Broadband laser-pumped plasma light source

Abstract

A light source in which plasma radiation is sustained in a gas-filled chamber by a CW laser focusing beam. The gas is an inert gas with a purity of at least 99.99%. The chamber includes a metal housing having at least one window made of MgF2 to output plasma radiation. Each window is positioned within a hole in the housing at the end of a sleeve and is soldered to the sleeve using glass cement, and each sleeve is welded to the hole in the metal housing at an external seam. The sleeve and the housing are made of an alloy in which the coefficient of linear thermal expansion (CLTE) matches the CLTE of the MgF2 and the direction perpendicular to the optical axis of the MgF2 crystal. The technical achievement is to extend the radiation spectrum of the light source into the VUV region.

Description

Technology Field

[0001] Cross-reference regarding related patents and applications

[0002] This patent application is a partial continuation of U.S. Patent Application 17 / 180,063, filed on February 19, 2021, which is now granted; said U.S. Patent Application 17 / 180,063 claims priority based on Russian Patent Application No. RU2020109782, filed on March 5, 2020; it is also a partial continuation of U.S. Application US 16 / 986,424, filed on August 6, 2020, which is now granted as U.S. Patent No. 10,964,523; said U.S. Application US 16 / 986,424 is a partial continuation of U.S. Application US 16 / 814,317, filed on March 10, 2020, which is now granted as U.S. Patent No. 10,770,282; and also October 2021 Priority is claimed based on the Russian patent application RU2021129398 filed on the 8th, and both the patent application and the patent are supplemented here by reference in their entirety.

[0003] Technology field

[0004] The present invention relates to a high-brightness broadband light source having a continuous light discharge function, a gas-filled chamber used therein, and a method for manufacturing the same. Background Technology

[0005] A stationary gas discharge maintained by laser radiation in a pre-generated, relatively high-density plasma is known as a continuous optical discharge (COD).

[0006] COD maintained in a gas-filled chamber by the focused beam of a continuous wave (CW) laser is realized in various gases, particularly in Xe at high gas pressures of up to 200 atm (Carlhoff et al., "Continuous Optical Discharges at Very High Pressure", Physica 103C, 1981, pp. 439-447). A COD-based light source with a plasma temperature of about 20,000 K (Raizer, "Optical discharges" Sov. Phys. Usp. 23(11), Nov. 1980, pp. 789-806) is one of the highest luminance continuous light sources over a wide spectral range from vacuum ultraviolet (VUV) to near-infrared.

[0007] One of the challenges related to generating high-intensity COD-based light sources is increasing the output of vacuum ultraviolet radiation, which in particular requires special transparency of optical materials used to output COD plasma broadband radiation from the chamber and shortwave boundary λb.

[0008] As disclosed in patent application JP2006010675 published on December 1, 2006, high light output in the VUV range is achieved in the photodischarge when the purity of the inert gas in the chamber is at least 99.99%. At the same time, the shortwave boundary of the light source radiation spectrum is determined by the material of the chamber exit window, which may be lithium fluoride (LiF), magnesium fluoride (MgF2), calcium fluoride (CaF2), sapphire (Al2O3), or quartz (SiO2).

[0009] Among these materials, LiF and MgF2 have the shortest transparent wave boundaries around 110 nm. Additionally, among the latter materials, MgF2 has better mechanical and thermal properties and better productivity, and consequently, its use is desirable for extending the radiation spectrum in the VUV range to 100 nm.

[0010] The device disclosed in patent application JP2006010675 uses pulsed mode excitation of photodischarge, and consequently, the disadvantage of said device lies in low average power and light source brightness. In the pulsed mode of photodischarge excitation, the optimal pressure of the chamber is about 1 atm, while the chamber temperature is close to room temperature, which eliminates problems related to sealing an exit window made of any of the aforementioned optical materials. However, in the case of a high-brightness plasma radiation source with a continuous photodischarge function, the situation is fundamentally different.

[0011] For example, as disclosed in Patent US 10964523 published in the Publication dated March 30, 2021 and supplemented herein by reference, the optimal continuous generation of COD plasma radiation characterized by a spectral luminance of 50 mW / (mm² nm average) or greater and a relative luminance instability σ of less than 0.1% is preferably achieved by having a possible maximum operating temperature of 600 to 900 K or greater on the inner surface of the chamber at an optimal gas pressure within the chamber of 50 atm or greater, and positioning the chamber wall at a distance of less than 5 mm, preferably 3 mm or less, from the plasma radiation region. A sealed bulb made of fused quartz and used as the chamber meets at least partially these criteria.

[0012] However, the transparent boundary λb of quartz 170 nm is the same as the other optical materials mentioned above, especially MgF2(λb It is inferior to 110 nm. At the same time, the option of replacing the precursor material with MgF2 is difficult due to its mechanical properties, whereas using an MgF2 window is also problematic because it is difficult to seal the window at high temperatures and high pressures.

[0013] In order to increase the operating temperature of the chamber, patent US 10109473, published in the publication dated October 23, 2018, proposes mechanically sealing the chamber window using a C-ring made of an elastic metal such as steel.

[0014] However, this solution method is mainly λb It is related to using a 145nm sapphire window. Applying an MgF2 window to this type of seal is problematic because the mechanical strength of the window is insufficient.

[0015] U.S. Patent 10609804, published in the Publication of May 31, 2020, describes a laser-pumped plasma light source comprising a metal column-shaped housing composed of two housing parts and a gas-filled chamber having coaxial inlet and outlet windows installed in a sealed manner on the ends of the housing. Each window, with a cylindrical side nickel-plated, is located inside a circular nickel-plated kobar sleeve and is soldered to the inner surface of the sleeve using Ag solder. Furthermore, each circular sleeve to which each window is soldered is soldered or welded to one of the housing parts on an outside seam. After the inner chamber parts (elliptical mirror and laser radiation shield) are installed, the housing part is welded together with the mounted window. After welding, the housing is vacuumed and gas is filled through a welded or sealed nozzle under pressure. The coefficient of linear thermal expansion (CLTE) of the kobar sleeve to which each window is soldered matches the CLTE of sapphire, and as a result, the use of a sapphire window in the chamber is proposed.

[0016] Commonly used quartz bulbs (λb Compared to 170 nm, the light source is when a sapphire window is used (λb It features a wider radiation spectrum in the VUV range (145 nm). In addition, it is equipped with a more powerful chamber that can increase the output of laser pumping and consequently increase the power of output radiation in both the UV and VUV ranges.

[0017] However, further expansion of the VUV spectrum is limited because it is difficult to apply an MgF2 window to this type of plasma light source. The CLTE of the MgF2 crystal differs significantly in the direction of the optical axis and in the direction perpendicular to the optical axis, and accordingly, is 13.7×10⁻⁶ / K and 8.48×10⁻⁶ / K. Consequently, the sealing connection between an isotropic metal circular sleeve and an anisotropic MgF2 crystal soldered thereto is unreliable when the chamber is heated to 600-900K, which is required to optimally generate radiation from the continuous photodischarge plasma. This unreliability of the seal stems from the fact that the CLTE of the metal solder (~20×10⁻⁶ / K) is also significantly different from the CLTE of MgF2. Furthermore, gas pressure applied to the window causes the sealed joint to shift and rupture, thereby reducing its reliability. Expanding the spectrum of a similar plasma light source in the VUV range has little effect due to the fact that the plasma radiation beam is formed solely by the reflection of plasma radiation by metal mirrors inside the chamber. The reflection coefficient of metal mirrors is low in the VUV range (less than 20% at a wavelength of 110 nm for aluminum). The presence of mirrors inside the chamber results in the position of the lens focusing the CW laser beam being located outside the chamber housing. This limits the focusing sharpness of the CW laser beam and reduces the brightness of the light source. Furthermore, the presence of mirrors does not allow for minimizing the dimensions of the space inside the chamber to suppress convective flow, which leads to instability in the existing radiated power. Another disadvantage of the above design is that the laser radiation beam propagates toward the exit window, requiring special measures to be taken to block it.

[0018] Therefore, it is necessary to generate a light source with higher brightness and stability that has a wider radiation spectrum in the VUV range and is free from the aforementioned disadvantages.

[0019] The technical problem and result of the present invention is to extend the radiation spectrum of a laser-pumped plasma light source into the VUV range, while simultaneously providing high brightness and stability of the broadband radiation of the laser-pumped plasma light source.

[0020] The present invention is essentially a minimum transmission boundary (λb The invention involves using an advanced optical material having a wavelength of 110 nm, namely MgF2, as a material for a window to output a plasma radiation beam from a chamber. This allows the radiation spectrum of a laser-pumped plasma light source to be expanded in the VUV range.

[0021] The gas inside the chamber belongs to an inert gas with a purity of at least 99.99% to remove self-absorption of VUV radiation caused by impurities.

[0022] Crystalline magnesium fluoride is anisotropic and characterized by weak double refraction. According to the present invention, in order to eliminate double refraction of the plasma radiation beam, the end surface of the axisymmetric sleeve and the adjacent MgF2 exit window surface are essentially perpendicular to the optical axis of the MgF2 crystal.

[0023] To provide high brightness and stability of the light source, the possibility of operating at a high temperature of at least 600K and a pressure of about 50 atm or more is achieved by soldering the chamber window with glass cement and sealing it. According to the present invention, the glass cement soldering process involves the application of a single-stage annealing of the joint at a temperature of at least 400°C, which results in the possibility of operating the joint at a temperature of up to 900K. The window is soldered to a separate metal part of the housing designed as a sleeve. After annealing, the metal part of the chamber housing is joined by welding in such a manner that the sealed joint is not exposed to another annealing that could reduce the reliability of the sealed joint.

[0024] To provide a reliable seal for the MgF2 exit window, the sleeve and housing are made of an iron-nickel alloy having a predefined CLTE, which matches the CLTE of the crystalline magnesium fluoride in a direction perpendicular to the optical axis of the crystal, such as the 47ND alloy.

[0025] To prevent window cracking caused by their irregular cooling, the window is soldered onto an axially symmetric metal sleeve end approximately 1 cm in length, instead of soldering it onto the housing portion of the chamber with a complex shape. Soldering is performed using a sealed joint component with a matching linear coefficient of thermal expansion (CLTE) that is optimally positioned in terms of gravity. Subsequently, the sleeve having the soldered window is welded to the housing at the outer seam. In another embodiment, the sleeve having the soldered window is welded to the housing portion, and after the inner chamber element is mounted, the housing is permanently welded together. At the same time, irregularities in heating and cooling of the assembled chamber structure are offset through the axially symmetric sleeve.

[0026] According to the present invention, a window is installed inside a gas-filled chamber. On the one hand, sealing reliability is improved due to the high pressure of the gas inside the chamber compressing the sealing element. On the other hand, the possibility of manufacturing the chamber with optimally minimized dimensions is realized when the chamber wall, including the optical element of the chamber, is located at a distance of less than 5 mm from the plasma radiation region. This suppresses turbulence of convective flow inside the chamber and provides high stability of the radiation source.

[0027] The inner chamber element includes a lens for focusing a CW laser beam. Such focusing lens preferably has an aspherical design and is positioned between the plasma emission region and the entrance window to enhance the brightness of the light source due to the sharpest possible focusing of the CW laser beam. For the same purpose, at least one retroreflector in the form of a spherical mirror centered, for example, in the plasma emission region, may be placed within the chamber and positioned opposite the exit window and / or on the axis of the focused laser beam. The exit window may also be a lens designed to reduce aberrations that distort the path of the plasma emission beam passing through the exit window, and / or to reduce the angular aperture of the outgoing plasma emission beam.

[0028] To prevent the generation of ozone and the absorption of plasma radiation beams, a vacuum or gas environment that does not absorb VUV radiation having a wavelength of 110 nm or more may be located outside the MgF2 outlet window. For this purpose, in one embodiment of the present invention, the chamber may be connected to an external chamber in a sealed manner, having an object filled with a vacuum or gas environment that does not absorb plasma radiation exiting the chamber through the MgF2 window and transports plasma radiation beams. Since the optimal temperature may be high, such as 600K or higher, the chamber may be connected to the external chamber in a sealed manner through a branch pipe that functions as a thermal bridge between the chamber and the external chamber. Additionally, a cooling radiator may be installed in the branch pipe to prevent heating of the external chamber.

[0029] Other embodiments of the present invention aim to further increase the brightness and stability of a laser-pumped plasma radiation source, as well as improve its performance.

[0030] The above-mentioned and other objects, advantages, and features of the present invention will become more apparent from the following non-limiting description of embodiments of the present invention provided as examples with reference to the accompanying drawings.

[0031] The essence of the present invention is explained by the drawings. Brief explanation of the drawing

[0032] FIGS. 1 and FIGS. 2 are cross-sectional views of a broadband laser pumping light source according to embodiments of the present invention. Figure 3 is a drawing showing the appearance of a broadband laser pumping light source. FIGS. 4 and 5 are drawings showing a broadband laser pumping light source according to embodiments of the present invention. Specific details for implementing the invention

[0033] Identical device elements are designated by the same reference number in the drawings.

[0034] These drawings do not encompass or limit the full scope of embodiments of such technical solutions, but are merely examples of specific embodiments of the present invention.

[0035] According to an embodiment of the present invention illustrated in FIG. 1, a broadband laser pumping light source comprises a chamber (1) filled with high-pressure gas and a plasma radiation region (2) maintained in the chamber by a focused beam (3) of a continuous wave (CW) laser (4). The chamber (1) comprises a metal housing (5) including a window (6a) for introducing the CW laser beam into the chamber and at least one window (6b) for outputting a plasma radiation beam (8) intended for subsequent use from the chamber.

[0036] The light source also includes means for initiating plasma ignition. As a means for plasma ignition, a pulsed laser system (9) may be used to generate at least one pulsed laser beam (10) focused in a chamber area designed for maintaining plasma radiation (2). In other embodiments of the present invention, ignition electrodes may be used as means for plasma ignition.

[0037] According to the present invention, a CW laser beam can be guided into the chamber by a dichroic mirror (11) and focused by a lens (12) placed in the chamber between the window (6a) and the plasma radiation region (2), such that the lens (12) provides sharp focusing of the CW laser beam and thereby increases the brightness of the light source. At the same time, the lens (12) can be used to focus a pulsed laser beam (10) at the start of plasma ignition.

[0038] The luminance of the light source is increased by minimizing the total aberrations of the optical system by ensuring the sharpest possible focusing of the CW laser beam using an optical system comprising the window (6a) and the focusing lens (12), preferably a focusing lens (12) having an aspherical design. The focusing lens (12) is preferably positioned at the smallest possible distance from the plasma radiation area (2), and the distance does not exceed 5 mm. To facilitate chamber design, the window (6a) can be made in the shape of a plate or lens having a spherical surface, for example, using simple manufacturing techniques. The aspherical lens (12) can be made of glass or quartz to facilitate its fabrication.

[0039] At least one window (6b) for outputting a plasma radiation beam (8) from the chamber is made of crystalline magnesium fluoride (MgF2). MgF2 is characterized by high productivity and, at the same time, has the shortest wavelength transmission boundary among optical materials. Therefore, the short wavelength boundary of the spectrum of the plasma radiation beam exiting the chamber is determined by the transmission limit of MgF2 in the vacuum ultraviolet (VUV) region, which is about 110 nm. In addition, the gas belongs to inert gases with a purity of at least 99.99% or is a mixture thereof to eliminate self-absorption of VUV radiation by gas impurities. This allows the radiation spectrum of the light source to be extended into the vacuum ultraviolet region.

[0040] In FIG. 1, the plasma radiation beam (8) is guided straight from the plasma radiation region (2) to a window (6b) made of MgF2 without reflection. In contrast to sources formed by metal mirrors inside a chamber where the plasma radiation beam has a reflection coefficient low in the VUV range (less than 20% at λ = 110 nm), this ensures that there is no blocking or suppression of VUV components in the spectrum of the plasma radiation beam.

[0041] Each of the above windows (6a, 6b) is positioned inside the chamber on the end of one of the sleeves (7a, 7b) closest to the plasma radiation region (2). Each of the above windows (6a, 6b) is soldered to one of the sleeves (7a, 7b) using glass cement (13). The window soldering performed during the annealing process ensures the possibility of operating the sealed joint and chamber assembly at a temperature of up to 900K, which is optimal for achieving high brightness and stability of the light source.

[0042] Each of the sleeves (7a, 7b) having soldered windows (6a, 6b) is positioned in one of the holes within the housing (5) and welded into the hole of the housing (5) on the external welding seams (14). Additionally, the internal portions of the axisymmetric sleeves (6a, 6b) are the external portions of the chamber that do not come into contact with the gas in which the chamber is filled. By placing the windows inside the chamber, this improves the reliability of the sealing joint due to the high pressure of the gas inside the chamber, which compresses the sealing material (glass cement (13)) and facilitates the sealing of the optical elements.

[0043] According to the present invention, the end surface of the sleeve (7b) and the surface of the adjacent MgF2 exit window (6b) are essentially perpendicular to the optical axis of the MgF2 crystal. The linear coefficient of thermal expansion (CLTE) of the glass cement (13), the material of the sleeves (7a, 7b), and the housing (5) matches the CLTE of the crystalline magnesium fluoride in a direction perpendicular to the optical axis of the MgF2 crystal. All of the above-mentioned factors provide high reliability and a long lifespan for the windows and chamber assembly. Preferably, the sleeves and chamber housings are made of a 47 ND iron-nickel alloy that meets these requirements.

[0044] The chamber (1) is filled with high-pressure gas through a soldered-welded pipe or through a gas port (15) designed to control the pressure and / or composition of the gas inside the chamber.

[0045] Accordingly, the present invention provides a method for manufacturing highly reliable chambers having an MgF2 window operating at high pressure (about 50 atm) and temperature (about 900°K) and for producing a brighter and more stable COD-based light source having the broadest radiation spectrum in the VUV range.

[0046] According to one embodiment of the present invention illustrated in FIG. 1, a vacuum or a gas environment such as helium or argon that does not absorb VUV radiation having a wavelength of 110 nm or more is located outside the MgF2 outlet window (6b) for outputting a plasma radiation beam (8) from the chamber. For this purpose, the chamber (1) may be connected in a sealed manner to an outer chamber (17) having an object to which the plasma radiation beam (8) is transported by a branch pipe (16).

[0047] In this case, the beam is carried out without ozone generation and without loss of the VUV component of plasma radiation.

[0048] High stability and high brightness of plasma radiation in continuous operation mode are achieved when the gas pressure inside the chamber is about 50 atm or higher and the chamber temperature is about 600 K or higher. Due to the high temperature of the chamber (1), the branch pipe (16) is designed to function as a thermal bridge between the chamber (1) and the outer chamber (17). For this purpose, at least some of the branch pipe (16) is made of a low thermal conductivity, for example, thin stainless steel. To cool at least some of the branch pipe (16) removed from the window (6b), it is designed as a cooling radiator (18) to prevent heating of the outer chamber (17). Sealed joints of the branch pipe (16) to the chamber (1) and the outer chamber (17) can be provided using a sealing gasket (19) which may be made of copper on at least the side of the heated chamber (1).

[0049] In the embodiment of the present invention illustrated in FIG. 1, all axisymmetric sleeves (7a, 7b) to which windows (6a, 6b) are soldered are welded to a single common housing portion (5). Additionally, a plasma radiation region (2) is located within a cavity of the housing (5) formed at the intersection of at least two holes, and within each of the at least two holes, one of the sleeves (7a, 7b) is located together with one of the windows (6a, 6b). The sleeves (7a, 7b) have a variable outer diameter, and the windows (6a, 6b) are located on the ends of the sleeves with a relatively small outer diameter.

[0050] A broadband laser pumping light source operates as described below. First, a chamber (1) of the light source is manufactured, comprising a metal housing (5) having at least two windows (6a, 6b) as in FIG. 1. At least one window (6b) is made of MgF2. The material of at least one of the windows (6a) may be glass having a CLTE that matches the CLTE of MgF2. The chamber housing is manufactured from a 47 ND precision alloy having a CLTE that matches the CLTE of MgF2. Each of the windows (6a, 6b) is soldered to one of the sleeves (7a, 7b) using glass cement (13) by applying annealing at a temperature of at least 400°C. Each sleeve to which the window is soldered is welded into a hole in the metal housing (5). High-pressure gas is filled into the chamber through a sealed pipe or through a gas port (15).

[0051] Broadband radiation of the COD plasma is generated as described below. The focused beam (3) of the CW laser (4) is guided into a chamber area (2) intended to maintain plasma radiation. Preferably, high-purity inert gases and mixtures thereof are used as gases. At least one pulsed laser beam (10) is generated by a pulsed laser system (9). The CW laser beam and the pulsed laser beam are introduced into the chamber (1) through a window (6a). At the same time, an optical system comprising the window (6a) and a focusing lens (12) provides sharp focusing of the laser beam. The pulsed laser system (9) is used to provide optical breakdown and to generate a starting plasma having a density exceeding the threshold density of the COD plasma, which has a value of about 10¹⁸ electrons / cm³. The concentration and volume of the starting plasma are sufficient to reliably maintain a continuous photovoltaic discharge by a CW laser focusing beam (3) with a relatively low power not exceeding 300W. In a fixed mode, broadband high-intensity radiation is output from the plasma radiation region (2) of the continuous photovoltaic discharge using at least one plasma radiation beam (8). The shortwave boundary of the plasma radiation spectrum exiting the chamber is determined by the MgF2 transmission limit, which is approximately 110 nm. The beam (8) exiting through the MgF2 exit window (7b) in the chamber is intended for subsequent use, for example, in an external chamber (17). The chamber (1) may be connected in a sealed manner to an external chamber (17) filled with a vacuum or gas environment that does not absorb VUV radiation exiting from the chamber (1). In operating mode, the temperature of the chamber (1) is preferably about 600K or higher. Additionally, thermal isolation between the chamber (1) and the outer chamber (17) is provided by a branch pipe (17) equipped with a cooling radiator (19) and designed as a thermal bridge.

[0052] In the embodiment of the present invention illustrated in FIG. 2, the chamber (1) comprises a welded metal housing (5) comprising at least two housing portions (5a, 5b), and in each of the at least two housing portions, a sleeve (7a, 7b) is welded together with a window (6a, 6b) soldered to the sleeve (7a, 7b).

[0053] After the internal chamber elements, including the focusing lens (12) together with the mounting or casing (20) and insert (21), are installed, the housing parts (5a, 5b) having windows (6a, 6b) are welded together with the welding seam (22). When welding the housing parts (5a, 5b), the axisymmetric sleeves (7a, 7b) welded to the housing parts together with the windows (6a, 6b) compensate for the irregular heating and cooling of the assembled chamber (1).

[0054] The appearance of the welding housing of the light source is schematically illustrated in Fig. 3.

[0055] To simplify the chamber design, the welds (14, 22) are located on the outer surface of the housing (5).

[0056] FIG. 4 schematically illustrates another embodiment in which an MgF2 window (6b) for outputting a plasma radiation beam (8) from a chamber is a lens designed to reduce the aperture angle of the plasma radiation beam or reduce aberrations that distort the path of the plasma radiation as it passes through the window (6b). Generally, the window (6b) is designed as a meniscus or other type of matching lens. This increases the brightness of the light source, minimizes the dimensions of the light source, and improves the ease of operation of the light source.

[0057] For a similar purpose of increasing the brightness of the light source, retroreflectors (23, 24), designed as spherical mirrors centered in the plasma radiation region (2), may be placed within the light source chamber as shown in FIG. 4. The retroreflectors (23, 24) are positioned opposite the MgF2 window and / or on the axis of the focused laser beam (3).

[0058] To eliminate the presence of undesirable CW laser radiation in the plasma radiation beam, the direction of the plasma radiation beam (8) is different from the direction of the CW laser beam (3) passing through the plasma radiation region (2). This prerequisite is easily implemented in the design of the chamber (1) in which the housing is designed as a cube or a rectangular prism, as shown in FIGS. 1, 2, 3, and 4, in which case the CW laser focusing beam (3) and each plasma radiation beam (8) are located on mutually orthogonal axes intersecting in the plasma radiation region (2).

[0059] In a preferred embodiment of the present invention, the axis of the CW laser focusing beam (3) is guided upward vertically, that is, against gravity or nearly vertically as illustrated in FIGS. 1, 2, and 4. The proposed design achieves maximum stability for the laser-pumped light source radiation output. This is generally due to the fact that the plasma radiation region (2) moves slightly from the focus toward the CW laser focusing beam (3) to the cross-section of the focused laser beam where the intensity of the CW laser focusing beam (3) is still sufficient to maintain the plasma radiation region (2). When the CW laser focusing beam (3) is guided upward from the bottom, the plasma radiation region (2), containing the hottest plasma with the lowest mass density, tends to float under the influence of a buoyant force. The rise of the plasma radiation region (2) ends at the position closest to the focus where the cross-section of the CW laser focusing beam (3) is relatively small and the laser radiation intensity is relatively high. On the one hand, this increases the brightness of the plasma radiation, and on the other hand, it equalizes the force acting on the plasma radiation region, and this equalization ensures high stability of the radiation output of the high-brightness laser pumping light source.

[0060] The stability of the output characteristics of the laser pumping light source is also affected by the magnitude of the pulse obtained under the action of buoyancy by the gas heated in the plasma radiation region (2). The less turbulence there is between the pulse obtained by the gas and the convection, the closer the plasma radiation region (2) is to the top wall of the chamber. Consequently, to ensure more stable output characteristics of the light source, the top wall of the chamber housing is located at a distance of 5 mm or less from the plasma radiation region (2).

[0061] Suppression of convective turbulence within the chamber and improvement of the stability of the light source output characteristics are achieved by reducing the internal volume of the chamber. For this purpose, in a preferred embodiment of the present invention, the chamber walls, as well as the focusing lens (13) and each window (6b) for outputting the plasma radiation beam, are positioned at a distance of 5 mm or less from the plasma radiation area.

[0062] Another embodiment of a light source according to the present invention is schematically illustrated in FIG. 5. In this embodiment, the chamber housing includes several windows (6b, 6c) for outputting several plasma radiation beams (8) from a chamber (1) required for specific applications of the light source.

[0063] As the CW laser (4), it is preferable to use a high-efficiency diode near-infrared laser that outputs radiation to the optical fiber (25). At the exit of the optical fiber (25), the magnifying laser beam is guided to a collimator (26), for example, in the form of a focusing lens. After the collimator (26) and the dichroic deflection mirror (11), the CW laser magnifying beam is guided into the chamber (1). The optical system, window (6a), and focusing lens (12) ensure sharp focusing of the CW laser beam (3) necessary to achieve high brightness of the light source.

[0064] In the embodiment of the invention illustrated in FIG. 5, ignition of the starting plasma is provided by a solid-state laser system comprising a first laser (27) for generating a first laser beam (28) in Q-switching mode and a second laser (29) for generating a second laser beam (30) in free-running mode. The pulsed laser having an active element (31) is equipped with an optical pumping source, for example, in the form of a flash lamp (32), and the pulsed laser preferably has common mirrors (33, 34) of the cavity. The first laser (27) is equipped with a Q-switch (35).

[0065] Two pulsed laser beams (28, 30) are guided into the chamber as shown in FIG. 5 and focused on a region (2) for maintaining plasma radiation. The first laser beam (28) is for starting plasma ignition or optical dielectric breakdown. The second laser beam (30) is for generating a plasma having a sufficiently high volume and density to maintain the plasma radiation region (2) by means of a CW laser focusing beam (3).

[0066] Preferably, the CW laser wavelength (λCW) is different from the wavelengths (λ1, λ2) of the first and second pulsed laser beams (28, 30). For example, the CW laser wavelength may have a radiation wavelength of λCW = 0.808 μm or 0.976 μm, and the pulsed laser may have a radiation wavelength of λ1 = λ2 = 1.064 μm. This allows the dichroic mirror (11) to be used to receive the laser beam (36) of the CW laser (4) and the pulsed laser beams (28, 30). Additionally, a tilt mirror (37) may be used to deliver the pulsed laser beams (28, 30) as shown in FIG. 5.

[0067] These embodiments of the present invention provide reliability of laser ignition and user-friendliness of the light source. Unlike sources using electrodes for starting plasma ignition, there is a possibility to optimize the chamber shape, reduce convective turbulence within the chamber, and minimize optical aberrations.

[0068] In addition, the device parts of this embodiment are identical to the device parts of the embodiments described above and have the same item numbers in FIG. 5, and their detailed description is omitted.

[0069] Generally, the proposed invention enables the expansion of the radiation spectrum in the VUV spectral region and ensures high brightness and stability of the laser-pumped plasma radiation source.

[0070] Industrial applicability

[0071] A high-brightness, high-stability laser pump light source designed according to the present invention can be used in various projection systems for spectrochemical analysis, spectral microanalysis of biological objects in the fields of biology and medicine, microcapillary liquid chromatography, inspection of optical lithography processes, spectrophotometric measurement, and inspection for other purposes.

Claims

Claim 1 As a laser pumping plasma light source, the laser pumping plasma light source comprises: a chamber filled with high-pressure gas; a plasma ignition means; and a plasma radiation region maintained within the chamber by a continuous wave (CW) laser focusing beam; The apparatus comprises at least one plasma radiation beam exiting from the chamber, the chamber comprising a metal housing having a window for introducing a CW laser beam into the chamber and at least one window for outputting a plasma radiation beam from the chamber, wherein the CW laser beam is focused by a lens installed within the chamber between the window and the plasma radiation region, the gas belongs to inert gases having a purity of at least 99.99% or a mixture thereof, and one or more windows for outputting the plasma radiation beam are made of crystalline magnesium fluoride (MgF2), each window is positioned inside the chamber at the end of a sleeve closest to the plasma radiation region, the sleeve is positioned within a hole in the metal housing, each window is soldered to the sleeve using glass cement, the sleeve is welded to the hole in the metal housing together with the window soldered to the sleeve, and the chamber is connected in a sealed manner to an external chamber having an object irradiated by plasma radiation through the window made of MgF2 using a branch pipe, and at least some of the branch pipes are made of low thermal conductivity. A laser-pumped plasma light source. Claim 2 A laser pumping plasma light source, wherein the surface of the cross-sectional portion of the sleeve and the adjacent surface of the window made of MgF2 are substantially perpendicular to the optical axis of the MgF2 crystal. Claim 3 A laser pumping plasma light source according to claim 1, wherein each sleeve and metal housing is made of a nickel-iron alloy such that the coefficient of linear thermal expansion (CLTE) matches the CLTE of the crystalline magnesium fluoride (MgF2) and the direction perpendicular to the optical axis of the MgF2 crystal. Claim 4 A laser-pumped plasma light source according to claim 1, wherein the shortwave boundary of the plasma radiation beam spectrum is determined by the MgF2 transmission boundary in the vacuum ultraviolet (VUV) region at 110 nm. Claim 5 A laser-pumped plasma light source according to claim 1, wherein a vacuum or gas environment that does not absorb vacuum ultraviolet (VUV) radiation having a wavelength of 110 nm or more is located outside the window made of MgF2. Claim 6 In paragraph 5, a laser-pumped plasma light source in which a cooling radiator is installed in the branch pipe. Claim 7 A laser-pumped plasma light source, wherein the plasma radiation beam is guided directly from the plasma radiation region without reflection to a window made of MgF2. Claim 8 A laser pumping plasma light source, wherein all of the sleeves are axisymmetric sleeves to which the windows are soldered, and the axisymmetric sleeves are welded to a metal housing formed as a single unit. Claim 9 A laser pumping plasma light source according to claim 1, wherein the plasma radiation region is located within a metal housing cavity formed by the intersection of at least two holes, and a sleeve with a window is present within each of the at least two holes. Claim 10 A laser pumping plasma light source according to claim 1, wherein at least one of the sleeves is located within a hole of the metal housing, the sleeve has a variable outer diameter, and the window is located at the end of the sleeve having a relatively small outer diameter. Claim 11 A laser pumping plasma light source according to claim 1, wherein the metal housing comprises at least two housing portions having the windows, and the housing portions are welded together after an inner chamber portion is installed. Claim 12 A laser-pumped plasma light source according to claim 11, wherein at least one retroreflector is disposed within the chamber in the form of a spherical mirror centered in the plasma radiation region, for example. Claim 13 A laser-pumped plasma light source according to claim 1, wherein welded portions are located outside the metal housing. Claim 14 A laser-pumped plasma light source, wherein the plasma ignition means is a solid-state laser system that generates two pulsed laser beams in Q-switching mode and free-running mode while the gas pressure inside the chamber is 50 bar or more and the internal surface temperature of the chamber is at least 600 K in continuous operation mode. Claim 15 A laser pumping plasma light source according to claim 1, wherein the CW laser focusing beam is guided vertically upward into the chamber and the top wall of the metal housing is located at a distance of 5 mm or less from the plasma radiation region. Claim 16 A laser pumping plasma light source according to claim 1, wherein the lens focusing the CW laser beam and the respective window outputting the plasma radiation beam are located at a distance of 5 mm or less from the plasma radiation region. Claim 17 A laser pumping plasma light source according to claim 1, wherein the window is a lens configured to reduce aberrations that distort the light path of a plasma radiation beam passing through the window and to reduce the angular aperture of a plasma radiation beam exiting the chamber. Claim 18 A laser pumping plasma light source according to claim 1, wherein the direction of the plasma radiation beam is different from the direction of the continuous wave (CW) laser beam that has passed through the plasma radiation region. Claim 19 A laser pumping plasma light source according to claim 1, wherein the chamber is designed as a rectangular prism, and the CW laser focusing beam and the plasma radiation beam have mutually orthogonal axes intersecting in the plasma radiation region. Claim 20 A laser pumping plasma light source according to claim 1, wherein the metal housing includes a sealed gas inlet or gas port designed to fill the chamber with gas and control the pressure and composition of the gas within the chamber.

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