CN116097396A

CN116097396A - Laser pumped plasma light source and method of generating light

Info

Publication number: CN116097396A
Application number: CN202180056803.7A
Authority: CN
Inventors: 亚历山大·安德烈耶维奇·拉什; 罗伯特·拉菲列维奇·加亚索夫; 丹尼斯·亚历山大罗维奇·格鲁什科夫; 尤里·鲍里索维奇·基柳欣; 弗拉基米尔·米哈伊洛维奇·克里夫松
Original assignee: Isteq Group Holdings Ltd; Isteq Private Ltd
Current assignee: Isteq Group Holdings Ltd; Isteq Private Ltd
Priority date: 2020-08-06
Filing date: 2021-07-26
Publication date: 2023-05-09
Also published as: WO2022031190A1; KR20230045077A; RU2752778C1; JP2023539424A; EP4193385A1; JP7430364B2

Abstract

The present invention relates to a plasma light source with continuous light discharge (COD). The light source comprises a gas-filled compartment with a region of radiant plasma maintained by a focused beam of a CW laser. The density of the gas particles in the compartment is less than 90.10 ¹⁹ cm ^‑3 And the temperature of the inner surface of the compartment is not lower than 600K. Preferably, the density of the gas particles is as low as possible and the temperature of the inner surfaces of the compartment in operation is as high as possible with a gas pressure in the compartment of about 50 bar or more. The technical result of the present invention is to provide COD maintenance conditions which are optimal for achieving high stability and high brightness of the radiation plasma, on the basis of which a broadband light source with ultra high brightness and stability is produced.

Description

Laser pumped plasma light source and method of generating light

Technical Field

The present invention relates to laser pumped plasma light sources that produce high brightness light in the Ultraviolet (UV), visible and Near Infrared (NIR) spectral bands and to methods of producing broadband radiation from continuous light discharge (COD) plasmas.

Background

A continuous light discharge is a stable gas discharge maintained by laser radiation in a pre-formed, relatively dense plasma. COD-based light sources with a plasma temperature of about 15000K are continuous light sources of highest brightness in a broad spectral range of about 0.1 μm to 1 μm (Raizer, "Optical Discharges", sov. Phys. Usp.23 (11), month 11 in 1980, pages 789 to 806). Such laser pumped plasma light sources are not only brighter than arc lamps, but also have a longer lifetime, making them more suitable for many applications.

The indicated temperature of the radiating plasma (about 15000K) is virtually fixed because when it is attempted to increase it by increasing the power of a Continuous Wave (CW) laser (within a factor of 2 to 10, but not of the order of magnitude), the plasma volume will increase and additional power will be released through the increased volume of the plasma-gas interface and radiation and thermal conduction of the surface. In other words, the plasma temperature is kept stable to a large extent by the COD itself and its conditions of presence. In this regard, in order to increase brightness to maintain COD, a high repetition rate pulsed laser is used, including in combination with the use of a CW laser, the power of which is not lower than the threshold power required to maintain COD, as is known from patent RU 2571433 issued on 12, 20, 2015.

However, with this method, there is a problem that the high-brightness laser pumping plasma light source has instability.

This disadvantage is largely overcome in the broadband light source known from us patent 9368337 issued 6/14 2016, in which the optically transparent COD plasma has an elongated shape along the axis of the CW laser beam. The plasma radiation is concentrated in the longitudinal direction, which results in a high brightness of the light source.

However, as the plasma radiation is longitudinally concentrated, a problem arises in that the laser radiation of the output beam of the plasma radiation is blocked. The problems of increasing brightness, increasing the absorption coefficient of the plasma for the laser radiation, and significantly reducing the numerical aperture of the blocked diverging laser beam passing through the plasma are solved, but the device does not fully solve the problem of light source brightness stability.

In the broadband light source known from us patent 9357627 issued 5.31 in 2016, the plasma radiation is concentrated in a direction other than the propagation direction of the laser beam. At the same time, since the light source configuration is optimized (where the laser beam is directed vertically upwards along the camera axis and the region radiating the plasma is immediately adjacent to the upper part of the compartment), the energy and spatial stability of the broadband plasma light source is improved by suppressing the convective vortices in the gas filled compartment.

The problems of improving the stability and control of the convection air flow, which causes instability of the brightness of the light source, are also solved by optimizing the geometry of the camera and the light source as a whole in several U.S. patents: 10008378 released on

month

6 and 26 of 2018; 10109473 published on 10, 23 in 2018; 9887076 published on 2018, 2, 6; 10244613 was released on the 3 rd month 26 th 2019.

However, it was not established for continuously generating plasma radiation with high spectral brightness (near the maximum achievable for this type of light source, higher than 50 mW/(mm) ² Sr·nm)), and optimal conditions to achieve low relative luminance instability σ (below 0.1%).

Disclosure of Invention

The technical problem to be solved by the present invention relates to creating an apparatus and a method for optimally generating broadband radiation from COD plasma and developing a highly stable high brightness plasma light source with laser pumping on the basis thereof.

Since the density of high temperature (-15000K) COD plasma is high, it is essential to the present invention to provide the highest possible light source brightness, said plasma density provided by the high pressure of the surrounding gas being equal to 50 bar to 100 bar or higher. A unique feature is to achieve such high pressures p (according to the ratio p c nT) while minimizing the density n of gas atoms, but using as high a gas temperature T as possible (in the range 600K to 900K or higher). Minimizing the gas density and the refraction associated with that density in turn effectively suppresses the instability of the light source brightness associated with turbulence of the convective gas flow in the gas-filled compartment. Therefore, the invention realizes the ultra-high brightness of the plasma light source with ultra-low brightness instability.

The technical result of the present invention is to provide COD maintenance conditions which are optimal for achieving high stability and high brightness of the radiation plasma, on the basis of which a broadband light source with ultra high brightness and stability is produced.

This object is achieved by a proposed laser pumped plasma light source comprising: a gas filled compartment, a plasma ignition device, a region of radiant plasma maintained in the compartment by a focused beam of a Continuous Wave (CW) laser, and at least one output beam of plasma radiation exiting the compartment, at least a portion of the gas filled compartment being optically transparent.

The light source is characterized by a density of gas particles passing through the compartment of less than 90.10 ¹⁹ cm ^-3 And the fact that the temperature of the inner surface of the compartment is not lower than 600K, an optimal continuous generation of the output beam of plasma radiation is achieved.

In a preferred embodiment of the invention, the optimal continuous production is characterized by a high spectral brightness of the light source, higher than 50 mW/(mm) ² Nm sr), and the relative standard deviation σ of luminance is low, below 0.1%.

In a preferred embodiment of the invention, the density of the gas particles is as low as possible and the temperature of the inner surfaces of the compartment in operation is as high as possible with a gas pressure in the compartment of about 50 bar or more.

In a preferred embodiment of the invention, the temperature of the inner surface of the compartment is not higher than 900K and the density of the gas particles is not less than 45.10 ¹⁹ cm ^-3 This corresponds to a gas pressure of not less than 16.5 bar at room temperature.

In one embodiment of the invention, the temperature of the inner surface of the compartment is not higher than 900K.

In one embodiment of the invention, the gas belongs to the group of inert gases comprising xenon, krypton, argon, neon or mixtures thereof.

In an embodiment of the invention, the gas is xenon and the wavelength of the CW laser is 808nm.

In one embodiment of the invention, at least a portion of the compartment arranged for outputting the output beam of the plasma is spherical and the region radiating the plasma is located in the centre of the spherical portion of the compartment.

In one embodiment of the invention, the radius of the inner surface of the spherical portion of the compartment is less than 5mm, preferably not more than 3mm.

In a preferred embodiment of the invention, the focused beam of the CW laser is directed into the compartment from bottom to top and the axis of the focused beam is oriented perpendicular or near perpendicular.

In a preferred embodiment of the invention, a part or portion of the compartment is located above a region of the irradiated plasma at a distance of not more than 3mm from it.

In a preferred embodiment of the invention, the compartment is provided with a heater.

In one embodiment of the invention, the transparent portion of the compartment is made of a material belonging to the group of sapphire, colorless sapphire (Al ₂ O ₃ ) Fused silica, crystalline quartz (SiO) ₂ ) Crystalline magnesium fluoride (MgF) ₂ ) Is made of a material of the group(s).

In one embodiment of the invention, the plasma ignition device comprises a solid state laser system that generates a pulsed laser beam in Q-switched mode and a pulsed laser beam in free-running mode, both pulsed laser beams being focused into the compartment.

In one embodiment of the invention, the beam of the CW laser and each output beam of plasma radiation exiting the compartment do not cross each other outside the region where the plasma is irradiated.

In one embodiment of the invention, the light source has three or more output beams of plasma radiation.

In another aspect, the invention relates to a method for generating light, the method comprising: the plasma is ignited within the gas filled compartment and the radiation plasma is sustained by a focused beam of a CW laser to produce at least one output beam of plasma radiation that exits the region of the radiation plasma through an optically transparent portion of the compartment.

The method is characterized in that the particle density is not more than 90.10 ¹⁹ cm ^-3 Is filled in the compartment and the plasma is maintained at a temperature of not less than 600K by the focused beam of the CW laser at the inner surface of the compartment.

In an embodiment of the invention, the gas pressure in the compartment in operation is close to 50 bar or more to provide a high spectral brightness of the light source, higher than 50 mW/(mm) ² ·nm·sr)。

In a preferred embodiment of the invention, the temperature of the inner surface of the compartment is as high as possible, with the density of the gas particles as low as possible, so that the relative standard deviation σ of the brightness is low, below 0.1%.

In a preferred embodiment of the present invention, the density of the gas particles is not less than 45.10 ¹⁹ cm ^-3 And maintaining the temperature of the interior surfaces of the compartment at a temperature not greater than 900K.

In a preferred embodiment of the invention, the focused beam of the CW laser is directed into the compartment from bottom to top along a vertical line.

In a preferred embodiment of the invention, the swirling of convection currents in the compartment is suppressed by placing the upper wall or a part of the compartment above a region of the radiating plasma at a distance as small as possible from it of not more than 3mm.

In a preferred embodiment, xenon is used to fill the compartments and the radiation plasma is maintained by a focused beam of a CW laser with a wavelength of 808nm.

In a preferred embodiment, ignition of the plasma is generated by focusing two pulsed laser beams generated by the solid state laser system in a free running mode and a Q-switched mode into the compartment.

Advantages and features of the 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.

Drawings

The essence of the invention is illustrated by the accompanying drawings, in which:

figure 1-a schematic view of a light source according to one embodiment of the invention,

figure 2-correlation of relative standard deviation of luminance with gas density,

figure 3-dependence of the brightness of a plasma light source on time,

FIG. 4-when CW laser wavelength lambda _CW =976 nm and λ _CW At 808nm, the spectral luminance of the light source is a function of the xenon pressure,

figures 5 and 6 show schematic views of a light source according to an embodiment of the invention,

fig. 7, 8-schematic diagrams of a light source with several beams of plasma radiation when the plasma is ignited by a laser and a discharge,

fig. 9A, 9B-spectral characteristics of the light source.

In the drawings, the matching elements of the device have the same reference numerals.

These drawings are not intended to be limiting and, in addition, are not intended to limit the scope of all alternatives for implementing the present technical solution, but are merely illustrative examples of specific cases of implementation thereof.

Detailed Description

The description is provided to illustrate how the invention may be implemented and is not intended to show the scope of the invention.

According to an example of embodiment of the invention shown in fig. 1, a laser pumped plasma light source comprises a high pressure gas filled compartment 1, at least a part of which is optically transparent. Fig. 1 shows an embodiment using a completely transparent compartment made of an optically transparent material, such as fused silica. The light source also comprises means for igniting the plasma, which means may be a pulsed laser system 2, generating at least one pulsed laser beam 3 focused into the compartment 1, i.e. into the area for sustaining the radiation plasma 4.

In other embodiments of the invention, the ignition electrode may be used as a means for igniting the plasma.

After ignition of the plasma, the focused beam 5 of the CW laser 6 maintains the region of the radiating plasma 4 in a continuous mode in the compartment. At least one output beam (or useful beam) 7 of plasma radiation is directed to an optical concentrator 8 and for subsequent use, leaves the compartment 1. The optical concentrator 8 forms a radiation beam 9 which is transmitted, for example via an optical fiber and/or a mirror system, to one or more optical consumer systems 10 which use broadband plasma radiation.

According to the invention, the density of the gas particles passing through the compartment 1 is less than 90.10 ¹⁹ cm ^-3 And the temperature of the inner surface of the compartment is not lower than 600K (preferably in the range 600K to 900K, or alternatively, if the higher temperature is not to the compartment)Life time and transparency thereof, and higher) to achieve an optimal continuous generation of the output beam 7 of plasma radiation.

The effect achieved by the invention is that the gas pressure increases with the temperature of the inner surface of the compartment due to factors for a given amount of gas in a given volume in the compartment. Since the temperature of the radiating plasma is virtually fixed (about 15000K and it is difficult to try to increase this temperature, since they only accompany an increase in the plasma volume) and the pressure in the plasma is equal to the pressure in the compartment, the density of the radiating plasma increases with an increase in the pressure in the compartment and therefore with an increase in the temperature of the compartment wall. The increase in the density of the radiation plasma results in an increase in the volumetric luminosity of the radiation plasma and thus in an increase in the brightness of the light source over a wide optical range, wherein the radiation plasma is virtually transparent.

By increasing the gas pressure at a given compartment temperature, the same increase in brightness can be obtained. However, in this case, the gas particle density and the refraction associated with this density will increase, which will lead to significant instability (fluctuation) of the brightness of the light source both in the region of the radiating plasma and at the periphery in the vortex.

Therefore, according to the present invention, it is preferable to increase the pressure of the gas not by increasing the density of the gas but by increasing the temperature thereof in order to ensure high brightness of the radiation and high stability of the brightness of the light source.

It should be noted that, as the temperature of the compartment and the gas increases, the turbulence of convection in the compartment also decreases for the following reasons. First, heating the compartment results in a decrease in the temperature gradient and gas density gradient in the compartment, which results in suppression of convection between the hotter regions of the plasma and the surrounding colder gas. Second, the nature of the gas flow is determined by the Reynolds number Re, and when the Reynolds number is less than the critical Reynolds number, eddy currents are suppressed. The reynolds number depends on the gas density ρ, the gas flow velocity ν, and the dynamic viscosity η:

Re≈ρ·ν/η (1)。

the dynamic viscosity increases with increasing temperature:

wherein eta ₀ Is at room temperature T ₀ (T ₀ A dynamic viscosity of the gas at 300 k). According to this, the reynolds number depends on the density of the gas, its velocity and temperature, as follows:

by increasing the absolute temperature of the compartment and the gas according to equation (3), the gas flow vortex can be suppressed. Other possibilities to suppress eddy currents and to improve the stability of the light source involve limiting the mass density p of the gas and its velocity v. In particular, the latter is achieved due to the reduced size of the compartments, since the acceleration of the gas heated in the region where the plasma is irradiated and floating up under the action of archimedes' force is limited by the size of the compartments.

According to (3), the lower the gas density, the lower the vortex of the convective gas flow. Furthermore, the lower the gas mass density ρ, the lower its refractive index and the lower the aberration associated with the refraction of light in the convective gas flow. Thus, the lower the density of the gas, the lower the instability of the brightness and other output parameters of the light source. As shown in fig. 2, fig. 2 shows the experimental correlation of the Relative Standard Deviation (RSD) σ of the light source luminance with both the mass density ρ and the number density n of the gas.

The number density n and the mass density ρ of the gas are related to each other as described by the following equation: n= (N) _A M) ρ, wherein M is the molar mass, N _A Is the avogalileo constant.

The relative standard deviation refers to (standard deviation of dataset)/(mean of dataset) ·100%. At a time interval of 10 ^-3 During the measurement period of seconds, the luminance dataset is sampled. This is sufficient to process a luminance signal having an oscillation frequency associated with the convection of the air stream of not more than 10Hz to 15HzAs shown in the enlarged section of fig. 3, fig. 3 shows the luminance of the plasma light source as a function of time.

To obtain the correlation shown in fig. 2, four identical sealed quartz compartments with electrodes for starting the ignition plasma were used, which were filled with xenon at different pressures, equivalent to room temperature: 11 bar, 17 bar, 23 bar and 29 bar. The correlation given in fig. 2 shows that the relative instability of the luminance sigma drops drastically by a factor of about 50 after a three-fold decrease in the gas density. Fig. 2 should be regarded as a qualitative illustration of the dependence of σ on gas density, since luminance instability may be further reduced by more than three times. In particular, this is achieved by using a CW laser control system with negative feedback in a plasma light source with electrodeless plasma ignition.

In FIG. 3, the time dependence of the brightness of the plasma light source is given, wherein 60.10 ¹⁹ cm ^-3 Which corresponds to a pressure of 22 bar at room temperature. The diameter of the inner surface of the spherical portion of the quartz cell arranged for the plasma radiation beam to leave is as small as 4mm. In this case, the relative standard deviation σ of the plasma light source luminance is in the range of 0.04% to 0.05%.

In general, the higher the gas pressure and thus the greater the pressure in the region where the plasma is emitted, the higher the brightness of the light source.

In order to make the relative standard deviation of the brightness sufficiently small (sigma < 0.1%), the density of the gas particles in the compartment is chosen to be less than 90.10 as determined experimentally ¹⁹ cm ^-3 This corresponds to a gas pressure of about 33.5 bar at room temperature. At the same time, in order to obtain a temperature in the range of 600K to 900K or more near the maximum value attainable (higher than 50 mW/(mm) ² Sr·nm)), the spectral brightness of the light source, the gas pressure and accordingly the density of the irradiated plasma should be sufficiently high to provide an optimal gas pressure of about 50 bar or more in operation. Thus, the density of the gas particles in the selected compartment is higher than experimentally determined 45.10 ¹⁹ cm ^-3 Lower limit of (2), which corresponds to a gas at room temperature of not less than 16.5 barBody pressure.

Thus, in order to provide a high spectral luminance and a low luminance relative standard deviation, the density of the gas particles should be as low as possible, while the temperature of the inner surfaces of the compartment in operation should be as high as possible in case a gas pressure of about 50 bar or more is provided in the compartment.

In one embodiment of the invention, the temperature of the inner surface of the compartment in operation is 600K and the density of the gas particles is 65.10 ¹⁹ cm ^-3 This corresponds to a gas pressure of 24.5 bar at room temperature and a gas pressure of 50 bar in operation.

In a preferred embodiment of the invention, the compartment may be operated at a temperature of its inner surface up to 860K and the density of the gas particles may be selected to be as low as 45.10 ¹⁹ cm ^-3 This corresponds to a gas pressure of 16.5 bar at room temperature and a gas of 50 bar in operation.

For illustration, fig. 4 shows the dependence of the spectral brightness of the light source on the pressure of xenon in the compartment at room temperature. In a stable mode of operation with a compartment temperature of 450K, measurements were made in the spectral range 600nm to 500 nm. In the indicated spectral range, the spectral luminance is about 25% lower than the maximum observed around a wavelength of about 400 nm. At wavelength lambda _CW =976 nm and λ _CW Two CW diode lasers with a radiation power of 65W were measured at 808nm.

The results of the study show that for both wavelengths of laser radiation, a high spectral brightness is achieved at room temperature at a gas pressure in the compartment of at least 25 bar. High stability of the radiation intensity (σ.ltoreq.0.1%) is maintained at room temperature at gas pressures in the compartments up to 36 bar.

Measurements indicate that as the compartment temperature increases to 600K or higher, there is a strong tendency to increase brightness while maintaining a high degree of stability of the output parameters of the light source.

According to the invention, it is preferred to use inert xenon gas as gas, which ensures safe operation and long lifetime of the light source. In addition, xe plasma is characterized by the highest optical output in a wide spectral range (including ultraviolet region, visible region, and infrared region) as compared with other inert gas radiation plasmas.

The preferred wavelength for the high efficiency CW diode laser is selected due to the following factors. Around the laser wavelength 976nm there is a strong Xe absorption line, where the lower states are filled with increasing temperature. Near 808nm, such lines are spaced farther from the absorption line, and therefore, at a given laser power, sufficient absorption to maintain a continuous light discharge is achieved at a higher plasma density and temperature than in the case of 976 nm.

Thus, in a preferred embodiment of the invention, the gas filling the compartment is xenon and the wavelength of the CW laser is 808nm.

Other embodiments of the present invention aim to further improve the stability of the output parameters of the light source, including the intensity, brightness, spectrum and spatial position of the radiated plasma, while ensuring that the brightness of the light source is as highest as possible.

In a preferred embodiment, the focused beam of the CW laser is directed into the compartment from bottom to top and the axis of the beam is oriented vertically parallel to gravity 11 (fig. 5), or nearly vertical. The stability of the light source is further improved in that the area of the radiation plasma 4 is generally moved slightly from the focal point towards the focused beam 5 of the CW laser to a cross-section of the focused laser beam in which the intensity of the focused beam 5 of the CW laser is still sufficient to maintain the area of the radiation plasma 4. When the focused beam 5 of the CW laser is directed from bottom to top, the region of the radiating plasma 4 containing the hottest mass density plasma and the lowest mass density plasma tends to float under the action of archimedes' force. Rising, the region of the radiation plasma 4 reaches a position closer to the focal point, where the cross section of the focused beam 5 of the CW laser becomes smaller and the laser radiation intensity becomes higher. This increases the brightness of the plasma radiation on the one hand and balances the forces acting on the region where the plasma is irradiated on the other hand, which ensures a high degree of stability of the light source.

To achieve these positive effects, the compartment 1 is preferably axisymmetric and the axis of the focused beam 5 of the CW laser is aligned with the symmetry axis of the compartment.

The stability of the output characteristics of the light source is also affected by the pulse amplitude obtained by the archimedes force of the gas heated in the region of the radiation plasma 4. The smaller the momentum obtained by the gas and the eddy currents of the convection, the closer the area of the radiating plasma 4 is to the upper wall of the compartment or to the portion of the compartment above the area of the radiating plasma 4. Thus, in order to improve the stability of the output characteristics of the light source in the embodiment shown in fig. 5, the part or portion 12 of the compartment is located on top of the area of the radiant plasma 4 at a distance from it as smallest as possible of less than 3mm, which does not have any negative effect on the lifetime of the compartment and its transparency.

Furthermore, the portion 12 of the compartment may be arranged for reflecting and focusing both the CW laser beam into the plasma 4, which passes through the region where the plasma is irradiated and the portion of the plasma irradiation. This reduces radiation losses and increases the efficiency of the light source. According to the present embodiment of the invention shown in fig. 5, the plasma-approaching portion 12 of the compartment contains a surface, which is a concave spherical mirror 13, the center of which is in the region of the radiation plasma 4.

In a preferred embodiment, at least a portion of the compartment 1 from which the output beam 7 for plasma radiation exits is spherical or approximately spherical, and the region of the radiated plasma 4 is located within the centre of symmetry of the spherical portion of the compartment 1, as shown in fig. 1 and 5. This minimizes chromatic and spherical aberration caused by the transparent walls of the compartment into the ray path of the plasma radiation.

In particular, by reducing the compartment size, suppression of aberrations associated with convective vortices is achieved. Thus, in one embodiment of the invention, the radius of the inner surface of the spherical portion of the compartment is less than 5mm, preferably not more than 3mm.

Figure 6 shows an embodiment of the compartment of the invention provided with a heater. The heater may consist of a heating coil 14 and a current source 15 connected to the heating coil through a temperature bridge 16 for providing a temperature difference between the heating coil 36 and the current carrying bus 17. Furthermore, the current carrying bus 17 may be provided with a heat exchanger (not shown), for example in the form of an air cooled radiator. The compartment may be composed of a spherical portion and a cylindrical portion on which the heating coil 14 is located. The compartment may also be equipped with a thermocouple for measuring the temperature of the compartment. In addition, the heating coil 14 may be accommodated in an insulating jacket (not shown).

The heater is designed to heat the pre-start-up of the compartment to an operating temperature, which facilitates ignition of the plasma and rapid transition of the light source to a steady state operating mode where the compartment reaches a preset optimum high temperature in the range 600K to 900K.

In one embodiment of the invention, the optical concentrator comprises a parabolic mirror 8 and a deflection mirror 18 for forming a plasma radiation beam 9, which is preferably transmitted by an optical fiber to an optical system using broadband plasma radiation.

In a preferred embodiment of the invention, the high brightness plasma light source comprises a control unit 19 with the function of automatically maintaining a given power of the plasma radiation output beam 7 (fig. 6). For this purpose, the light source is equipped with a power meter 20 to which a small fraction of the luminous flux from the plasma radiation beam 9 is supplied by means of a coupling (not shown). Preferably, the control unit is connected to the heater 15, the power meter 20 and the power supply unit of the CW laser 6. Maintaining a specific power of the plasma radiation beam 9 is performed by the control unit 19 according to a feedback circuit between the power meter 20 and the power supply unit of the CW laser 6. Furthermore, the control unit 19 can be made to have the function of achieving thermal stabilization of the compartment at its optimal high temperature. The present embodiment of the invention improves the power and brightness stability of the laser pumped plasma light source in a long-term continuous mode of operation.

As shown in fig. 6, in a preferred embodiment of the present invention, a CW laser 6 with an optical fiber output is used. At the output of the optical fiber 21, the expanded laser beam is directed to a collimator 22, for example in the form of a condenser lens. After the collimator 22, the expanded parallel beam 23 of the CW laser is directed by means of a deflection mirror 24 to a focusing optical element 25 (for example in the form of an aspherical lens) so that the CW laser beam 5 is sharply focused, which is necessary to ensure a high brightness of the light source.

In a preferred embodiment of the invention, the solid-state laser system 2 is used for reliably igniting a plasma, the solid-state laser system comprising a first laser 26 for generating a first laser beam 27 in a Q-switched mode and comprising a second laser 28 for generating a second laser beam 29 in a free-running mode. The pulsed laser with the

active element

30, 31 is provided with an optical pump source, for example in the form of a flash lamp 32, and preferably has a common cavity mirror 33, 34. The first laser 26 is provided with a Q-switch 35. Two pulsed laser beams 27, 29 are focused into the compartment in the region for sustaining the radiation plasma 2 (fig. 6). The first laser beam 27 is used for optical breakdown. The second laser beam 29 is used to generate a plasma of sufficient volume and density to stably maintain the region of the radiating plasma 4 by the focused beam 5 of the CW laser.

Preferably, the wavelength λ of the CW laser _CW A wavelength lambda different from the first and second pulse laser beams 27, 29 ₁ Wavelength lambda ₂ . For example, the wavelength of the CW laser may be λ _CW =808 nm or 976nm, and the emission wavelength of the pulsed laser may be λ ₁ ＝λ ₂ =1064nm. This allows CW laser beam 23 and pulsed laser beam 27, pulsed laser beam 29 to be input into the compartment using dichroic mirror 24. For transmitting the pulsed laser beam 27, 29 a rotating mirror 36 (fig. 6) may be additionally used.

Fig. 1, 5 and 6 show that when the plasma is ignited using a pulsed laser system 2, the compartment 1 allows the plasma radiation to be output at all azimuth angles. In one embodiment, the output beam of plasma radiation exits the compartment at a spatial angle of at least 9sr or greater than 70% of the total solid angle. In this case, the opening angle (flat angle with respect to the plane of the drawing) of the output beam 7 of plasma radiation is not less than 90 °.

The light source according to the invention is not limited to this embodiment, except for the output beam 7 which outputs plasma radiation to the optical concentrator 8 in all azimuth angles. In other embodiments of the invention, the light source may have at least three

concentric output beams

7a, 7b, 7c of plasma radiation, as shown in fig. 7, fig. 7 showing a cross section of the light source in a horizontal plane through the region of the radiating plasma 4. The laser beam in fig. 7 that ignites and maintains the continuous light discharge is located below the plane of the drawing. For many industrial applications, it is desirable to use several (especially three) beams of plasma radiation from a single light source. In the present embodiment, the laser pumping light source chamber 1 may be accommodated in a housing 37 provided with three

optical concentrators

8a, 8b, 8c. The three

optical concentrators

8a, 8b, 8c form a plasma radiation beam 9a, a plasma radiation beam 9b, a plasma radiation beam 9c, e.g. the plasma radiation beam 9a, the plasma radiation beam 9b, the plasma radiation beam 9c is transmitted by optical fibers to an optical consumer system 10a, an optical consumer system 10b, an optical consumer system 10c using broadband plasma radiation. This allows one light source to be used for three or more optical consumer systems, ensuring compactness of the system and uniformity of broadband radiation parameters for all optical channels.

Fig. 8 shows another version of a light source with three radiation output channels, wherein two

ignition electrodes

38, 39 are used as plasma ignition means connected to a high voltage pulsed power supply (not shown). Parts of the apparatus in this embodiment that are the same as those in the above-described embodiment (fig. 7) have the same reference numerals in fig. 8 and their detailed descriptions are omitted.

In a preferred embodiment of the invention, the transparent portion of the compartment is made of quartz. In other embodiments, the transparent portion of the compartment may be made of an optically transparent material belonging to the group of sapphire, colorless sapphire, fused silica, crystalline magnesium fluoride.

The method of generating light from COD plasma using the proposed laser pumped plasma light source shown in fig. 1, 5, 6, 7 and 8 is as follows. Using particlesParticle density of less than 90.10 ¹⁹ cm ^-3 Which corresponds to a pressure of 35.5 bar at room temperature, fills compartment 1. A focused beam 5 of a CW laser 6 is directed into the compartment 1. The plasma is ignited by means of a plasma ignition device, which may be an ignition electrode or a pulsed laser system 2. The concentration and volume of the initial plasma is sufficient to reliably maintain a continuous optical discharge by the focused beam 5 of the CW laser 6. In a steady state stable mode of operation, the focused beam passing through the CW laser maintains a region of the radiation plasma at a temperature in the range 600K to 900K or alternatively higher at the inner surface of the compartment. At least one output beam of plasma radiation is directed from the region where the plasma 4 is irradiated through the optically transparent portion of the compartment 1.

By heating the walls of the compartment to a specific temperature, an increase of the pressure of the gas around the region where the plasma is irradiated by a multiple, a double to a triple or more is achieved. Since the pressure of the plasma is equal to the pressure in the compartment, the density of the radiating plasma increases due to the heating of the walls of the compartment, which results in an increase of the volumetric luminosity of the radiating plasma and thus in an increase of the brightness of the light source over a wide optical range. In this case, the increase of the gas pressure and the brightness of the light source is achieved without increasing the gas density and the refraction proportional thereto, which leads to a significant instability of the brightness of the light source under vortex flow. As shown above, when considering equation (3), suppression of convective vortex can be achieved in the proposed method for generating light by increasing the gas temperature T, decreasing or limiting its density ρ, and decreasing the gas flow velocity v.

In order to achieve a high spectral luminance (higher than 50 mW/(mm) ² Nm sr)) such that the gas pressure in the compartment in operation is close to 50 bar or higher.

In order to achieve a low relative instability of the luminance sigma below 0.1%, the temperature of the inner surface of the compartment is provided as high as possible with the lowest possible density of gas particles.

By positioning the upper wall or a portion of the compartment at a distance of no more than 3mm from the region of the radiating plasma which is as minimal as possible, the velocity v of the gas flow rising from the region of the radiating plasma is reduced as much as possible. In one embodiment, the dimensions of the compartments are chosen such that the walls of the compartments are located at a distance of no more than 3mm from the region where the plasma is radiated, which helps to suppress the swirling of convection currents in the compartments.

The invention thus allows to approach the maximum achievable for this type of light source at high brightness to achieve a high degree of stability of the laser pumped plasma light source.

In one embodiment of the method, the compartment is heated after ignition of the plasma during the process of bringing the light source into a stable operation mode due to the radiation power of the CW laser entering the compartment.

In another embodiment, compartment 1 is rapidly heated to a temperature in the range of 600K to 900K before the plasma is ignited by an external heater (fig. 6), including

elements

14, 15, 16, 17. This facilitates ignition of the plasma and shortens the time for the light source to reach a stable operating mode, simplifying its design and enhancing ease of use. The specific temperature of the inner surface of the compartment is maintained by the radiant power of the heater and the CW laser.

To further increase the stability of the light source, the focused beam of the CW laser is directed into the compartment from bottom to top along a vertical line, which increases the brightness and spatial stability of the region where the plasma is irradiated. In this case, the CW laser beam is preferably focused at the centre of symmetry of the portion of the compartment from which the plasma radiation output beam passes. This reduces optical aberrations that may distort the path of the beam as the broadband plasma radiation passes through the transparent wall of the compartment and reduce the brightness of the light source as it is transmitted.

In order to achieve the highest possible brightness of the light source, xenon is preferably used, and the laser is a continuous diode laser with a wavelength of 808nm (fig. 4).

The Xe plasma of continuous light discharge (COD) is characterized by the highest light output in a wide spectral range including the visible light region and the near infrared region, compared to other inert gas radiation plasmas. The characteristic spectrum of a light source using Xe as an inert gas is shown in fig. 9A.

The use of other inert gases, especially heavy inert gases Kr, ar, ne, or the addition of other inert gases to xenon at a ratio of 20% or less, does not have any significant effect on COD optical properties. COD radiation in the ultraviolet, visible and near infrared regions of the spectrum is close to blackbody radiation and is primarily determined by plasma temperature. Meanwhile, in the Vacuum Ultraviolet (VUV) region of 100nm to 200nm, absorption or self-absorption of radiation by xenon is observed. Thus, in case the radiation spectrum is extended to the VUV region, it may be preferable to use other inert gases, especially Kr, ar or mixtures thereof. As shown in fig. 9B, fig. 9B shows the VUV spectrum of the light source when the xenon and kr+ar gas mixtures are used for comparison.

Generally, according to the invention, an inert gas (preferably xenon, krypton, argon, neon or mixtures thereof) is used as the gas filling the compartment.

In one 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 where the plasma is irradiated (fig. 6). The two pulsed laser beams 27, 29 achieve a photo-induced breakdown and generate an initial plasma with a density higher than the threshold density of a continuous photo-discharge plasma, the value of the threshold density of which is about 10 ¹⁸ Electrons/cm ³ . In this embodiment, reliable laser ignition and ease of use of the light source are achieved. The geometry of the compartment may be optimized, the eddy currents of the convective gas flow therein reduced and the optical aberrations minimized, and the spatial angle at which the plasma radiation is concentrated increased compared to a source that uses electrodes to initiate ignition of the plasma.

In general, the claimed invention may: the brightness of the laser pumped plasma radiation source is improved and a high degree of stability is ensured.

INDUSTRIAL APPLICABILITY

The high brightness, highly stable laser pump light source made in accordance with the present invention can be used in a variety of projection systems for spectrochemical analysis, spectroscopic microscopic analysis of biological objects in biology and medicine, microcapillary liquid chromatography, inspection of optical lithography processes, spectrophotometry, and other purposes.

Claims

1. A laser pumped plasma light source comprising: a gas-filled compartment, a plasma ignition device, a region of radiant plasma maintained in the compartment by a focused beam of a Continuous Wave (CW) laser, and at least one output beam of plasma radiation exiting the compartment, at least a portion of the gas-filled compartment being optically transparent, wherein

The density of the gas particles passing through the compartment is less than 90.10 ¹⁹ cm ^-3 And the fact that the temperature of the inner surface of the compartment is not lower than 600K, an optimal continuous generation of the output beam of plasma radiation is achieved.

2. The light source according to claim 1, wherein the optimal continuous production is characterized by a high spectral luminance of the light source, higher than 50 mW/(mm) ² Nm sr), and the relative standard deviation σ of the brightness is low, below 0.1%.

3. A light source according to claim 1 or 2, wherein the density of the gas particles is as low as possible and the temperature of the inner surface of the compartment in operation is as high as possible with a gas pressure in the compartment of about 50 bar or more.

4. A light source as claimed in any one of the preceding claims, wherein the density of the gas particles is not less than 45-10 ¹⁹ cm ^-3 This corresponds to a gas pressure of not less than 16.5 bar at room temperature.

5. A light source according to any one of the preceding claims, wherein the temperature of the inner surface of the compartment is not higher than 900K.

6. A light source according to any one of the preceding claims, wherein the gas comprises xenon, krypton, argon, neon and/or mixtures thereof.

7. A light source as claimed in any one of the preceding claims, wherein the gas is xenon and the wavelength of the CW laser is 808nm.

8. A light source as claimed in any one of the preceding claims, wherein at least a portion of the compartment from which the output beam for the plasma exits is spherical and the region of the radiated plasma is located at the centre of the spherical portion of the compartment.

9. A light source according to claim 8, wherein the radius of the inner surface of the bulbous portion of the compartment is less than 5mm, preferably no more than 3mm.

10. The light source of any one of the preceding claims, wherein the focused beam of the CW laser is directed into the compartment from bottom to top and an axis of the focused beam is directed to be perpendicular or near perpendicular.

11. A light source as claimed in any one of the preceding claims, wherein a portion or part of the compartment is located above a region of the radiating plasma at a distance of no more than 3mm therefrom.

12. A light source as claimed in any one of the preceding claims, wherein the compartment is provided with a heater.

13. A light source according to any of the preceding claims, wherein the transparent portion of the compartment is made of a material belonging to the group of sapphire, colorless sapphire (Al ₂ O ₃ ) Fused silica, crystalline quartz (SiO) ₂ ) Crystalline magnesium fluoride (MgF) ₂ ) Is made of a material of the group(s).

14. A light source as claimed in any one of the preceding claims, wherein the plasma ignition device comprises a solid state laser system which generates a pulsed laser beam in Q-switched mode and a pulsed laser beam in free running mode, the two pulsed laser beams being focused into the compartment.

15. A light source as claimed in any one of the preceding claims, wherein the beam of the CW laser and each output beam of plasma radiation exiting the compartment do not cross each other outside the region of the radiating plasma.

16. A light source according to any one of the preceding claims, having three or more output beams of plasma radiation.

17. A method for generating light, the method comprising: igniting a plasma within a gas filled compartment and sustaining the radiant plasma by a focused beam of a CW laser to produce at least one output beam of plasma radiation exiting a region of the radiant plasma through an optically transparent portion of the compartment, wherein

Using particle densities of not more than 90.10 ¹⁹ cm ^-3 Is filled in the compartment, and

the plasma is maintained by a focused beam of the CW laser at a temperature of not less than 600K at the inner surface of the compartment.

18. A method according to claim 17, wherein the gas pressure in the compartment in operation is close to 50 bar or more to give a light source with a high spectral brightness, above 50 mW/(mm) ² ·nm·sr)。

19. A method according to claim 17 or 18, wherein the temperature of the inner surface of the compartment is as high as possible with the density of the gas particles as low as possible, so that the relative standard deviation σ of luminance is low, below 0.1%.

20. The method according to any one of claims 17 to 19, wherein a particle density using a density of gas particles is not less than 45.10 ¹⁹ cm ^-3 And maintaining the temperature of the interior surface of the compartment at a temperature of no more than 900K.

21. The method of any one of claims 17 to 20, wherein the focused beam of the CW laser is directed into the compartment from bottom to top along a vertical line.

22. A method according to any one of claims 17 to 21, wherein the swirling of convection in the compartment is suppressed by placing an upper wall or a portion of the compartment above a region of the radiant plasma at a distance of no more than 3mm therefrom.

23. The method of any one of claims 17 to 22, wherein xenon is used to fill the compartment and a radiation plasma is maintained by the focused beam of the CW laser at 808nm wavelength.

24. The method of any one of claims 17 to 23, wherein the ignition plasma is generated by focusing two pulsed laser beams generated by a solid state laser system in free running mode and Q-switched mode into the compartment.