BROADBAND LASER-PUMPED PLASMA LIGHT SOURCE

Abstract

A light source with radiating plasma sustained in the gas-filled chamber by a focused beam of CW laser. The gas is inert gas with a purity of at least 99.99%. The chamber contains a metal housing with at least one window made of MgF.sub.2 for outputting a plasma radiation. Each window is located in a hole of the housing on the end of a sleeve and is soldered to the sleeve by means of glass cement, and each sleeve is welded to the hole of the metal housing on the outside seam. The sleeves and the housing are made of an alloy with a coefficient of linear thermal expansion (CLTE), matched with the CLTE of the MgF.sub.2 crystal in the direction perpendicular to the optical axis of the MgF.sub.2 crystal. The technical result consists in expanding the radiation spectrum of the light source into the VUV region.

Claims

1. A laser-pumped plasma light source, comprising: a chamber filled with a 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 that contains a metal housing with a window for introducing into the chamber a beam of the CW laser and with at least one window for outputting a beam of plasma radiation from the chamber, wherein the beam of the CW laser is focused by a lens installed in the chamber between the window and the region of radiating plasma, the gas belongs to inert gases with a purity of at least 99.99% or is a mixture thereof, at least one window for outputting the beam of plasma radiation is made of crystalline magnesium fluoride (MgF.sub.2), each window is located on an inner side of the chamber on an end of a sleeve closest to the region of radiating plasma, the sleeve located in a hole of the housing, each window is soldered to the sleeve by means of glass cement and the sleeve with the window soldered to it is welded to the hole of the metal housing.

2. The light source according to claim 1, wherein a surface of the end face of the sleeve and an adjacent surface of the MgF.sub.2 window are substantially perpendicular to an optical axis of the MgF.sub.2 crystal.

3. The light source according to claim 1, wherein each sleeve and the housing are made of a nickel-iron alloy with a coefficient of linear thermal expansion (CLTE) matched with the CLTE of the crystal magnesium fluoride in a direction perpendicular to an optical axis of the MgF.sub.2 crystal.

4. The light source according to claim 1, wherein a short-wave boundary of a spectrum in the beam of plasma radiation is determined by a MgF.sub.2 transmission boundary in a vacuum ultraviolet (VUV) region, being equal to 110 nm.

5. The light source according to claim 1, wherein a vacuum or gas environment, which does not absorb VUV radiation with a wavelength of 110 nm and more, is located outside the MgF.sub.2 window.

6. The light source according to claim 5, wherein the chamber filled with the high-pressure gas is sealingly connected to an outside chamber with objects that are irradiated through the MgF.sub.2 window by plasma radiation, said outside chamber is sealingly connected by means of a branch pipe made as a thermal bridge and equipped with a cooling radiator.

7. The light source according to claim 1, wherein the beam of plasma radiation is directed from the region of the radiating plasma to the MgF.sub.2 window directly without reflections.

8. The light source according to claim 1, wherein all sleeves are axisymmetric sleeves with the windows soldered to them, the axisymmetric sleeves are welded to the housing made in one piece.

9. The light source according to claim 1, wherein the region of radiating plasma is located in a housing cavity formed by an intersection of at least two holes in each of which there is a sleeve with a window.

10. The light source according to claim 1, wherein at least one the sleeves is located in the hole of the housing, said sleeve has a variable outer diameter and the window is located at the end of the sleeve with a smaller outer diameter.

11. The light source according to claim 1, wherein the housing contains at least two housing parts with the windows, said housing parts are welded together after internal chamber parts are installed.

12. The light source according to claim 11, in the chamber of which at least one retroreflector is placed, for example, in ϕ form of a spherical mirror centered in the region of radiating plasma.

13. The light source according to claim 1, wherein welds are outside the housing.

14. The light source according to claim 1, wherein the means for plasma ignition is a solid-state laser system generating two pulsed laser beams in Q-switching mode and in free-running mode, while in a continuous mode of operation a gas pressure in the chamber is around 50 bar or higher with a temperature of the chamber's inside surface of at least 600 K.

15. The light source according to claim 1, wherein the focused beam of the CW laser is directed into the chamber vertically upwards and an upper wall of the housing is located at a distance from the region of radiating plasma of no more than 5 mm.

16. The light source according to claim 1, wherein the lens focusing the beam of the CW laser and each window for outputting the beam of plasma radiation are located at a distance from the region of radiating plasma of no more than 5 mm.

17. The light source according to claim 1, wherein the window is a lens arranged for reducing aberrations which distort a path of rays of the beam of plasma radiation passing through the window, and for reducing the angular aperture of the beam of plasma radiation exiting the chamber.

18. The light source according to claim 1, wherein a direction of the beam of plasma radiation differs from a direction of the CW laser beam having passed through the region of radiating plasma.

19. The light source according to claim 1, wherein the chamber housing is designed as a rectangular prism, while the focused beam of the CW laser and the beams of plasma radiation have mutually orthogonal axes which intersect in the region of radiating plasma.

20. The light source according to claim 1, wherein the housing contains either a sealed gas inlet or a gas port designed to fill the chamber with gas and to control the pressure and composition of the gas in the chamber.

Description

BRIEF DESCRIPTION OF FIGURES

[0029] The essence of invention is explained by drawings wherein:

[0030] FIG. 1, FIG. 2—cross-section of the broadband laser-pumped light source according to embodiments of this invention.

[0031] FIG. 3—external view of the broadband laser-pumped light source.

[0032] FIG. 4, FIG. 5—diagram of the broadband laser-pumped light source according to embodiments of this invention.

[0033] Identical device elements are designated by the same reference numbers on the drawings.

[0034] 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.

EMBODIMENTS OF INVENTION

[0035] According to the example of invention embodiment shown in FIG. 1, the broadband laser-pumped light source comprises a chamber 1 filled with gas at high pressure, with a region of radiating plasma 2 sustained in the chamber by a focused beam 3 of a continuous wave (CW) laser 4. The chamber 1 contains a metal housing 5 comprising 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 contains a means for starting plasma ignition. As the means for plasma ignition a pulsed laser system 9 can be used generating at least one pulsed laser beam 10 focused in the chamber region designed for sustenance of the radiating plasma 2. In other embodiments of invention, igniting electrodes can be used as the means for plasma ignition.

[0037] According to the invention, the CW laser beam can be directed into the chamber by means of a dichroic mirror 11 and focused by means of a lens 12 placed in the chamber between the window 6a and the region of radiating plasma 2, which provides for sharper focusing of the CW laser beam and thereby increases the light source brightness. The lens 12 can be simultaneously used to focus the pulsed laser beam 10 at the time of starting plasma ignition.

[0038] The light source brightness is increased by ensuring the sharpest possible focus of the CW laser beam using an optical system which comprises the window 6a and the focusing lens 12, preferably with an aspherical design, in order to minimize total aberrations of the said optical system. The focusing lens 12 is preferably positioned at the smallest possible distance from the region of radiating plasma 2, the distance not exceeding 5 mm. In order to facilitate the chamber design, the window 6a can be made using a simple manufacturing technique, for example, in the shape of a plate or lens with a spherical surface. The aspherical lens 12 can be made of glass or quartz to facilitate its manufacturing.

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

[0040] In FIG. 1 the beam of plasma radiation 8 is directed from the region of radiating plasma 2 into the window 6b made of MgF2 straight and without reflections. In contrast to sources where the beam of plasma radiation is formed by an intrachamber metal mirror, whose coefficient of reflection is low in the VUV range (less than 20% at λ=110 nm), this ensures the absence of a cut-off or suppression of the VUV component in the spectrum of the beam of plasma radiation.

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

[0042] Each of the sleeves 7a, 7b with the soldered window 6a, 6b is positioned in one of the holes in the housing 5 and is welded into the hole of the housing 5 on the outside welding seams 14. Further, the internal parts of the axisymmetric sleeves 6a, 6b are the external part of the chamber which is not in contact with the gas it is filled with. Along with the placement of windows on the chamber inside, this improves reliability of the sealed joint due to the high pressure of gas in the chamber which compresses the sealing material (glass cement 13) and facilitates the sealing of optical elements.

[0043] According to the invention, the surface of the end of sleeve 7b and the surface of the MgF2 exit window 6b adjacent to it are essentially perpendicular to the optical axis of the MgF2 crystal. The coefficients of linear thermal expansion (CLTE) of the glass cement 13, material of the sleeves 7a, 7b and the housing 5 are matched with the CLTE of the crystal magnesium fluoride in the direction perpendicular to the optical axis of the MgF2 crystal. All of the mentioned above provides for high reliability and longer lifetime of the windows and the chamber assembly. Preferably, the sleeves and the chamber housing are made of the 47 ND iron-nickel alloy which meets these requirements.

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

[0045] Thus, the present invention provides for manufacturing highly reliable chambers with MgF2 windows to operate at high pressures (around 50 atm) and temperatures (around 900° K) and for creating brighter and more stable COD-based light sources with the broadest spectrum of radiation in the VUV range.

[0046] According to an embodiment of invention shown in FIG. 1, a vacuum or gas environment, such as helium, argon, etc., which does not absorb VUV radiation with wavelengths of 110 nm and higher, is located outside the MgF2 exit window 6b intended for outputting the beam of plasma radiation 8 from the chamber. For this purpose the chamber 1 can be sealingly connected to an outside chamber 17 with objects which the beam of plasma radiation 8 is carried to, by means of a branch pipe 16.

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

[0048] High stability and high brightness of the radiating plasma in the continuous mode of operation is achieved when the pressure of gas in the chamber is around 50 atm or higher, while the chamber temperature is around 600 K or higher. Due to the high temperature of the chamber 1 the branch pipe 16 is designed with the function of a thermal bridge between the chamber 1 and the outside chamber 17. For this purpose at least a part of the branch pipe 16 is made with a low thermal conductivity, for example, of thin stainless steel. In order to cool the part of branch 16 removed from the window 6b, it is designed as a cooling radiator 18 which prevents heating of the outside chamber 17. The sealed joint of the branch pipe 16 to the chamber 1 and the outside chamber 17 can be provided using sealing gaskets 19 which can be made of copper, at least, on the side of the heated chamber 1.

[0049] In the embodiment of invention, FIG. 1, all the axisymmetric sleeves 7a, 7b with the windows 6a, 6b soldered to them, are welded to the single common housing part 5. Further, the region of radiating plasma 2 is positioned in the cavity of housing 5 formed by the intersection of at least two holes, in each of which one of the sleeves 7a, 7b with one of the windows 6a, 6b is located. The sleeves 7a, 7b have a variable outside diameter, while the windows 6a, 6b are located on the end of sleeves with the smaller outside diameter.

[0050] The broadband laser-pumped light source is operated as described below. First, the chamber 1 of the light source is manufactured, comprising the metal housing 5, with at least two windows 6, 6b, FIG. 1. At least one window 6b is made of MgF2. The material of at least one of the windows 6a can be glass with a CLTE matched with the CLTE of MgF2. The chamber housing is manufactured from the 47 ND precision alloy with a CLTE also matched with the CLTE of MgF2. Each of the windows 6a, 6b, is soldered to one of the sleeves 7a, 7b, using glass cement 13 with the application of annealing at the temperature of at least 400° C. Each sleeve with the window soldered to it is welded into the hole of metal housing 5. The chamber is filled with gas at high pressure either through the sealed tubulation or through the gas port 15.

[0051] Broadband radiation of COD plasma is generated as described below. The focused beam 3 of the CW laser 4 is directed into the region 2 of the chamber intended for sustaining the radiating plasma. Preferably, inert gases of high purity and mixtures thereof are used as the gas. By means of the pulsed laser system 9 at least one pulsed laser beam 10 is generated. The beam of CW laser and the pulsed laser beam are introduced into the chamber 1 through the window 6a. At the same time, the optical system comprising the window 6a and the focusing lens 12 provides for sharp focusing of the laser beams. The pulsed laser system 9 is used to provide the optical breakdown and to generate the starting plasma with a density which exceeds the threshold density of COD plasma having a value of around 1018 electrons/cm3. The concentration and volume of the starting plasma are sufficient for reliable sustenance of a continuous optical discharge by the focused beam of CW laser 3 with a relatively low power not exceeding 300 W. In stationary mode broadband high-brightness radiation is output from the region of radiating plasma 2 of the continuous optical discharge using at least one beam 8 of plasma radiation. The short-wave boundary of the spectrum of plasma radiation exiting the chamber is determined by the MgF2 transmission limit which is approximately 110 nm. The beam 8 exiting the chamber through the MgF2 exit window 7b is intended for subsequent use, for example, in the outside chamber 17. The chamber 1 can be sealingly connected to the outside chamber 17 filled with a vacuum or gas environment which does not absorb the VUV radiation exiting the chamber 1. In working mode the temperature of chamber 1 is preferably around 600 K or higher. Further, thermal isolation between the chamber 1 and the external chamber 17 is provided by means of the branch pipe 17 which is designed with the thermal bridge function and equipped with the cooling radiator 19.

[0052] In the embodiment of invention shown in FIG. 2 the chamber 1 contains the welded metal housing 5 comprising at least two housing parts 5a, 5b, to each of which the sleeve 7a, 7b is welded with the window 6a, 6b soldered to it.

[0053] After the internal chamber elements, which include the focusing lens 12 with a mounting or casing 20 and an insert 21, are installed, the housing parts 5a, 5b with the windows 6a, 6b are welded together with a welding seam 22. During the welding of housing parts 5a, 5b the axisymmetric sleeves 7a, 7b welded to them with the windows 6a, 6b cancel out the irregular heating and cooling of the assembled chamber 1.

[0054] The external view of the welded housing of the light source is schematically shown in FIG. 3.

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

[0056] In FIG. 4 another embodiment is schematically shown where the MgF2 window 6b for outputting the beam of plasma radiation 8 from the chamber is a lens designed with the function of reducing the angular aperture of the beam of plasma radiation or reducing the aberrations which distort the path of rays of plasma radiation when they pass through the window 6b. Generally, the window 6b is designed as a meniscus or another type of matching lens. This increases the brightness of radiation source, minimizes the dimensions of light source and improves its ease of operation.

[0057] For the similar purpose of increasing light source brightness, retroreflectors 23, 24 designed as spherical mirrors with the center in the region of radiating plasma 2 are placed in the light source chamber, FIG. 4. The retroreflectors 23 and 24 are positioned opposite the MgF2 window 6b and on the axis of the focused laser beam 3.

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

[0059] In the preferred embodiments of invention, the axis of the focused beam of CW laser 3 is directed vertically upwards, i.e. against the force of gravity, FIG. 1, FIG. 2, FIG. 4, or close to vertical. The proposed design achieves the highest stability of the power of laser-pumped light source radiation. This is due to the fact that typically the region of radiating plasma 2 is slightly shifted from the focal point towards the focused beam 3 of CW laser up to the cross-section of focused laser beam where the intensity of the focused beam 3 of CW laser is still sufficient to sustain the region of radiating plasma 2. When the focused beam 3 of CW laser is directed from the bottom upwards, the region of radiating plasma 2 that contains the hottest plasma with the lowest mass density, tends to float under the influence of the buoyant force. The rising region of radiating plasma 2 ends up in the location closest to the focal point where the cross-section of the focused beam 3 of CW laser is smaller, and the laser radiation intensity is higher. On the one hand, this increases the brightness of plasma radiation, and on the other hand, it equalizes the forces acting on the region of radiating plasma, which ensures high stability of radiation power of the high-brightness laser-pumped light source.

[0060] The stability of output characteristics of the laser-pumped light source is also influenced by the size of the pulse acquired under the action of the buoyant force by the gas heated in the region of radiating plasma 2. The pulse acquired by the gas and the turbulence of convective flows are the less, the closer the region of radiating plasma 2 to the top chamber wall. Consequently, to ensure more stable output characteristics of the light source the top wall of chamber housing is positioned at a distance of no more than 5 mm from the region of radiating plasma 2.

[0061] The suppression of convective flow turbulence in the chamber and improvement of stability of the light source output characteristics is achieved by reducing the internal volume of the chamber. For this purpose, in the preferred embodiments of invention the chamber walls, as well as the focusing lens 13 and each window 6b for outputting the beam of plasma radiation are positioned at a distance of no more than 5 mm from the region of radiating plasma.

[0062] One more embodiment of the light source according to the present invention is schematically shown in FIG. 5. In this embodiment the chamber housing contains several windows 6b, 6c for outputting several beams of plasma radiation 8 from the chamber 1 which is required for certain applications of the light source.

[0063] Preferably, a high-efficiency diode near-infrared laser with the output of radiation to an optical fiber 25 is used as the CW laser 4. At the exit of optical fiber 25, the expanding laser beam is directed to the collimator 26, for example, in the form of a collecting lens. After the collimator 26 and the dichroic deflecting mirror 11 the expanded beam of CW laser is directed into the chamber 1. The optical system, window 6a and focusing lens 12 ensure sharp focusing of the beam 3 of CW laser required to achieve a high brightness of the light source.

[0064] In the embodiment of invention, FIG. 5, the starting ignition of plasma is provided by a solid-state laser system which contains a first laser 27 for generating the first laser beam 28 in Q-switching mode and a second laser 29 for generating the second laser beam 30 in free-running mode. Pulsed lasers with active elements 31 are equipped with optical pumping sources, for example, in the form of flash lamps 32 and preferably have the 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 directed into the chamber and focused in the region intended for the sustenance of radiating plasma 2, FIG. 5. The first laser beam 28 is intended for starting plasma ignition or for optical breakdown. The second laser beam 30 is intended to create plasma, the volume and density of which are high enough for stationary sustenance of the region of radiating plasma 2 by the focused beam 3 of the CW laser.

[0066] Preferably, the CW laser wavelength λCW, is different from wavelengths λ1, λ2 of the first and second pulsed laser beams 28, 30. For example, the CW laser wavelength can be λCW=0.808 μm or 0.976 μm and the pulsed lasers can have a wavelength of radiation λ1=λ2=1.064 μm. This allows to use the dichroic mirror 11 for introducing the laser beam 36 of the CW laser 4 and the pulsed laser beams 28, 30. Additionally, a tilt mirror 37 can be used to transfer the pulsed laser beams 28, 30, FIG. 5.

[0067] This embodiment of invention provides for reliability of laser ignition and for user-friendliness of the light source. In contrast to sources using electrodes for starting plasma ignition, the possibility is achieved to optimize chamber geometry, reduce turbulence of convective flows in the chamber and minimize optical aberrations.

[0068] Otherwise, the device parts in this embodiment are the same as in the embodiments described above, have the same item numbers in FIG. 5, and their detailed description is omitted.

[0069] Generally, the proposed invention allows for expanding the radiation spectrum in the VUV spectral region and ensuring high brightness and stability of the laser-pumped plasma radiation source.

INDUSTRIAL APPLICABILITY

[0070] High-brightness high-stability laser-pumped 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.