of aberrations is achieved when the parts of external and internal surfaces of the chamber, through which beam 5 of plasma radiation exits the chamber, are the parts of two concentric spheres, that may be difficult to implement.

[0079] In particular, to simplify the chamber fabrication process, preferably, a part of the internal surface of tube 11 is cylinder-shaped, as shown in Fig. 1.

[0080] A significant reduction in aberrations is achieved in preferred embodiments of the invention, in which a portion of the tube 11 arranged for exit of the beam of plasma radiation 5 has an axis of symmetry, a center of symmetry, a cylindrical shape of an internal surface, a barrel-like or a toroidal shape of an external surface and said center of symmetry is located at the region of radiating plasma 2, Fig. 1. In these embodiments of the invention, aberrations are reduced by using a relatively simple and easy-to-fabricate chamber 1.

[0081] Fig. 2a shows a schematic diagram of a homocentric beam from quasi-zero- dimensional radiating plasma region 2, passing through the walls of circle-and-cylinder-shaped tube 11. The beam opening angle immediately near radiating plasma region 2 is designated with optical rays 15. At interfaces, i.e. on surfaces of tube 11 of the chamber, the rays are refracted in accordance with Snell’s refraction law:

[0082] ni sinOi = n₂ sinO₂ (1)

[0083] where ni — refraction index of the medium, from which the light falls on the interface, n₂ — refraction index of the medium, to which the light spreads after having passed the interface, 0i — light incidence angle, i.e. the angle between a ray incident on the surface and a normal to the surface, 0₂ — light refraction angle, i. e. the angle between a ray passed through the surface and a normal to the surface

[0084] Let us designate rays 15 passed through transparent tube 11 of chamber as rays 15’, which are displaced from rays 15 and parallel to it, as shown in Fig. 2a. The bigger an angle to the normal, the grater the displacement is. As a result of passing the cylinder-shaped tube of the chamber, the beam becomes astigmatic, i.e. the rays passed through the chamber wall cease to converge to a point. From the perspective of rays 15’ exited the chamber, an extension of which is shown with dotted lines 15”, the quasi-zero-dimensional source of radiation (imaginary center of rays 15’) takes on a disk shape 2’ (Fig. 2a) due to aberrations, and as a result, the surface area visible from the outside of the chamber significantly increases. Thus, when using a simple circle-and-cylinder-shaped tube of the chamber, aberrations substantially reduce brightness of the light source in the directions other than normal to the tube surface.

[0085] If the external surface of tube 11 is implemented according to an embodiment of the invention, Fig. 2b, rays 15’, after passing through the tube walls, are not only displaced from rays 15, but also inclined to the propagation direction of rays 15 near radiating plasma region 2. As a result, a beam with opening angle designated with rays 15’ remains almost homocentric, and radiating plasma region 2’ visible from the side of rays 15’, which have passed the tube of the chamber, remains quasi-zero-dimensional.

[0086] Thus, the light source remains quasi-zero-dimensional for optical consumer systems using the beam passed through accordingly shaped tube 11 of the chamber, Fig. 2b. This provides evidence of efficient elimination of the aberrations, which may significantly reduce brightness of the light source in the tube configurations shown as an example in Fig. 2a.

[0087] In general, the external surface of the tube is shaped in such a way that eliminates chromatic and spherical aberrations.

[0088] According to the calculations made, for the radiating plasma region of elliptical shape with dimensions 0.1 x 0.2 mm and the tube of the chamber with a cylinder-shaped internal surface, for example, with a radius of approximately 3 mm, and toroid-shaped external surface of optimized configuration, for example, with a curvature radius close to 20 mm, only 11% of the light source brightness reduction is possible within a rather wide solid angle compared to the spherical chamber. For example, it may be particularly beneficial if the radius R of curvature of the toroid-shaped external surface lies within a range between 2 and 4 times of a quantity that depends on the radius r of curvature of the inner surface of the tube 11 and its thickness d. The expression to determine a suitable radius of curvature R may be given by R= F * r * (1 + r/d), wherein the factor F lies within the range of 2 to 4. After having passed the wall of the tube, the rays thus appear to originate from the point-like light source, resulting in an efficient elimination of aberrations and improved brightness.

[0089] The laser-pumped plasma light source operates as follow. Focused beam 3 of CW laser 4 is directed into chamber 1 comprising tube 11, the ends of which are tightly connected to bottom 12 and cap 13 of the chamber, Fig. 1. Cap 13 is intended for filling the chamber with high-pressure gas, for example more than 10 atm. Xenon. Other inert gases and their mixtures may be used for filling, including those containing metal vapors, for example, mercury, or various gas mixtures, including those containing halogens. Pulsed laser system 9 generates at least one pulsed laser beam 10 focused in region 2 of the chamber which region is intended for sustaining radiating plasma 2. The beams of CW laser 4 and pulsed laser system 9 are introduced into chamber 1 through a focusing optical element 16 and bottom 12 of the chamber. Pulsed laser system 9 provides for the optical breakdown and generation of an initial plasma with a density higher than the threshold plasma density of the continuous optical discharge (COD) having a value in the order of 10¹⁸ electrons/cm³. Concentration and volume of the initial plasma are sufficient to stationarily sustain the COD with a focused beam 3 of CW laser 4 of a relatively low power, not exceeding 300 W. In stationary mode, high- brightness broadband radiation is output from radiating plasma region 2 of the COD by at least one output beam 5 of plasma radiation intended for subsequent use. Beam 5 of plasma radiation exits the chamber through tube 11, the external surface of which is shaped to reduce aberrations that distort the path of rays of plasma radiation when they pass through the tube wall.

[0090] Fig. 1 shows that according to present invention tube 11 of the chamber, except for its near-end parts used to seal the chamber, is intended for exit of beam 5 of plasma radiation from the chamber in all azimuths. This means that in azimuth plane passing through the region of radiating plasma 2 perpendicular to the axis of beam 3 of the CW laser, the beam of plasma radiation exits the chamber in all azimuths from 0° to 360°. Preferably, the opening angle (in the plane of the drawing in Fig. 1) of beam 5 is not less than 90°. This means that the beam 5 of useful plasma radiation exits from chamber 1 to optical collector 6 in a solid angle, which is not less than 9 sr or more than 70% of the full solid angle.

[0091] In order to provide high brightness of the laser-pumped light source, a sharp focusing of beam 3 of the CW laser is required. This, in turn, requires to minimize aberrations, in particular, spherical aberration of the focusing optical system. According to the present invention, beam 3 of CW laser 4 is focused in chamber 1 by means of an optical system comprising bottom 12 of the chamber and focusing optical element 16. A mirror, for example, off-axis parabolic mirror or, preferably, a lens 16 due to its small size, as shown in Fig. 1, may be used as a focusing optical element. [0092] In order to simplify the design of the chamber, its bottom 12 is, preferably, made in the form of an optical element, quite a simple to make it commercially available, for example, in the form of a plate with spherical and/or flat surfaces. According to the present embodiment, the optical element 16 located outside the chamber and having a more complicated shape than the bottom of the chamber incorporates the function of minimizing total aberrations of the optical system comprising optical element 16 itself and the bottom 12 of the chamber.

[0093] For illustrative purposes, Fig. 3a shows a schematic diagram of the optical system intended for focusing the laser beam, which comprises bottom 12 of the chamber in the form of a flat-convex spherical lens and focusing optical element 16 in the form of a flat-convex aspherical lens. Preferably, the bottom of the chamber and the aspherical lens are made of different materials, which allows for optimizing characteristics of the optical system of these two elements more flexibly.

[0094] The calculation results in Fig. 3b show that an optical system realized in accordance with the present embodiment, in general, allows for focusing about 90% of the laser beam power in a spatial region with a radius of as small as 2.5 pm at a distance d of « 4 mm from the bottom of the chamber.

[0095] However, the present invention admits other embodiments, in which the sharp focusing of beam 3 of the CW laser is provided by only one focusing lens, in particular, aspherical, which is the bottom 12 the chamber.

[0096] In a preferred embodiment of the invention, the axis of the focused beam 3 of CW laser 4 is directed close to vertical or vertically upward, that is, against the force of gravity 17, FIG. 4. If implemented according to the proposed embodiment, the highest radiation power stability of the laser-pumped light source is achieved. This is associated with the fact that radiating plasma region 2 is typically displaced from the focus towards focused beam 3 of the CW laser up to that cross-section of the focused laser beam where the intensity of focused beam 3 of the CW laser is still high enough to sustain radiating plasma region 2. When focused laser beam 3 of the CW laser is directed from the bottom upwards, radiating plasma region 2, containing the highest-temperature and low mass density plasma, tends to float under the action of the buoyant force. Radiating plasma region 2, when rising, ends up in the location closest to the focus where the cross-section of focused beam 3 of the CW laser is smaller, and the laser radiation intensity is higher. On the one hand, this increases brightness of the plasma radiation, and on the other hand, it equalizes the forces acting on the radiating plasma region, which ensures high stability of radiation power of the high-brightness laser-pumped plasma light source.

[0097] To realize these positive effects, preferably, chamber 1 is axially symmetric, and the axis of focused beam 3 of the CW laser is aligned with the axis of symmetry of the chamber.

[0098] The turbulence of convective flows in the chamber is suppressed, in particular, by means of reducing its dimensions. This is easily realizable in the proposed design of the laser- pumped light source, the embodiments of which are characterized in that the radius of the internal cylinder-shaped surface of the tube is less than 5 mm, preferably, not exceeding 3 mm. [0099] Stability of output parameters of the laser-pumped light source is also influenced by a value of momentum acquired under the action of the buoyant force by the gas heated in radiating plasma region 2. The closer the region of plasma radiation 5 to the upper wall of the chamber, the smaller the momentum acquired by the gas and the turbulence of the convective flows. In this regard, to increase stability of output characteristics of the light source in the embodiment shown in Fig. 4, a part or part 18 of cap 13 of the chamber is located close to the radiating plasma region 2 at a distance less than 3 mm, minimum possible in order to avoid sensible negative effect on the light source lifetime.

[0100] In this regard, part 18 of the cap may be made of a refractory material such as tungsten, molybdenum or alloys based thereof.

[0101] Part 18 of the cap may also be made with the function of reflecting and focusing in radiating plasma region 2 of laser radiation passed through the radiating plasma region, and broadband plasma radiation. This increases the plasma temperature, and brightness and efficiency of the light source. According to this embodiment of the invention, shown in Fig. 4, part 18 of the cap is made in the form of a concave spherical mirror 19 with a center (of the spherical mirror surface) in the radiating plasma region 2.

[0102] In the embodiments of the invention, tube 11 and bottom 12 of the chamber may be fabricated as an integral unit from a single piece of material, Fig. 4.

[0103] In other embodiments, tube 11 and bottom 12 of the chamber are tightly sealed using refractory glass fiber reinforced cement ensuring long lifetime of the light source at high temperatures, above 900 K.

[0104] The near-end parts of tube 11, i.e. parts near the axial ends of the cylindrical tube, are used to seal the chamber. In this case, cap 13 and tube 11 of the chamber are tightly sealed by means of brazing with the use of a high-temperature braze, preferably, with a melting point not less than 900 K. Before brazing, the near-end part of the tube 11 of the chamber is metalized. [0105] The cap of the chamber may include or consist of several pieces or parts made of either metal or ceramics.

[0106] Preferably, tube 11 and bottom 12 of the chamber are made of a material from the group comprising sapphire, leuco-sapphire, fused and crystalline quartz, which have the most distinguished optical, physical, chemical and mechanical properties.

[0107] A detailed example of the light source according to the present invention is shown as a schematic diagram in Fig. 5. In this embodiment of the invention, for starting plasma ignition, a solid-state laser system is used, comprising first laser 20 to generate first laser beam 21 in the Q-switching mode and second laser 22 to generate second laser beam 23 in the free-running mode. The pulsed lasers with active elements 24, 25 are equipped with optical pumping sources, for example, in the form of flash lamps 26 and, preferably, have common mirrors 27, 28 of the cavity. First laser 20 is equipped with a Q-switch 29.

[0108] Two pulsed laser beams 21, 23 are focused in the chamber, in the region intended to sustain radiating plasma 2, Fig. 4. First laser beam 21 is intended for starting plasma ignition or optical breakdown. Second laser beam 23 in intended to create plasma, the volume and density of which are enough to stationarily sustain radiating plasma region 2 with focused beam 3 of the CW laser.

[0109] Preferably, a high-efficiency near-infrared diode laser with the output to an optical fiber 29 is used as a CW laser 4. At the exit of optical fiber 29, the expanding laser beam is directed to collimator 30, for example, in the form of a collecting lens. After collimator 30, expanded parallel beam 31 of the CW laser is directed to focusing optical element 16, for example, in the form of an aspherical collecting lens. The focusing optical system comprising optical element 16 and bottom 12 of the chamber ensures sharp focusing of beam 3 of CW laser 4 required to achieve high brightness of the light source.

[0110] Preferably, the wavelength of the CW laser Aw is different from wavelengths Xi, X₂ of first and second pulsed laser beams 21, 23. For example, a wavelength of the CW laser may be cw= 0.808 pm or 0.976 pm, and the pulsed lasers may have wavelengths XI=X₂=1.064 pm. This allows using dichroic mirror 32 to introduce laser beam 31 of the CW laser and beams 21, 23 of the pulsed lasers. To transfer beams 21, 23 of the pulsed laser beams, a rotating mirror 33, Fig. 5, may also be used.

[0111] Optical collector 6, to which beam 5 of plasma radiation is directed, forms a beam 7 of plasma radiation that is transferred, for example, with the use of rotating mirror 34 and another optical system, including optical fiber, to an optical consumer system, which uses broadband plasma radiation.

[0112] In the embodiments of the invention, the cap of the chamber is equipped with a heater, which includes or consists of, for example, a heating coil 36, a current source 37, which is connected to the former through a temperature bridge 38 intended to provide a temperature difference between heating coil 36 and current-carrying busbars 39. Furthermore, currentcarrying busbars 39 may be equipped with a heat exchanger (not shown), for example, in the form of air-cooled radiators. Cap 13 of the chamber may also be equipped with a thermocouple 40 to measure the chamber temperature. Besides, cap 13 of the chamber with heating coil 36 may be placed in a heat-insulating enclosure (not shown).

[0113] Heater 36 is intended for pre-start heating of the chamber up to an operating temperature, that facilitates the starting plasma ignition and ensures fast onset of the steady running mode of the light source with the preset optimally high temperature of the chamber, which is, preferably, in a range of 600 to 900 K.

[0114] In the preferred embodiment of the invention, the high-brightness light source contains a control unit 41, which incorporates the function of automated maintaining the preset power in beam 7 of plasma radiation directed to the consumer system, Fig. 5. For this purpose, the light source is equipped with a power meter 42, to which, using a coupler (not shown), a small part of the luminous flux from beam 7 of plasma radiation directed to the consumer system is supplied. Preferably, the control unit is connected with heater 35, thermocouple 40, power meter 42, pulsed laser system 9, and the power supply unit of CW laser 4. The preset power in beam 7 of plasma radiation is maintained by control unit 41 via a feedback circuit between power meter 42 and the power supply unit of CW laser 4. Besides, control unit 41 may incorporate the function of temperature stabilization of the chamber at an optimally high temperature. In this embodiment of the invention, high stability of power and brightness of the laser-pumped light source is achieved.

[0115] Along with the output of beam 5 of plasma radiation to optical collector 6, which collects radiation in all azimuths, Fig.1, the light source according to the present invention is not limited to this embodiment. In other embodiments, plasma radiation can be collected by a multichannel optical collector, comprising at least three channels 6a, 6b, 6c, as shown in Fig. 6, where the light source cross-section is located in the horizontal plane passing through radiating plasma region 2. The laser beams in Fig. 6, which produce the ignition and maintain the continuous optical discharge, are located below the drawing plane. The use of a multichannel optical collector for one light source is required for a number of industrial applications. In the embodiment of the invention, chamber 1 of the laser-pumped plasma light source may be placed in a casing 43, which is equipped with optical collector comprising three channels 6a, 6b, 6c, each of which receives the corresponding parts 5a, 5b, 5c of the beam of plasma radiation. The channels 6a, 6b, 6c of optical collector form the beams of plasma radiation 7a, 7b, 7c transferred, for example, via an optical fiber to optical consumer systems 8a, 8b, 8c, which use broadband plasma radiation. This allows for the use of one light source for three and more optical consumer systems, ensuring small size of the system and equivalence of the parameters of broadband plasma radiation in all optical channels.

[0116] In another embodiment of the invention, shown in Fig. 7, the chamber 1 is placed into an external shell 44 with a socket 45. The socket may be used to attach chamber 1 and may be partially filled with a sealing material 46. The tightly sealed connections are shown in Fig. 7 with solid lines.

[0117] To minimize aberrations, the external shell, preferably, has a spherical part with a center in radiating plasma region 2.

[0118] Focusing optical element 16, which, in the particular case, is a lens, is, preferably, also placed into the external shell. In this case, focusing lens 16 is fixed in a rim 47, which in its turn is fixed, for example, by means of glass fiber reinforced cement or brazing on the nearend part of tube 11 of the chamber 1, Fig. 7.

[0119] In order to eliminate convective flows outside the chamber 1 and increase brightness stability of the light source, the external shell is, preferably, evacuated.

[0120] In the embodiments of the invention, the external bulb may be made from optical material which incorporates the function of filter cutting off the radiation with wavelengths below 240-260 mm, which produces ozone, i. e. may be used in ozone-free modifications of the laser-pumped light source.

[0121] If implemented according to the proposed embodiment, increased durability, brightness and operation stability of the laser-pumped light source is achieved.

[0122] In general, the proposed invention allows for creating electrode-free high-brightness broadband light sources with highest spatial and power stability and capability of collecting plasma radiation in a solid angle exceeding 9 sr.

[0123] Particular aspects of the subject-matter disclosed herein are set out in the following numbered clauses. The claims of the present disclosure or of any divisional application might be directed to one or more of these aspects. [0124] 1. A laser-pumped plasma light source, comprising: a chamber filled with high- pressure gas, a means for plasma ignition, a region of radiating plasma sustained in the chamber by a focused beam of a continuous wave (CW) laser, at least one beam of plasma radiation exiting the chamber, means for igniting the plasma, characterized in that the means for plasma ignition is a pulsed laser system generating at least one pulsed laser beam focused in the chamber; the chamber consists of a tube, a bottom and a cap; one end of the tube is hermetically connected to the bottom, while the other end of the tube is hermetically connected to the cap, which is equipped with a gas inlet; the tube and the bottom are made from an optically transparent material; the bottom is arranged for introducing the focused beam of the CW laser and each pulsed laser beam into the chamber and the tube, excluding of its end parts, is arranged for the exit of the beam of plasma radiation from the chamber and is configured for reducing aberrations which distort a path of rays of the beam of plasma radiation passing through a tube wall, by the fact that a portion of the tube arranged for exit of the beam of plasma radiation has an axis of symmetry, a center of symmetry, a cylindrical shape of an internal surface, a barrel-like or a toroidal shape of an external surface, and said center of symmetry is located at the region of radiating plasma.

[0125] 2. The laser-pumped light source according to clause 1, wherein the tube and the bottom of the chamber are either sealed by means of glass cement, or made as one piece from a single piece of material.

[0126] 3. A laser-pumped plasma light source, comprising: the chamber filled with high- pressure gas, the means for plasma ignition, the region of radiating plasma sustained in the chamber by the focused beam of the continuous wave (CW) laser, at least one beam of plasma radiation exiting the chamber, means for igniting the plasma, characterized in that means for plasma ignition is a solid-state laser system that contains a first laser for generating a first laser beam in a Q-switched mode and contains a second laser for generating a second laser beam in a free-running mode the chamber consists of the tube, bottom and cap; one end of the tube is hermetically connected to the bottom, and the other end of the tube is hermetically connected to the cap, which is equipped with a gas inlet; the tube and the bottom of the chamber are made of optically transparent material; the bottom is arranged for introducing the focused beam of the CW laser and each pulsed laser beam into the chamber and the tube, excluding of its end parts, is arranged for the exit of the beam of plasma radiation from the chamber.

[0127] 4. A laser-pumped plasma light source, comprising: the chamber filled with high- pressure gas, the region of radiating plasma sustained in the chamber by the focused beam of the continuous wave (CW) laser, at least one beam of plasma radiation exiting the chamber, means for igniting the plasma, characterized in that the means for plasma ignition is a pulsed laser system generating at least one pulsed laser beam focused in the chamber, the chamber consists of the tube, a bottom and a cap; one end of the tube is hermetically connected to the bottom, while the other end of the tube is hermetically connected to the cap, which is equipped with a gas inlet; the tube and the bottom are made from an optically transparent material; the tube and the bottom of the chamber are made of optically transparent material; the bottom is arranged for introducing the focused beam of the CW laser and each pulsed laser beam into the chamber, the tube is arranged for the exit of the beam of plasma radiation from the chamber, and the chamber is located in an external bulb.

[0128] 5. The light source according to clause 4, wherein the external bulb is equipped with a base on which the chamber is fixed.

[0129] 6. The light source according to clause 4 or 5, wherein the beam of the CW laser is focused in the chamber by means of an optical system comprising the chamber bottom and a focusing optical element fixed on the chamber.

[0130] 7. The light source according to any of clauses 4-6, wherein the external bulb is hermetically sealed.

[0131] 8. The light source according to any of clauses 4-7, wherein the external bulb is evacuated.

[0132] 9. The light source according to any of clauses 4-8, wherein the external bulb has a spherical part centered in the region of radiating plasma.

[0133] 10. The light source according to any of clauses 4-9, wherein the external bulb is arranged to prevent ozone generation.

[0134] While specific embodiments are disclosed herein, various changes and modifications can be made without departing from the scope of the invention. The present embodiments are to be considered in all respects as illustrative and non-restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

[0135] High-brightness high-stability laser-pumped plasma light sources designed according to the present invention can be used in a variety of projection systems, for spectrochemical analysis, spectral microanalysis of bio objects in biology and medicine, microcapillary liquid chromatography, for inspection of the optical lithography process, for spectrophotometry and for other purposes.

Claims

What is claimed is: A laser-pumped plasma light source, comprising: a chamber (1) filled with high-pressure gas, a means for plasma ignition, a region of radiating plasma (2) sustained in the chamber (1) by a focused beam of a continuous wave, CW, laser (4), and a beam of plasma radiation (5) exiting the chamber, characterized in that the chamber (1) comprises a tube (11), a bottom (12) and a cap (13); one end of the tube (11) is hermetically connected to the bottom (12), while the other end of the tube (11) is hermetically connected to the cap (13); the tube (11) and the bottom (12) are made from an optically transparent material; the bottom (12) is arranged for introducing the focused beam of the CW laser (4) into the chamber (1); the tube (11) is configured to allow the beam of plasma radiation (5) to exit from the chamber over at least 70% of azimuthal angles in a plane perpendicular to the beam of the CW laser (4) and passing through said region of radiating plasma (2); and the cap (13) is equipped with a gas inlet (14). The light source according to claim 1, wherein a portion of the tube (11) arranged for exit of the beam of plasma radiation (5) has an axis of symmetry and a center of symmetry, wherein said center of symmetry is located at the region of radiating plasma, wherein the portion of the tube preferably further has a cylindrical shape of an internal surface and/or a barrel-like or a toroidal shape of an external surface. The light source according to any of the preceding claims, wherein the beam of the CW laser is focused in the chamber by means of an optical system comprising the chamber bottom (12) and a focusing optical element (16) with a surface configured to minimize total aberrations of the optical system. The light source according to claim 3, wherein the focusing optical element (16) is an aspherical lens. The light source according to claim 3 or 4, wherein the focusing optical element (16) is fixed in a rim (47), which in turn is fixed to an end part of the tube (11) at the end of the tube (11) at which the bottom (12) is provided. The light source according to any of the preceding claims, wherein bottom (12) of the chamber has the form of a lens. The light source according to any of the preceding claims, wherein a part (18) or a detail of the cap (13) is designed as a concave spherical mirror with a center in the region of radiating plasma (2). The light source according to claim 7, wherein the concave spherical mirror radius is not more than 5 mm. The light source according to any of the preceding claims, wherein the focused beam (3) of the CW laser (4) is directed into the chamber (1) vertically upwards. The light source according to any of the preceding claims, wherein a part of the cap (13) is made of a refractory material such as tungsten, molybdenum or alloys based thereof. The light source according to any of the preceding claims, wherein a radius of the internal surface of the tube (11) is not more than 5 mm, preferably not more than 3 mm. The light source according to any of the preceding claims, wherein the tube (11) and/or the bottom (12) of the chamber are made from a material belonging to a group of sapphire, leuco sapphire, fused quartz, crystalline quartz. The light source according to any of the preceding claims, wherein to seal the chamber, the end parts of the tube (11) are used, and the tube (11) and the bottom (12) of the chamber are sealed with glass cement, and/or the cap (13) of the chamber and the tube (11) are sealed using soldering. The light source according to any of the preceding claims, wherein the cap (13) is equipped with a gas inlet (14) arranged for filling the chamber with the gas or/and controlling a pressure and a composition of the gas in the chamber. The light source according to any of the preceding claims, wherein the cap (13) is made from a metal. The light source according to any of the preceding claims, wherein the cap (13) of the chamber is equipped with a heater (35). The light source according to any of the preceding claims, wherein the means for plasma ignition is a pulsed laser system (9) generating at least one pulsed laser beam (21, 23) focused in the chamber, wherein the bottom (12) is arranged for introducing the at least one pulsed laser beam (21, 23) into the chamber (1). The light source according to claim 17, wherein the pulsed laser system is a solid-state laser system (9) generating a pulsed laser beam (21) in Q-switching mode and a pulsed laser beam (23) in free-running mode, the two pulsed laser beams being focused into the chamber. The light source according to any of the preceding claims, wherein the chamber (1) is located in an external bulb (44). The light source according to any of the preceding claims, further comprising an optical collector (6). The light source according to claim 20, wherein the optical collector (6) is a multichannel collector comprising at least three channels (6a, 6b, 6c). The light source according to any of the preceding claims, wherein the tube (11) excluding its end parts is arranged for exit of the beam of plasma radiation (5) from the chamber (1) in all azimuths. A laser-pumped plasma light source, comprising: a chamber (1) filled with high-pressure gas, a means for plasma ignition, a region of radiating plasma (2) sustained in the chamber (1) by a focused beam of a continuous wave, CW, laser (4), and a beam of plasma radiation (5) exiting the chamber, characterized in that the chamber (1) comprises a tube (11), a bottom (12) and a cap (13); one end of the tube (11) is hermetically connected to the bottom (12), while the other end of the tube (11) is hermetically connected to the cap (13); the tube (11) and the bottom (12) are made from an optically transparent material; the bottom (12) is arranged for introducing the focused beam of the CW laser (4) into the chamber (1); the tube (11) excluding its end parts is configured to allow the beam of plasma radiation (5) to exit from the chamber in all azimuthal angles in a plane perpendicular to the beam of the CW laser (4) and passing through said region of radiating plasma (2); wherein a portion of the tube arranged for exit of the beam of plasma radiation has an axis of symmetry, a center of symmetry, a cylindrical shape of an internal surface, a barrel-like or a toroidal shape of an external surface, and said center of symmetry is located at the region of radiating plasma; and the cap (13) is equipped with a gas inlet (14). A method for reducing aberrations in a laser-pumped plasma light source, wherein the method comprises: providing a chamber (1) comprising a tube (11), a bottom (12) and a cap (13), wherein the cap (13) is equipped with a gas inlet (14) and the bottom (12) is arranged for introducing a focused beam of a continuous wave, CW, laser (4) into the chamber, wherein one end of the tube is hermetically connected to the bottom, while the other end of the tube is hermetically connected to the cap, wherein the chamber (1) is filled with high-pressure gas and wherein the tube (11) and the bottom (12) are made from an optically transparent material, sustaining a radiating plasma in a radiating plasma region (2) in the gas-filled chamber (1) by the focused beam of the CW laser (4), wherein the tube (11) excluding its end parts is arranged to allow a beam of plasma radiation to exit from the chamber in all azimuthal angles in a plane perpendicular to the beam of the CW laser (4) and passing through said radiating plasma region (2), and reducing aberrations which distort a path of rays of the beam of plasma radiation (5) passing through a wall of the tube (11), by providing the tube (11) with a shape that has an axis of symmetry, a center of symmetry, a cylindrical shape of an internal surface, and a barrel or a toroidal shape of an external surface, wherein said center of symmetry is located at the region of radiating plasma. The method according to claim 24, wherein the beam of the CW laser (4) is focused by means of an optical system including the bottom (12) and an aspherical lens (16) with a surface minimizing total aberrations of the optical system. The method according to claim 24 or 25 wherein the beam of plasma radiation (5) exits the chamber in a solid angle of not less than 9 sr.