FIELD OF INVENTION

[0001] The present invention relates to laser-pumped plasma light sources producing high- brightness light in the ultra-violet (UV), visible and near infrared (NIR) spectral bands and to methods of generating broadband radiation from the plasma of continuous optical discharge (COD).

BACKGROUND OF INVENTION

[0002] Continuous optical discharge is a stationary gas discharge sustained by laser radiation in pre-created relatively dense plasma. COD-based light sources with a plasma temperature of about 15,000 K are among the highest brightness continuous light sources in a wide spectral range between about 0.1 pm and 1 pm (Raizer, “Optical Discharges,” Sov. Phys. Usp. 23(11), Nov. 1980, pp. 789-806). Compared to arc lamps, such laser-pumped plasma light sources not only have a higher brightness, but also a longer lifetime, making them preferable for numerous applications.

[0003] The indicated temperature of the radiating plasma, about 15,000 K, is practically fixed, since when an attempt is made to increase it by increasing the power of a continuous wave (CW) laser (within 2-10 times, but not by many orders of magnitude), the plasma volume will increase, and the additional power will be released by radiation and thermal conductivity from the increased volume and surface of the plasma-gas interface. In other words, the plasma temperature is largely stabilized by the COD itself, by the conditions of its existence. In this regard, in order to increase the brightness to sustain the COD, pulsed lasers with a high repetition rate are used, including in conjunction with the use of a CW laser, the power of whichis not lower than the threshold power required to sustain the COD, as is known, for example, from patent RU 2571433, issued on December 20, 2015.

[0004] However, with this approach, there is a problem of instability of a high-brightness laser- pumped plasma light source.

[0005] This drawback is largely overcome in the broadband light source known from US patent 9368337, issued on June 14, 2016, in which the optically transparent COD plasma has a shape elongated along the axis of the CW laser beam. Plasma radiation is collected in the longitudinal direction, which results in a high brightness of the light source.

[0006] However, with longitudinal collection of plasma radiation, the problem of blocking laser radiation in an output beam of plasma radiation arises. Solving the problems of increasing the brightness, increasing the absorption coefficient of laser radiation by the plasma, and significantly reducing the numerical aperture of the blocked diverging laser beam that has passed through the plasma, this device does not completely solve the problem of light source brightness stability.

[0007] In the broadband light source known from US Patent 9357627, issued on May 31, 2016, plasma radiation is collected in directions other than the directions of propagation of the laser beam. Along with this, due to the optimization of light source configuration in which the laser beam is directed vertically upward along the camera axis and the region of radiating plasma is in the immediate vicinity of the upper part of the chamber, the energy and spatial stability of the broadband plasma light source is increased by suppressing the turbulence of convective flows in the gas-filled chamber.

[0008] The problem of increasing the stability and control of convective gas flows, the turbulent flow of which leads to instability of the brightness of the light source was also solved by optimizing the geometry of the camera and the light source as a whole in a number of US patents: 10008378, issued on June 26, 2018; 10109473, issued on October 23, 2018; 9887076, issued on February 06; 2018, 10244613, issued on March 26, 2019.

[0009] However, the optimal conditions for obtaining continuous generation of plasma radiation with high spectral brightness, close to the maximum achievable for light sources of this type, more than 50 mW/(mm ² 'sr-nm), and low relative brightness instability G, less than 0.1% were not determined.

SUMMARY

[0010] The technical problem to be solved by the invention relates to the creation of devices and methods for the optimal generation of broadband radiation from the COD plasma and the development on their basis of highly stable high-brightness plasma light sources with laser pumping.

[0011] The essence of the invention is to provide the highest possible brightness of the light source due to the high density of high-temperature (-15000 K) COD plasma, said plasma density provided by the high pressure of the surrounding gas, equal to 50-100 bar or higher. A distinctive feature is that such high pressures p are provided (according to the ratio p oc nT) at a minified density n of gas atoms but using as high as possible gas temperature T (in the rangefrom 600 to 900 K or higher). Minimizing the gas density and the refraction, associated with this density, in turn, provides highly efficient suppression of the light source brightness instability, associated with the turbulence of convective gas flows in gas-filled chamber. Thus, the invention provides the achievement of ultra-high brightness of the plasma light source with ultra-low instability of its brightness.

[0012] The technical result of the invention consists in providing COD sustaining conditions, which are optimal for achieving high stability and high brightness of the radiating plasma, inthe creation on this basis of broadband light sources with ultra-high brightness and stability.

[0013] Achievement of the purpose is possible by means of the proposed laser-pumped plasma light source, comprising: a gas filled chamber, at least a part of which is optically transparent, 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 at least one output beam of plasma radiation exiting the chamber.

[0014] The light source is characterized in that an optimal continuous generation of the output beam of plasma radiation is achieved by a fact that a density of gas particles in the chamber is less than 90- 10 ¹⁹ cm' ³ , and a temperature of an inner surface of the chamber is not less than 600 K.

[0015] In a preferred embodiment of the invention, the optimal continuous generation is characterized by a high spectral brightness of the light source, more than 50 mW/(mm ² -nm-sr), and by a low relative standard deviation of the brightness <5 less than 0.1 %.

[0016] In a preferred embodiment of the invention, the density of gas particles is as low as possible and the temperature of the inner surface of the chamber at operation is as high as possible under providing a gas pressure in the chamber of about 50 bar or more.

[0017] In a preferred embodiment of the invention, the temperature of the inner surface of the chamber is not more than 900 K, and the density of gas particles is not less than 45 -IO ¹⁹ cm' ³ , which corresponds to a gas pressure at room temperature of not less than 16.5 bar.

[0018] In an embodiment of the invention, the temperature of the inner surface of the chamber is not more than 900 K.

[0019] In an embodiment of the invention, the gas belongs to the group inert gases, includes xenon, krypton, argon, neon, or mixtures thereof.

[0020] In the embodiment of the invention, the gas is xenon and a wavelength of the CW laser is 808 nm.

[0021] In an embodiment of the invention, at least a part of the chamber arranged for outputting of the output beam of plasma is spherical, and the region of radiating plasma is located in a center of the spherical part of the chamber.

[0022] In an embodiment of the invention, a radius of an internal surface of the spherical part of the chamber is less than 5 mm, preferably not more than 3 mm.

[0023] In a preferred embodiment of the invention, the focused beam of the CW laser is directed into the chamber from bottom to top, and an axis of the focused beam is directed vertically or close to vertical.

[0024] In a preferred embodiment of the invention, a part or a detail of the chamber is located above the region of radiating plasma at a distance from it, not more than 3 mm.

[0025] In a preferred embodiment of the invention, the chamber is provided with a heater.

[0026] In an embodiment of the invention, a transparent part of the chamber is made from a material belonging to a group of sapphire, leucosapphire (AI2O3), fused quartz, crystalline quartz (SiCh), crystalline magnesium fluoride (MgF2).

[0027] In an embodiment of the invention, the means for plasma ignition comprises a solid-state laser system generating a pulsed laser beam in Q-switching mode and a pulsed laser beam in free-running mode the two pulsed laser beams being focused into the chamber.

[0028] In an embodiment of the invention, the beam of the CW laser and each output beam of plasma radiation exiting the chamber do not cross each other outside the region of radiating plasma.

[0029] In an embodiment of the invention, the light source have three or more output beams of plasma radiation.

[0030] In another aspect, the invention relates to a method for light generation comprising: plasma igniting within a gas filled chamber an radiating and plasma sustaining by a focused beam of a CW laser to produce at least one output beam of plasma radiation exiting from a region of radiating plasma through an optically transparent part of the chamber.

[0031] The method is characterized in that the chamber is filled with a gas with a particles density of not more than 90- 10 ¹⁹ cm' ³ and the plasma is sustained by the focused beam of CW laser at a temperature of an inner surface of the chamber not less than 600 K.

[0032] In embodiments of the invention, a gas pressure in the chamber at operation is close to 50 bar or more to provide a high spectral brightness of a light source, more than 50 mW/(mm ² -nm-sr).

[0033] In a preferred embodiment of the invention, the temperature of the inner surface of the chamber is as high as possible at the lowest possible density of gas particles to provide a low relative standard deviation of a brightness G less than 0.1 %.

[0034] In the preferred embodiment of the invention, the chamber is filled with a gas with a particles density of a density of gas particles of at not less than 45- 10 ¹⁹ cm' ³ , and the temperature of the inner surface of the chamber is maintained at a temperature of not more than 900 K.

[0035] In a preferred embodiment of the invention, the focused beam of the CW laser is directed into the chamber from bottom to top along a vertical.

[0036] In a preferred embodiment of the invention, a turbulence of convective flows in the chamber is suppressed by placing an upper wall or a part of the chamber above the region of radiating plasma at a minimum possible distance from it, of not more than 3 mm.

[0037] In a preferred embodiment, the chamber is filled with xenon and radiating plasma is sustained by the focused beam of the CW laser with a wavelength of 808 nm.

[0038] In a preferred embodiment, the plasma igniting is produced by focused into the chamber two pulsed laser beams generated by a solid-state laser system in a free-running mode and in a Q-switched mode.

[0039] The advantages and features of the present invention will become more apparent from the following non-limiting description of exemplary embodiments thereof, given by way of example with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0040] The essence of the invention is explained by the drawings, in which:

[0041] Fig. 1- a schematic representation of a light source in accordance with an embodimentof the present invention,

[0042] Fig. 2 - dependence of the relative standard deviation of brightness on the gas density,

[0043] Fig. 3 dependence of the brightness of the plasma light source on time,

[0044] Fig. 4- spectral brightness of the light source as a function of the xenon gas pressure for CW laser wavelengths _cw = 976 nm and _cw = 808 nm,

[0045] Fig. 5, Fig. 6 show schematic representations of a light source in accordance with embodiments of the invention,

[0046] Fig. 7, Fig. 8 - schematic of a light source with several beams ofplasma radiation when plasma is ignited by a laser and an electric discharge,

[0047] Fig. 9 A, Fig. 9B - spectral characteristics of the light source.

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

[0049] These drawings do not cover and, moreover, do not limit the entire scope of options for implementing this technical solution, but are only illustrative examples of particular cases of its implementation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] This description is provided to illustrate how the invention can be implemented and in no way to demonstrate the scope of this invention.

[0051] According to the example of invention embodiment shown in Fig. 1, the laser-pumped plasma light source comprises the high-pressure gas filled chamber 1, at least part of which is optically transparent. Fig. 1 shows an embodiment with a completely transparent chamber manufactured from an optically transparent material, e.g. fused quartz. The light source also contains a means for igniting the plasma, which can be a pulsed laser system 2, generating at least one pulsed laser beam 3, which is focused into the chamber 1, namely into the region intended for sustaining the radiating plasma 4.

[0052] In other embodiments of the invention, ignition electrodes may be used as the means for igniting the plasma.

[0053] After plasma ignition, the region of radiating plasma 4 is sustained in the chamber in a continuous mode by a focused beam 5 of a CW laser 6. At least one output beam (or useful beam) of plasma radiation 7 directed to the optical collector 8 and intended for subsequent use, exits the chamber 1. The optical collector 8 forms the radiation beam 9 transmitted, for example, via an optical fiber and/or a system of mirrors to one or more optical consumer systems 10, which uses broadband plasma radiation.

[0054] In accordance with the present invention, the optimal continuous generation of the output beam of plasma radiation 7 is achieved by a fact that a density of gas particles in the chamber 1 is less than 90- 10 ¹⁹ cm' ³ and a temperature of an inner surface of the chamber is not less than 600 K, preferably in a range from 600 to 900 K or, optionally, higher if higher temperature does not have any negative impact on the lifetime of the chamber and its transparency.

[0055] The effect achieved by the invention is due to the factor that for a given amount of gas in a given volume of the chamber, the gas pressure increases with the temperature of inner surface of the chamber. Since the temperature of the radiating plasma is practically fixed (about 15000 K, and attempts to raise this temperature are difficult, since they are accompanied only by an increase in the plasma volume) and the pressure in the plasma is equal to the pressure in the chamber, the density of the radiating plasma increases with increasing pressure in the chamber, and hence with increasing temperature of the chamber wall. An increase in the density of the radiating plasma leads to an increase in the volumetric luminosity of the radiating plasma and, as a consequence, to an increase in the brightness of the light source in a wide optical range, where the radiating plasma is practically transparent.

[0056] The same increase in brightness can be obtained by increasing the gas pressure at a given temperature of the chamber. However, in this case, the gas-particles density and the refraction associated with this density will increase, which, in a turbulent flow, both in the radiating plasma region and at the periphery, will lead to significant instability (fluctuations) of the brightness of the light source.

[0057] Thus, in accordance with the present invention, it is preferable to increase the pressure of the gas not by increasing the density of the gas, but by increasing its temperature in order to ensure high brightness of the radiation with high stability of the brightness of the light source.

[0058] It should be noted that with an increase in the temperature of the chamber and gas, the turbulence of convective flows in the chamber also decreases for the following reasons. First, heating the chamber leads to a decrease in temperature gradients and gas density gradients in the chamber, which leads to suppression of convective flows between the hotter region of the plasma and the surrounding colder gas. Second, the nature of the gas flow is determined by the Reynolds number Re, and turbulence is suppressed when the Reynolds number becomes less than the critical one. The Reynolds number depends on the gas density p, gas flow rate v, and dynamic viscosity rp

Re « p-v/r| (1).

[0059] The dynamic viscosity increases with increase of temperature: ~ _o V ^r / ^r o (2),

[0060] where r|g is the dynamic viscosity of the gas at room temperature T _o (T ₀ ~300 K). In accordance with this, the Reynolds number depends on the density of the gas, its velocity and temperature as follows:

Re « V^o/r -p-v/rio (3).

[0061] In accordance with formula (3), suppression of gas flow turbulence is possible by increasing the absolute temperature T of the chamber and gas. Other possibilities for suppressing of turbulence and increasing the stability of the light source involve limiting the mass density of the gas p and its velocity v. The latter is realized, in particular, due to a decrease in the dimensions of the chamber, since the acceleration of the gas heated in the region of radiating plasma and floating up under the action of the Archimedean force is limited by the dimensionsof the chamber.

[0062] In accordance with (3), the lower the gas density, the lower the turbulence of the convective gas flow. In addition, the lower the gas mass density p, thelower its refractive index and the lower the aberrations associated with the refraction of light in the convective gas flow. Accordingly, the lower the density of the gas, the less is the instability of brightness and other output parameters of the light source. It is illustrated in Fig. 2, which shows the experimental dependence of Relative Standard Deviation (RSD) of the light source brightness <5 on both the mass density p and number density n of the gas.

[0063] The number density n and mass density p of the gas are related to each other as described by the following equation: n = N _A /M) p, where M is molar mass, N _A is the Avogadro constant.

[0064] Relative Standard Deviation means (Standard Deviation of a Data Set) / (Mean of a Data Set) • 100%. The brightness data set was sampled during the measurements with a time interval of 10' ³ sec. That is quite enough for processing the brightness signal, the oscillation frequency of which associated with the convection of the gas flow does not exceed 10-15 Hz, as can be seen in the enlarged fragment of Fig. 3, which shows the dependence of the brightness of the plasma light source on time.

[0065] To obtain the dependence shown in Fig. 2, four identical sealed quartz chambers were used, with electrodes for the starting ignition of plasma, the chambers filled with xenon at various pressures, equal at room temperature to: 11, 17, 23 and 29 bar. The dependence given in Fig. 2 demonstrates a dramatic, by approximately 50 times, drop in the relative instability of brightness <5 following a threefold decrease in the gas density. Figure 2 should be regarded as a qualitative illustration of the dependency of <5 on the gas density, given that further multiple, more than threefold, decrease in the brightness instability is possible. It is achieved, in particular, by the use in the plasma light sources of electrodeless plasma ignition, as well as a CW laser control system with negative feedback.

[0066] In Fig. 3 the time dependence of the brightness is given for a plasma light source with a chamber filled at a xenon gas density of 60- 10 ¹⁹ cm' ³ , which corresponds to a pressure of 22 bar at room temperature. The spherical part of the quartz chamber arranged for the exit of the beam of plasma radiation has a diameter of inner surface as small as 4 mm. The relative standard deviation of the plasma light source brightness <5 in this case is in the range from 0.04 to 0.05%.

[0067] In general, the higher the gas pressure and thus the pressure in the region of the emitting plasma, the higher the brightness of the light source.

[0068] In order to provide the relative standard deviation of brightness to be sufficiently small, <5 < 0.1%, the density of gas particles in the chamber is chosen below the experimentally determined upper limit of 90-10 ¹⁹ cm' ³ , which corresponds to a gas pressure of about 33.5 barat room temperature. At the same time, to obtain the spectral brightness of the light source close to the maximum achievable, more than 50 mW/(mm ² -sr-nm) at the temperatures in the range from 600 to 900 K or higher, the gas pressure and, accordingly, the density of the radiating plasma should be high enough to provide optimal gas pressure of about 50 bar or more at operation. For this purpose, the density of gas particles in the chamber is selected above the experimentally determined lower limit of 45- 10 ¹⁹ cm' ³ , which corresponds to a gas pressureat room temperature of not less than 16,5 bar.

[0069] Thus, to provide a high spectral brightness and low relative standard deviation of the brightness, the density of gas particles should be as low as possible while the temperature of the inner surface of the chamber at operation should be as high as possible under providing a gas pressurein the chamber of about 50 bar or more.

[0070] In an embodiment of the invention, the temperature of the inner surface of the chamberat operation is 600 K and the density of gas particles is 65 -IO ¹⁹ cm' ³ , which corresponds to a gas pressure of 24.5 bar at room temperature and 50 bar at operation.

[0071] In preferable embodiment of invention, the chamber can operate at its inner surface temperature as high as 860 K and the density of gas particles may be chosen as low as 45 -IO ¹⁹ cm' ³ , which corresponds to a gas pressure of 16.5 bar at room temperature and 50 bar at operation. [0072] For illustration, Fig. 4 shows the dependence of the spectral brightness of the light source on the pressure of xenon gas in the chamber at room temperature. The measurements were carried out in the spectral range of 600-500 nm at the stationary mode of operation with chamber temperature of 450 K. In the indicated spectral range, the spectral brightness is about 25% lower than in the maximum observed near wavelengths of about 400 nm. The measurements were made for two CW diode lasers with a radiation power of 65 W at wavelengths _cw = 976 nm and _cw = 808 nm.

[0073] The research results show that for both wavelengths of laser radiation, high spectral brightness is achieved at a gas pressure in the chamber of at least 25 bar at room temperature. High stability of the radiation intensity, <5 < 0.1%, is sustained at a gas pressure in the chamberup to 36 bar at room temperature.

[0074] The measurements showed a confident tendency to increase the brightness while sustaining high stability of the output parameters of the light source with an increase in the chamber temperature to 600 K and higher. [0075] In accordance with the invention, the use of inert xenon is preferred as the gas, which ensures safe operation and a long lifetime of the light source. In addition, compared to the radiating plasma of other inert gases, Xe plasma is characterized by the highest optical o in a wide spectral range, including UV, visible and IR regions.

[0076] The choice of the preferred wavelength of high-efficient CW diode laser is due to the following factors. Near the laser wavelength 976 nm, there are strong absorption lines of Xe, in which the lower state is populated as the temperature rises. Near 808 nm, such lines are spaced farther from the absorption lines and, therefore, at a given laser power, sufficient absorption to sustain a continuous optical discharge is achieved at a higher plasma density andtemperature than in the case of 976 nm.

[0077] Accordingly, in a preferred embodiment of the invention, the gas, filling the chamber, is xenon and a wavelength of the CW laser is 808 nm.

[0078] Other embodiments of the invention are aimed at further increasing the stability of the output parameters of the light source, which include intensity, brightness, spectrum, and spatial position of the radiating plasma while ensuring the highest possible brightness of the source.

[0079] In a preferred embodiment, a focused beam of CW laser is directed into the chamber from bottom to top, and the axis of said beam is directed vertically parallel to the force of gravity 11, Fig. 5, or close to vertical. The further improvement the light source stability is dueto the fact that usually the region of radiating plasma 4 is slightly shifted from the focus towards focused beam 5 of the CW laser to that cross section of the focused laser beam where the intensity of focused beam 5 of the CW laser is still sufficient to sustain the region of radiating plasma 4. When focused beam 5 of a CW laser is directed from bottom to top, the region of radiating plasma 4, which contains the hottest and lowest mass density plasma, tends to float under the action of the Archimedean force. Ascending, the region of radiating plasma 4 reaches a place closer to the focus, where the cross section of focused beam 5 of the CW laser is smallerand the intensity of laser radiation is higher. On the one hand, this increases the brightness of the plasma radiation, and on the other hand, it balances the forces acting on the region of radiating plasma, which ensures high stability of the light source.

[0080] To realize these positive effects, it is preferable that chamber 1 is axisymmetric and the axis of focused beam 5 of the CW laser be aligned with the axis of symmetry of the chamber.

[0081] The stability of the output characteristics of the light source is also influenced by the magnitude of the pulse acquired under the action of the Archimedean force by a gas heated inthe region of radiating plasma 4. The momentum acquired by the gas and the turbulence of convective flows are the less, the closer the region of radiating plasma 4 is to the upper wall ofthe chamber or to the part of the chamber located above the region of radiating plasma 4. Therefore, in order to increase the stability of the output characteristics of the light source in the embodiment shown in Fig. 5, part or detail 12 of the chamber is located on top of the regionof radiating plasma 4 at the minimum possible distance from it, less than 3 mm, which does not have any negative impact on a lifetime of the chamber and its transparency.

[0082] Also, part 12 of the chamber can be arranged for reflection and focusing into the plasma4 both the CW laser beam, which has passed through the region of radiating plasma, and the part of the plasma radiation. This reduces the radiative losses and increases the efficiency of the light source. In accordance with this embodiment of the invention shown in Fig. 5, part 12 of the chamber close to the plasma contains a surface that is a concave spherical mirror 13 withcenter in the region of radiating plasma 4.

[0083] In a preferred embodiment, at least the part of the chamber 1 intended for the exit of output beam of plasma radiation 7 is spherical or nearly spherical, and the region of radiating plasma 4 is located in the center of symmetry of the spherical part of chamber 1, as shown in Fig. 1 and Fig. 5. This minimizes chromatic and spherical aberrations, caused by the transparent walls of the chamber into the paths of rays of plasma radiation.

[0084] The suppression of aberrations associated with the turbulence of convective flows is achieved, in particular, by reducing the chamber size. Therefore, in an embodiment of the invention, the radius of the inner surface of the spherical part of the chamber is less than 5 mm, preferably not more than 3 mm.

[0085] Fig. 6 shows an embodiment of the invention in which the chamber is equipped with a heater. The heater can consist of a heating coil 14 and a current source 15 connected to the heating coil through a temperature bridge 16 intended to provide a temperature difference between heating coil 36 and current-carrying busbars 17. Additionally, current-carrying busbars 17 can be provided with a heat exchanger (not shown), for example in the form of aircooled radiators. The chamber can consist of a spherical part and a cylindrical part, on whicha heating coil 14 is located. The chamber can also be equipped with a thermocouple to measurethe temperature of the chamber. In addition, heating coil 14 may be housed in a heat insulating] acket (not shown).

[0086] The heater is designed for pre-starting heating of the chamber to the operating temperature, which facilitates the ignition of the plasma and provides a quick transition of the light source to the steady-state operating mode with a preset optimum high temperature of the chamber, which is in the range from 600 to 900 K.

[0087] In an embodiment of the invention, the optical collector includes a parabolic mirror 8 and a deflecting mirror 18 intended to form a beam of plasma radiation 9, preferably transported by optical fiber to an optical system that uses broadband plasma radiation.

[0088] In a preferred embodiment of the invention, the high-brightness plasma light source comprises a control unit 19 with the function of automatically sustaining a given power in the output beam of plasma radiation 7, Fig. 6. For this, the light source is equipped with a power meter 20, to which a small part of the light flux from beam of plasma radiation 9 is supplied with a coupler (not shown). Preferably, the control unit is connected to a heater 15, a power meter 20, and a power supply unit of CW laser 6. Maintaining the specified power in the beamof plasma radiation 9 is carried out by control unit 19 according to the feedback circuit between power meter 20 and the power supply unit of CW laser 6. In addition, control unit 19 can be made with the function of thermal stabilization of the chamber at its optimum high temperature. This embodiment of the invention improves the stability of power and brightness of the laser- pumped plasma light source in a long-term continuous mode of operation.

[0089] As shown in Fig. 6, in a preferable embodiment of the invention, the CW laser 6 with fiber-optic output is used. At output of optical fiber 21, the expanding laser beam is directed to collimator 22, for example, in the form of a condenser lens. After collimator 22, expanded parallel beam 23 of the CW laser is directed by means of a deflecting mirror 24 to a focusing optical element 25, for example, in the form of an aspherical lens, providing sharp focusing ofthe CW laser beam 5, which is necessary to ensure high brightness of the light source.

[0090] In preferable embodiment of the invention, a solid-state laser system 2, which contains a first laser 26 for generating a first laser beam 27 in the Q -switched mode and contains a second laser 28 for generating a second laser beam 29 in a free-running mode, is used for reliable plasma ignition. Pulsed lasers with active elements 30, 31 are equipped with sources of optical pumping, for example, in the form of flash lamps 32 and preferably have common cavity mirrors 33, 34. First laser 26 is equipped with a Q-switch 35. Two pulsed laser beams 27, 29 are focused into the chamber, in the region intended to sustain radiating plasma 2, Fig. 6. First laser beam 27 is intended for optical breakdown. Second laser beam 29 is intended to create a plasma, the volume and density of which are sufficient for stationary maintenance of the region of radiating plasma 4 by a focused beam 5 of a CW laser.

[0091] Preferably the wavelength of the CW laser X _cw is different from the wavelengths l, A2 of first and second pulsed laser beams 27, 29. As an example, the wavelength of the CW laser may be CW = 808 nm or 976 nm, and the pulsed lasers may have an emission wavelength of X1=X2= 1064 nm. This allows dichroic mirror 24 to be used to input CW laser beam 23 and pulsed laser beams 27, 29 into the chamber. To transport pulsed laser beams 27, 29, a rotary mirror 36 can be additionally used, Fig. 6.

[0092] Fig. 1, Fig. 5, Fig. 6 show that when using a pulsed laser system 2 for plasma ignition, chamber 1 allows the output of plasma radiation in all azimuths. In an embodiment, the exit ofthe output beam of plasma radiation from the chamber is carried out into a spatial angle of at least 9 sr or more than 70% of the total solid angle. In this case the opening angle of the outputbeam 7 of plasma radiation (flat angle with respect to the plane of the drawing) is not less than90°.

[0093] Along with the output of the output beam of plasma radiation 7 to the optical collector 8 in all azimuths, the light source according to the present invention is not limited to this embodiment only. In other embodiments of the invention, the light source may have at least three homocentric output beam of plasma radiation 7a, 7b, 7c, as illustrated in Fig. 7, which shows a cross-section of a light source in a horizontal plane passing through the region of radiating plasma 4. The laser beams in Fig. 7, which ignite and sustain a continuous optical discharge, are located below the plane of the drawing. The use of several, in particular three beams of plasma radiation from a single light source is required for a number of industrial applications. In this embodiment, the laser pumped light source chamber 1 can be housed in a housing 37, which is equipped with three optical collectors 8a, 8b, 8c. Three optical collectors 8a, 8b, 8c form beams of plasma radiation 9a, 9b, 9c, transported, for example, by optical fiber to optical consumer systems 10a, 10b, 10c, using broadband plasma radiation. This allows the use of one light source for three or more optical consumer systems, ensuring the compactness of the system and the identity of the parameters of broadband radiation in all optical channels.

[0094] Fig. 8 shows another version of a light source with three radiation output channels, in which two ignition electrodes 38, 39 are used as a means for plasma ignition, connected to a high-voltage pulsed power supply (not shown). The parts of device which in this embodiment are the same as those in the above-described embodiment (Fig. 7), have in Fig. 8 the same reference numbers and their detailed description is omitted.

[0095] In a preferred embodiment of the invention, the transparent part of the chamber is made of quartz. In other embodiments, the transparent part of the chamber can be made of an optically transparent material belonging to the group of sapphire, leucosapphire, fused silica, crystalline silica, crystalline magnesium fluoride.

[0096] A method for light generation from a COD plasma using the proposed laser-pumped plasma light source shown in Fig. 1, Fig. 5, Fig. 6, Fig. 7, Fig. 8 is as follows. A chamber 1 is filled with a gas with a particles density of less than 90- 10 ¹⁹ cm' ³ , which corresponds to a pressure 35.5 bar at room temperature. The focused beam 5 of the CW laser 6 is directed into the chamber 1. With the help of means for plasma ignition, which can be either ignition electrodes or a pulsed laser system 2, plasma is ignited. The concentration and volume of initial plasma are sufficient to reliably sustain a continuous optical discharge by a focused beam 5 of a CW laser 6. In a steady-state stationary mode of operation, the region of radiating plasma is sustained by a focused beam of a CW laser at a temperature of the inner surface of the chamberin the range from 600 to 900 K or optionally higher. At least one output beam of plasma radiation is directed from a region of radiating plasma 4 through an optically transparent part of the chamber 1.

[0097] By heating the walls of the chamber to the specified temperature, a multiple, two to three or more times increase in the pressure of the gas surrounding the region of radiating plasma is provided. Since the pressure in the plasma is equal to the pressure in the chamber, the density of the radiating plasma is increased due to the heating of the chamber walls, which leads to an increase in the volumetric luminosity of the radiating plasma and, as a consequence, to an increase in the brightness of the light source in a wide optical range. In this case, an increase in the gas pressure and the brightness of the light source is achieved without increasing the gas density and the proportional to it refraction, which at a turbulent flow lead to significantinstability of the light source brightness. As shown above when considering formula (3), the suppression of convective flow turbulence is possible by increasing the gas temperature T, decreasing or limiting its density p and decreasing the gas flow velocity v, which is implemented in the proposed method for generating light.

[0098] To achieve a high spectral brightness of a light source, more than 50 mW/(mm ² -nm-sr)a gas pressure in the chamber at operation is provided close to 50 bar or more.

[0099] To achieve a low relative instability of a brightness <5 less than 0.1 %.the temperature of the inner surface of the chamber is provided as high as possible at the lowest possible density of gas particles

[0100] The velocity of the gas flow v ascending from the region of radiating plasma is minimized by positioning the upper wall or part of the chamber at the minimum possible, not exceeding 3 mm, distance from the region of radiating plasma. In an embodiment, the size of the chamber is chosen so that the walls of the chamber are located at a distance from the region of radiating plasma not exceeding 3 mm, which helps to suppress the turbulence of convectiveflows in the chamber.

[0101] Thus, the invention allows, at high brightness, close to the maximum achievable for sources of this type, to provide high stability of the laser pumped plasma light source.

[0102] In an embodiment of the method, the chamber is heated after ignition of the plasma inthe process of bringing the light source to a stationary mode of operation due to the radiation power of a CW laser entering the chamber.

[0103] In another embodiment, prior to the ignition of the plasma by an external heater, including elements 14, 15, 16, 17, Fig. 6, the chamber 1 is rapidly heated to a temperature ranging from 600 to 900 K. This facilitates the ignition of the plasma and reduces the time the light source reaches a stationary mode of operation, simplifying its design and increasing easeof use. The specified temperature of the inner surface of the chamber is maintained by the radiation power of the CW laser and the heater.

[0104] In order to further increase the stability of the light source, a focused beam of CW laser is directed into the chamber from bottom to top along the vertical, which increases the brightness and spatial stability of the region of radiating plasma. In this case, the CW laser beam is preferably focused in the center of symmetry of that part of the chamber through whichthe output beam of plasma radiation passes out. This reduces optical aberrations, which can distort the path of the beams when broadband plasma radiation passes through the transparent walls of the chamber and reduce the brightness of the light source when transporting its radiation.

[0105] To achieve the maximum possible brightness of the light source, xenon gas is preferably used, and the laser is a continuous diode laser with a wavelength of 808 nm, Fig. 4.

[0106] Compared to the radiating plasma of other inert gases, Xe-plasma of the continuous optical discharge (COD) is characterized by the highest optical output in a broad spectral range that includes the visible and the near-infrared regions. The characteristic spectrum of a light source with the use of Xe as the inert gas is shown in Fig. 9A.

[0107] Using other inert gases, in particular, heavy inert gases Kr, Ar, Ne, or adding other inert gases to xenon with the ratio of 20% or less, does not produce any noticeable effect on the COD optical characteristics. COD radiation in the UV, visible and near-infrared spectral regions is close to black-body radiation and is mainly determined by the plasma temperature. At the same time, in the vacuum ultraviolet (VUV) region of 100 - 200 nm, absorption or self-absorption of radiation in xenon is observed. Therefore, in the case of expanding the radiation spectrum into the VUV region, using other inert gases, in particular, Kr, Ar or mixtures thereof, may be preferable. It is illustrated in Fig. 9B, which shows the VUV spectra of a light source when using xenon gas and Kr+Ar gas mixture for the purpose of comparison.

[0108] Generally, in line with the invention, an inert gas, preferably xenon, krypton, argon, neon or mixtures thereof, is used as the gas for filling the chamber.

[0109] In an embodiment of the invention, the plasma is ignited by two pulsed laser beams 27, 29 of a solid-state pulsed laser system 2, focused in the region of radiating plasma, Fig. 6. Two pulsed laser beams 27, 29 provide optical-induced breakdown and the creation of an initial plasma, the density of which is higher than the threshold density of a continuous optical discharge plasma, which has a value of about 10 ¹⁸ electrons/cm ³ . In this embodiment, the reliability of laser ignition and ease of use of the light source are achieved. In contrast to sources using electrodes for starting plasma ignition, it is possible to optimize the geometry of the chamber, reduce the turbulence of convective gas flows in it and minimize optical aberrations, as well as increase the spatial angle of the plasma radiation collection.

[0110] In general, the claimed invention makes it possible to: increase the brightness and ensure high stability of the laser pumped plasma radiation source.

INDUSTRIAL APPLICABILITY

[0111] High-brightness, highly stable laser pumped light sources made in accordance with the present invention can be used in various projection systems, for spectrochemical analysis, spectral microanalysis of biological objects in biology and medicine, in microcapillary liquid chromatography, for inspection of the optical lithography process, for spectrophotometry and other purposes.