CN118103946A - Laser pump light source and method for laser ignition of a plasma - Google Patents

Abstract

In a laser pumped plasma light source (100) with a plenum (1), the means for plasma ignition is a solid state laser system (7) that produces two pulsed laser beams (8, 9) focused into the plenum (1): the first beam (8) is generated in a free-running mode and the second beam (9) is generated in a Q-switch mode. The solid state laser system (7) comprises a single active element (10), a radiation source (11) for pumping the active element (10) and optical resonators (12, 13) providing multiple channels of laser beams through the active element (10). One optical resonator (13) is provided with a Q-switch (14) mounted on the path of the second beam (9) such that the Q-switch overlaps only a portion (15 b) of the cross-section of the laser beam.

Description

Laser pump light source and method for laser ignition of a plasma

Technical Field

The present invention relates to a broadband light source with Continuous Optical Discharge (COD) and a method of initiating ignition of a plasma sustained by continuous laser radiation.

Background

One of the challenges associated with creating a high brightness COD-based light source relates to reliable start-up ignition of the COD.

For example, from patent US9368337 published in 2016, 6 and 14, it is known that in a laser pumped plasma source, two needle electrodes located on the axis of a transparent chamber are used to initiate plasma ignition, and a short arc discharge is generated between the two needle electrodes. The CW laser beam is focused in the gap between the two electrodes in the center of the cavity. The light source is characterized by high brightness and ease of use. The latter is mainly due to the fact that quartz chambers or bulbs with two electrodes containing gas, in particular high pressure Xe (10 atm or higher, i.e. 1MPa or higher), are commercially available products.

However, relatively cold electrodes located near the high temperature plasma region can cause turbulence to the convective gas flow within the chamber, thereby compromising the spatial and energy stability of the laser pumped plasma light source. Furthermore, the electrodes present in the vicinity of the radiating plasma region are characterized by a "dead" space angle that limits the emission of plasma radiation. Furthermore, sputtering of the electrode material may lead to a decrease in the transparency of the bulb wall and correspondingly to degradation of the light source over time.

The high brightness broadband light source of patent US9357627, 31, published 2016 largely overcomes this disadvantage. In an embodiment thereof, after ignition of the COD, the laser beam focus area and the corresponding radiation plasma area move from the gap between the ignition electrodes towards the chamber wall. By selecting the relative positions of the laser beam, the chamber axis and the radiating plasma region, a high spatial and power stability of the broadband laser pumped plasma source is provided.

However, the need to move the radiating plasma region complicates the light source design and operation. Furthermore, this makes sharp focusing using the laser beam more difficult, which may limit the realization of high brightness of the light source. Drawbacks of chambers containing electrodes also include complex techniques for sealing metal/glass joints and complex chamber shapes that create stress concentrations, which result in lower chamber strength when operated at high gas pressures.

The above-mentioned drawbacks do not exist with the electrodeless laser pumped plasma light source known from published patent application JPS61193358, 8, 27 in 1986, in which a laser is used to initiate plasma ignition and COD maintenance.

However, the laser radiation threshold power required for plasma ignition is typically about ten to several hundred kilowatts or more, while the laser radiation intensity sufficient to maintain COD is typically only a few tens of watts. Thus, using the same laser with high output power for plasma ignition and COD maintenance can result in reduced light source life (when full laser power is used for COD maintenance), or be redundant, expensive, and therefore impractical if only a small fraction of the full laser power is used to maintain COD.

Patent US10057973 published on 8/21 2018 proposes to overcome this challenge by using a single CW laser with a power of less than 250 watts and a wavelength of less than 1.1 pm. It is suggested to provide COD ignition and maintenance by sharp focusing of the CW laser beam with a focal zone cross-over size of less than 1-15 microns (i.e., 1-15 μm) and a focal zone length of 6 microns or less (i.e., 6 μm or less).

However, this solution is not versatile, as the requirements for laser focusing are very high and high functional reliability of the proposed light source cannot be guaranteed. Furthermore, the laser power of about 250 watts provided to the light source may be too high for various applications.

The light source known from patent FR2554302 published 5/3 1985 overcomes these drawbacks, in which a focused pulsed laser beam for initial plasma ignition or optical breakdown is used as a means of plasma ignition, CW laser being used for COD maintenance. The method eliminates the problem of the service life of the laser pumping plasma light source.

However, both plasma ignition and ensuring high brightness of the laser pumped plasma source require sharp focusing of the laser beam. Thus, extremely precise adjustments to the pulse and CW laser focus areas are required. This results in complexity and poor reliability of laser ignition, making stable COD ignition in high brightness light sources problematic.

The light source in patent US10244613 published 5, 25, 2017 partially overcomes these drawbacks. In an embodiment of the invention, the light beam of one or more ignition lasers and the light beam of one or more CW lasers for COD maintenance are introduced into an optical fiber for transmitting the radiation of said lasers into a condensing or focusing optical system. In the apparatus, if the wavelengths of the laser light are similar, superposition of the focused regions of the pulsed laser light and the CW laser light is achieved.

However, if the wavelengths of the pulse laser light and the CW laser light are different, their focal regions may diverge due to chromatic aberration. Furthermore, the transmission of high power laser pulses (hundreds kW) through the optical fiber for reliable COD ignition may lead to fiber damage, which determines a disadvantage of this solution.

The closest technical solution, occasionally referred to herein as a prototype, is the light source in patent RU2732999 (likewise US10770282B 2) published 9/28/2020, wherein the plasma ignition device is a solid state laser system, which can generate two laser beams focused in a room. The laser beam generated in Q-switched mode is designed to optically break down the gas. At the same time, the laser beam generated in free-running mode, which is designed to generate a plasma after optical breakdown, is not itself optically broken down, the volume and density of which are sufficient to keep it stable by a continuous laser. In other words, the combined action and free running of the two laser beams generated in the giant pulse mode results in the formation of a plasma whose combustion is picked up by the continuous laser. Thus, a reliable electrodeless ignition of the laser pumped plasma light source is provided. This allows to create a highest brightness broadband light source with high spatial and energy stability, collecting plasma radiation at large spatial angles.

However, the presence of two active elements complicates the solid-state laser system and the light source, reducing the reliability and convenience of its operation. When using a common resonator of a pulsed laser system, there is a rather strict requirement for parallelism of the ends of the two active elements that are made into a rod shape. Another problem with obtaining two parallel beams is the difference in thermal effects in the different rods. Furthermore, the use of two pulsed laser beams that are spatially separated complicates their introduction into the chamber along with a continuous laser beam, requiring the use of dichroic mirrors. The latter in turn results in the necessity to use polarized laser radiation through dichroic mirrors to reduce reflection losses and impose limitations on the wavelength selection of the firing and continuous laser radiation, making it difficult to use pulsed and CW lasers with similar radiation wavelengths.

Disclosure of Invention

The technical problem to be solved by the present invention relates to a method and an apparatus for creating a highly reliable laser ignition for continuous optical discharge and on the basis of this developing a high brightness, high stability laser pumped plasma light source.

The technical result of the invention is to ensure a simplified light source design, to improve its reliability and ease of use, and to create an electrodeless high brightness broadband light source with high spatial and power stability on this basis.

This object is achieved by a laser pumped plasma light source comprising: a plenum, a region of radiant plasma maintained in the plenum by a focused beam of Continuous Wave (CW) laser, at least a portion of the plenum being optically transparent, and means for plasma ignition.

The light source is characterized in that the means for plasma ignition is a solid state laser system, generating two pulsed laser beams and focusing into the chamber; one of the two pulsed laser beams is generated in free-running mode and the other beam is generated in Q-switched mode.

A solid state laser system includes a single active element (i.e., only one active element), a radiation source for pumping the active element, and an optical resonator that provides multiple channels (i.e., repeated channels) for the intracavity laser beam to pass through the active element; the optical resonator is equipped with a Q-switch mounted in the path of the intracavity laser beam such that the Q-switch overlaps only a partial cross section of the intracavity laser beam.

In an embodiment of the invention, the Q-switch is made of chromium doped yttrium aluminum garnet crystal Cr ⁴⁺: a saturable absorber made of YAG.

In an embodiment of the invention, the Q-switch overlaps a small fraction of the cross-section of the laser beam in the cavity by no more than 30% of its area.

In an embodiment of the invention, the focusing optical element focuses the pulsed laser beam and the CW laser beam into the chamber, the CW laser beam on the focusing optical element not intersecting the pulsed laser beam on the focusing optical element.

In an embodiment of the invention, the deflection mirror is mounted on the beam path of the CW laser outside the pulse laser beam path.

In an embodiment of the present invention, the deflecting mirror is installed on the pulse laser beam passing path other than the CW laser beam passing path.

In an embodiment of the invention, the output power of the CW laser is no more than 30 watts sufficient to ignite and sustain a radiating plasma.

In an embodiment of the present invention, the axis of the focused beam of the CW laser is vertically upward or nearly vertically oriented at an angle of no more than 10 degrees from vertical.

In the embodiment of the present invention, the density of the indoor gas particles is less than 90.10 ¹⁹cm^-3, which corresponds to a gas pressure of 33atm (i.e., 3.3 MPa) at room temperature, and the temperature of the indoor surface is not less than 600K.

In the embodiment of the present invention, the temperature of the indoor surface is not more than 900K, and the density of gas particles is not less than 45.10 ¹⁹cm^-3, which corresponds to the gas pressure of 16.5atm (i.e., 1.6 MPa) at room temperature.

In a preferred embodiment of the invention, the radiation plasma is characterized by a hyperspectral brightness (greater than 50 MW/(mm ². Nm. Sr) and a relatively low instability of the brightness sigma (less than 1%).

In a preferred embodiment of the invention, the gas belongs to a group of inert gases. The group includes the elements xenon, krypton, argon, neon, but the gas may also be a mixture of these elements.

In another aspect, the invention relates to a method of plasma ignition in a laser pumped plasma light source, comprising: a focused beam of Continuous Wave (CW) laser is directed into a plenum, at least a portion of which is optically transparent, plasma ignition and stable maintenance of a radiating plasma by using the focused beam of CW laser.

The method is characterized in that the plasma is ignited by a solid state laser system with a single active element, which generates two parallel pulsed laser beams focused into the chamber; one of the two pulsed laser beams is generated in free-running mode and the other pulsed laser beam is generated in Q-switch mode by a Q-switch mounted in the optical cavity, overlapping only a partial cross-section of the laser beam within the cavity. In other words, the first beam is generated in the free-running mode and the second beam is generated in the Q-switched mode.

In an embodiment of the invention, the focusing optical element focuses the pulsed laser beam and the CW laser beam to an area for sustaining a radiation plasma, the CW laser beam on the focusing optical element being disjoint from the pulsed laser beam on the focusing optical element.

When the proposed form of light source is employed, since a laser system having only one active element for plasma ignition is used, the number of elements of the laser system is reduced, the design is simplified as much as possible, and the reliability of the device and the radiation source for laser plasma ignition is increased. The problems associated with the focal combination of two pulsed laser beams due to the different prismatic power of the two active elements and the possible separation of the two active elements due to thermal effects are eliminated compared to the prototype.

At the same time, since the two pulsed laser beams are generated by one active element, the maximum convergence of the two pulsed laser beams in space allows the pulsed and continuous laser beams to be injected into the chamber without using a dichroic mirror, thereby eliminating the need to use polarized laser radiation passing through the dichroic mirror.

In addition, cr ⁴⁺: a passive Q-switch in the form of a saturated absorber in the form of a YAG crystal (particularly a slab) provides automatic operation of the dual beam laser system.

These also simplify the design of the radiation source, improving its reliability and ease of operation.

Reliable ignition of the continuous optical discharge is achieved due to the following factors. The optical breakdown is provided by a laser beam generated in the Q-switched modulation mode. However, igniting COD with only one laser beam is problematic. One of the reasons is that it is difficult to combine the focal region of the continuous laser with an optical breakdown region, which is typically very small in size, not exceeding about 50 μm. Even if the focal areas of the pulsed and continuous laser beams are combined, it is still difficult to fire COD with only one laser beam. This is because the optical breakdown produced by laser radiation is explosive. Explosive processes, especially shock waves, can lead to sustained optical discharge quenching for low power (typically no more than 300 watts) continuous lasers. According to the invention, this problem is solved by the fact that: the laser beam generated in the free-running mode and unable to perform optical breakdown by itself provides plasma ignition after optical breakdown by the laser beam generated in the Q-switched mode. The parameters of the laser beam generated in the free-running mode are selected such that the optical discharge maintained by it is itself free from explosion phenomena, while being resistant to disturbances caused by previous optical breakdowns. At the same time, the laser beam generated in free-running mode provides a plasma of sufficient volume and density to achieve reliable stable maintenance by a continuous laser of low power (up to 30 watts) after the end of the ignition laser pulse.

Thus, reliable electrodeless ignition of continuous optical discharge is achieved. The elimination of the electrode reduces turbulence of the convective gas flow near the region where the plasma is emitted, simplifies the chamber, allows optimizing its design to reduce turbulence of the convective gas flow and minimize optical aberrations, particularly when the plasma radiation is output through the transparent portion of the chamber, and increases the spatial angle at which the plasma radiation is collected.

According to the invention, the continuous generation of plasma radiation with a high spectral brightness (greater than 50 MW/(mm ². Nm. Sr) and a relative brightness sigma. Instability (less than 0.1%) is achieved by the fact that the density of the gas particles in the chamber should be as low as possible and that the temperature of the chamber interior surface should be as high as possible during operation, while ensuring a chamber pressure of about 50atm. And more.

All this makes it possible to create a highest brightness broadband light source with a large spatial angle of plasma radiation collection, characterized by the highest spatial and energy stability.

The specific objects, features and advantages of the present invention, as well as the invention itself, will be more readily understood from the following description of the embodiments of the invention, as illustrated in the accompanying drawings.

Drawings

The technical nature and the operating principle of the proposed device are explained by means of the accompanying drawings, in which:

Figure 1 is a schematic view of a light source with a pulsed solid state laser system for plasma ignition and a cross-sectional view of a plasma laser ignition device within its optical resonator region,

Fig. 2 is a characteristic waveform diagram of radiation intensity of a solid-state laser system for plasma ignition.

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

These drawings do not cover and do not limit the full scope of options for implementing the technical solution, but are merely illustrative examples of the particular circumstances in which the technical solution is implemented.

Detailed Description

The description is intended to be illustrative of embodiments of the invention and is not intended to be limiting of the scope of the invention.

According to an example of the invention (fig. 1), a laser pump light source 100 comprises a chamber 1 filled with a high pressure gas. At least a portion of the chamber 1 is optically transparent. Fig. 1 shows a variation of a chamber made of an optically transparent material such as fused silica. In the chamber 1 there is a region of the radiation plasma 2 maintained in the chamber by a focused beam 3 of a continuous laser 4 (i.e. a CW laser).

At least one plasma radiation beam 5 directed towards the optical radiation collection system 6 for further use is emitted from the chamber 1. The optical radiation collection system 6 may comprise axisymmetric ellipsoidal mirrors (fig. 1) forming a plasma radiation beam that is transmitted, for example, by an optical fiber or mirror system, to an optical system using broadband plasma radiation.

The light source also comprises a plasma ignition device using a solid state laser system 7 having the function of generating two laser beams 8, 9 which are focused into the chamber 1. One of the two laser beams 8 (first beam) is generated in the free-running mode and the other laser beam 9 (second beam) is generated in the Q-switched mode. In this case the solid state laser system comprises only one active element 10 (i.e. a single active element), a radiation source 11 (e.g. a compact pulsed xenon lamp for pumping the active element) and an optical resonator with mirrors 12, 13 and a Q-switch 14. The optical resonator provides multiple passes (i.e., repeated passes) of the laser radiation flux 15 through the active element 10. Since the resonator with mirrors 12, 13 can be considered as a cavity between the mirrors, the flux 15 can be referred to as an "intracavity laser beam". In this case, the Q-switch 14 is mounted in the path of the intracavity laser radiation flux 15 such that it covers only a portion of the aperture of the laser radiation flux 15.

A portion of the intracavity laser radiation flux 15a is free of the Q-switch 14 in its path which results in the generation of the laser beam 8 in free running mode.

Another part of the intracavity laser radiation flux 15b has a Q-switch 14 in its path which results in the generation of the laser beam 9 in Q-switched mode.

As can be seen from the cross section A-A of the solid state laser system 7 in fig. 1, in an embodiment of the invention the apertures of the laser radiation fluxes 15a and 15b and the respective apertures of the laser radiation beams 8, 9 coming out of the resonator are in the form of circular segments of different cross sections.

The laser beam 9 generated in Q-switched mode and the laser beam 8 generated in free-running mode are focused into the chamber 1, i.e. into a chamber area designed for sustaining an emitted plasma.

The laser beam 9 generated in Q-switched mode is designed for initial plasma ignition or optical breakdown in the chamber 1. The laser beam 8 generated in the free-running mode is designed to ignite a plasma after optical breakdown of the laser beam 8 generated in the Q-switched mode. In this case, the Q-switch 14 preferably covers only a small portion 15b of the aperture of the laser radiation flux 15, preferably not more than 30% of its area (at the mirror 13), in order to ensure an optimal energy ratio of the laser beams 8 and 9 from the point of view of plasma ignition.

The laser beam 8 generated in the free-running mode is designed to ignite a plasma after optical breakdown of the laser beam 9 generated in the Q-switched mode. In this embodiment of the invention, the pulsed laser radiation generated in the solid-state laser system 7 has a wavelength λ ₁ =1.064 μm.

Q-switch 14 may be passive, made of orthotropic material, and represents a saturated absorber, for example, to incorporate chromium: cr ⁴⁺: YAG aluminum garnet crystal form. The Q-switch 14 may be formed in the form of a plate, not limited to this option. In other embodiments of the invention, the Q-switch 14 may be active.

In a preferred embodiment of the present invention, all laser beams are focused by a single focusing optical element 16 (e.g., the form of a condensing lens 16, not limited to this option) to a chamber region for sustaining an emission plasma 2 (i.e., a radiation plasma).

In the embodiment of the apparatus shown in fig. 1, the continuous laser beam 17 is directed into the chamber using a turning mirror 18 mounted outside the path of the pulsed laser beams 8, 9. In these embodiments of the invention, it is also possible to direct the pulsed laser beams 8, 9 into the chamber using another rotating mirror mounted directly outside the beam path. The continuous laser beam 17 directed to the focusing optical element 16 does not intersect the beams 8, 9 of the solid state laser system also directed to the focusing optical element 16.

In this embodiment of the invention, the known limitations associated with the use of dichroic mirrors are alleviated.

The method of plasma ignition in a laser pumped plasma light source is implemented as follows. As shown in fig. 1, the focused beam 3 of the continuous laser 4 is directed into the chamber 1 with high pressure gas, for example using a total reflection turning mirror 18. Xenon or other inert gases and mixtures thereof (including metal vapors such as mercury) and various gas mixtures (including halogen-containing gas mixtures) are used as efficient plasma forming mediums.

The starting ignition of the plasma is provided by a solid-state laser system 7 with one active element 10. After switching on the radiation source 11 designed for pulsed pumping of the active element 10, two parallel laser beams 8, 9 are generated, focused into the chamber into a region designed for sustaining the emission plasma 2. In this case, by using a Q-switch 14 mounted in a resonator with mirrors 12, 13, one pulsed laser beam 8 is generated in free running mode and the other pulsed laser beam 9 is generated in Q-switch mode, providing multiple channels of laser radiation flux 15 through the active element 10. The Q-switch 14 is mounted in the path of the intracavity laser radiation flux 15 such that it covers only part of the aperture of the laser radiation flux 15 b. In this case, the pulsed laser beam 9 generated in Q-switched mode is used to provide optical breakdown, after which the pulsed laser beam 8 generated in free-running mode is used to ignite a plasma, the volume and density of which are sufficient to maintain a stable plasma by the focused beam 3 of the continuous laser 4.

In the fixed mode, high brightness broadband radiation is output from the region of the continuous optical discharge where the plasma 2 is emitted by at least one beam of useful plasma radiation, which exits through an optically transparent portion of the chamber 1 and is used further.

In the example of the present invention, the air pressure Xe in the room temperature is 30atm (i.e., 30 MPa); the wavelength of the continuous laser is λ _CW =0.808 μm, and the power varies from 30 watts to 100 watts.

The characteristic time dependence of the laser radiation power produced by a solid state laser system is shown in fig. 2. In this example, the energy of the laser beam generated in the peak free running mode is about 150mJ for about 100 microseconds with a radiation wavelength of λ ₁ =1.064 μm. The laser beam generated in the passive Q-switched mode has a time delay characterized by a laser pulse energy of 3mJ for a duration of 20ns. The characteristic dimension of the optical breakdown plasma is 50-100 μm.

The optical breakdown mode does not provide reliable ignition of a continuous optical discharge. Thus, after optical breakdown, the plasma is ignited by a laser beam generated in free-running mode, whose volume (up to 1mm ³) and density (exceeding 10 ¹⁸cm^-3) are sufficient to maintain a stable plasma by the focused beam of the continuous laser. Preferably, as shown in fig. 2, the laser beam firing pulse generated in the free running mode ends no earlier than 50 microseconds after the end of the laser beam firing pulse generated in the Q-switched mode. The time of about 50 microseconds ensures attenuation of the interference from optical breakdown and evolution of the plasma size and density to a value sufficient to maintain a stable plasma by a focused beam of relatively low power continuous laser light.

Other embodiments of the present invention are directed to further improvements to laser pumped plasma light sources.

Since the radiation power of the solid-state laser system 7 at the time of generating the giant pulse does not allow its radiation to be transmitted using optical fibers, in an embodiment of the present invention, only the continuous laser has an optical fiber radiation output (not shown).

Meanwhile, the output of the continuous laser radiation 4 is preferably performed in an optical fiber (not shown). At the output end of the optical fiber, the expanded laser beam is directed to a collimator (not shown) after which the expanded parallel beam of the continuous laser light is directed to a focusing optical element 16, for example in the form of an aspherical collecting lens. The focusing optical element 16 provides a sharp focusing of the light beam 3 of the continuous laser 4, which is necessary to ensure a high brightness of the light source.

Due to the laser ignition of the plasma and the absence of ignition electrodes, in a preferred version of the invention, a useful plasma radiation beam 5 is output from the chamber in all directions, as shown in fig. 1. This means that in the azimuthal plane passing through the region of the emitted plasma 2 perpendicular to the axis of the beam 3 of the continuous laser, useful plasma radiation exits along all azimuths from 0 to 360 degrees. In a preferred embodiment of the invention, the useful plasma radiation beam 5 has a flat opening angle (in fig. 1, in the plane of the drawing) of at least 90 °. This means that the output of the useful plasma radiation beam 5 from the chamber 1 to the radiation collection system 6 is at a spatial angle of at least 9sr or more than 70% of the total solid angle.

In this embodiment, the axis of the focused beam 3 of the continuous laser is vertically upwards, i.e. opposite or near vertical to gravity, within ±10°. Preferably, the chamber 1 is axisymmetric and the axis of the focused beam 3 of the continuous laser is aligned with the symmetry axis of the chamber. When performed in the proposed form, a maximum stability of the radiation power of the laser pump light source is achieved.

According to the present invention, in the mode of maintaining the radiation plasma, if a higher temperature does not have a significant negative effect on the intensity of the chamber and its transparency, the temperature of the inner surface of the chamber is in the range of 600 to 900K or more. The positive effect achieved by the invention is due to the following facts: for a given amount of gas in a given volume of chamber, the gas pressure increases with the temperature of the chamber. Since the temperature of the emitted plasma is virtually fixed (about 15000K and trying to raise it is difficult because they only accompany an increase in the plasma volume) and the pressure in the plasma is equal to the pressure in the chamber, the density of the emitted plasma increases with an increase in the pressure in the chamber, which means an increase in the chamber wall temperature. An increase in the density of the emitted plasma results in an increase in the volumetric luminosity of the emitted plasma and thus in an increase in the brightness of the light source over a wide optical range, wherein the emitted plasma is virtually transparent.

At a given room temperature, the brightness can also be increased by increasing the air pressure. In this case, however, the gas density and the refraction associated with this density will increase, which will lead to significant instability (fluctuation) of the brightness of the light source in the region and periphery of the plasma emitted.

In order to make the relative luminance instability sufficiently small (σ.ltoreq.0.1%), the density of the indoor gas particles was selected to be lower than the experimentally determined upper limit of 90.10 10 ¹⁹cm^-3, which corresponds to a gas pressure of 33.5atm (i.e., 3.3 MPa) at room temperature. At the same time, in order to obtain a light source spectral luminance close to the maximum achievable at a specific temperature (greater than 50 MW/(mm ². Nm. Sr), the gas pressure and accordingly the density of the emitted plasma must be high enough to ensure an optimal gas pressure of about 50 bar or higher in steady state operation.

Thus, to ensure high spectral brightness and low relative brightness instability, the density of the gas particles should be as low as possible, and the temperature of the indoor surface should be as high as possible during operation, while ensuring that the gas pressure in the room is about 50 bar or more.

According to the present invention, it is preferable to use inert xenon as a gas, which can ensure safe operation and long life of the light source. In addition, xe plasmas are characterized by the highest light output over a broad spectral range, including the ultraviolet, visible, and near infrared regions, as compared to other inert gas emitting plasmas.

Preferably, a high efficiency near infrared diode laser is used as the continuous laser 4. The preferred wavelength is selected from the two wavelengths 976nm and 808nm, and the high efficiency diode laser is due to the following factors. The strong Xe absorption line is located near the laser wavelength of 976nm, where the lower state increases with increasing temperature. Near 808nm, these lines are farther from the absorption line, and therefore, at a given laser power, sufficient absorption to sustain an optical discharge is achieved at a higher plasma density and temperature than in the 976nm case.

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

In other variations, a high efficiency solid state or fiber laser may be used as the continuous laser. In this case, the wavelengths of radiation from the continuous laser and the solid state laser system may be close or coincident.

The invention also enables the possibility of achieving maximum brightness of the broadband laser pump light source, in particular by optimizing the shape and size of the electrodeless chamber, while ensuring a high stability of the output parameters. Thus, in a preferred embodiment of the invention, the outer and inner surfaces of the chamber or transparent portion thereof have the shape of concentric spheres and the plasma-emitting region 2 is located in the center of these concentric spheres, as shown in fig. 1. In this version of the invention, aberrations that distort the ray path in the useful plasma radiation beam 5 are eliminated, increasing its brightness.

In order to ensure a plasma radiation output in a broad spectral range from ultraviolet to near infrared, the optically transparent portion of the chamber is preferably made of the following materials: crystalline magnesium fluoride (MgF ₂), crystalline calcium fluoride (CaF ₂), crystalline or colorless sapphire (Al ₂O₃), fused silica or crystalline quartz.

In general, the present invention can ensure high reliability of plasma ignition maintained by laser radiation and on the basis of this create an electrodeless high brightness broadband light source with the highest spatial and energy stability and can collect plasma radiation at spatial angles greater than 9 sr.

When the proposed form of the light source is used, the design is simplified as much as possible, and the reliability of the device for plasma laser ignition and the radiation source as a whole is improved. In contrast to the prototype, the problem of combining the focus of the two pulsed laser beams and separating the two active elements of the laser system during operation is eliminated. The limitations associated with the use of dichroic mirrors, which are commonly used to inject several laser beams into a chamber, are eliminated. In general, when the light source is performed in the proposed form, a reliable ignition of the COD is achieved. This makes it possible to create an electrodeless high brightness broadband laser pumping source featuring the highest spatial and energy stability while optimizing the conditions for maintaining COD.

INDUSTRIAL APPLICABILITY

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

Claims (14)

1. A laser pumped plasma light source (100), comprising: a plenum (1), at least a portion of the plenum (1) being optically transparent; a region (2) of radiating plasma maintained in the chamber by a focused beam (3) of a continuous wave laser (4), the continuous wave laser (4) being hereinafter referred to as a CW laser; and means for plasma ignition, said laser pumped plasma light source (100) being characterized by

The means for plasma ignition is a solid state laser system (7) that generates two pulsed laser beams (8, 9) focused into the chamber (1); one (8) of the two pulsed laser beams is generated in free-running mode and the other pulsed laser beam (9) is generated in Q-switched mode;

The solid state laser system (7) comprises a single active element (10), a radiation source (11) for pumping the active element (10) and an optical resonator (12, 13) providing a plurality of channels of an intracavity laser beam (15) through the active element, the optical resonator being provided with a Q-switch (14) mounted on the path of the intracavity laser beam such that the Q-switch overlaps only a portion (15 b) of the cross section of the intracavity laser beam.

2. The light source (100) of claim 1, wherein the Q-switch (14) is made of chromium doped yttrium aluminum garnet crystal Cr ^{4 +}: a saturable absorber made of YAG.

3. The light source (100) according to claim 1 or 2, wherein the Q-switch (14) overlaps a small portion (15 b) of the cross-section of the intra-cavity laser beam (15) by no more than 30% of its area.

4. A light source (100) according to any one of claims 1 to 3, wherein the pulsed laser beam (8, 9) and the beam (17) of the CW laser (4) are focused into the chamber by a focusing optical element (16), and the beam (17) of the CW laser (4) directed towards the focusing optical element (16) does not intersect the pulsed laser beam (8, 9) directed towards the focusing optical element (16).

5. The light source (100) according to any one of claims 1 to 4, wherein a deflection mirror (18) is mounted on the path of the beam (17) of the CW laser (4) outside the path of the pulsed laser beam (8, 9).

6. The light source (100) according to any one of claims 1 to 4, wherein the deflection mirror (18) is mounted on the passing path of the pulsed laser beam (8, 9) outside the beam passing path of the CW laser (4).

7. The light source (100) according to any one of claims 1 to 6, wherein the output power of the CW laser (4) sufficient to ignite and sustain a radiation plasma is not more than 30 watts.

8. The light source (100) according to any of claims 1 to 7, wherein the axis of the focused beam (3) of the CW laser is vertically upwards or near vertical and is at an angle of no more than 10 degrees to the vertical.

9. The light source (100) according to any one of claims 1 to 8, wherein the density of gas particles in the chamber (1) is less than 90-10 ¹⁹cm^-3, which corresponds to a gas pressure of 33atm at room temperature, and wherein the temperature of the inner surface of the chamber is not lower than 600K.

10. The light source (100) according to any one of claims 1 to 8, wherein the temperature of the inner surface of the chamber is not more than 900K and the density of the gas particles is not less than 45-10 ¹⁹cm^-3, which corresponds to a gas pressure of 16.5atm at room temperature.

11. The light source (100) according to any one of claims 1 to 10, wherein the radiation plasma is characterized by a hyperspectral brightness (greater than 50 MW/(mm ² nm sr) and a low relative instability of the brightness σ (less than 1%).

12. The light source (100) according to any one of claims 1 to 11, wherein the gas belongs to a group of inert gases comprising xenon, krypton, argon, neon, or wherein the gas comprises a mixture of xenon, krypton, argon, neon.

13. A method for plasma ignition in a laser pumped plasma light source (100), comprising: directing a focused beam (3) of a Continuous Wave (CW) laser (4) into a plenum (1), at least a portion of which is optically transparent, igniting and stably sustaining a radiating plasma (2) by the focused beam (3) of the CW laser (4), characterized by

The plasma is ignited by a solid state laser system (7) with a single active element (10) which generates two parallel pulsed laser beams (8, 9) focused into the chamber; one (8) of the two pulsed laser beams is generated in free running mode and the other pulsed laser beam (9) is generated in Q-switch mode by a Q-switch (14) mounted in the optical cavity, overlapping only a part (15 b) of the cross section of the intra-cavity laser beam (15).

14. Method according to claim 13, wherein the pulsed laser beam (8, 9) and the beam (3) of the CW laser (4) are focused by a focusing optical element (16) into the region (2) for sustaining the radiation plasma, and the beam (17) of the CW laser directed towards the focusing optical element does not intersect the pulsed laser beam (8, 9) directed towards the focusing optical element (16).