CN115210849A

CN115210849A - Laser pumping plasma light source and plasma ignition method

Info

Publication number: CN115210849A
Application number: CN202180018909.8A
Authority: CN
Inventors: D·B·阿布拉门科; R·R·盖亚索夫; V·M·克里夫森; A·A·拉什; Y·B·基里尤金; D·A·格卢什科夫
Original assignee: Istek Private Ltd; RND ISAN Ltd
Current assignee: Istec Group Holdings Ltd; Istek Private Ltd
Priority date: 2020-03-05
Filing date: 2021-02-26
Publication date: 2022-10-18
Also published as: JP6885636B1; CA3166712A1; JP2021141037A; WO2021177859A1; KR20220133979A; IL294983A; EP4115441A1

Abstract

The light source comprises a gas-filled chamber having a region of radiating plasma sustained by a focused beam of a CW laser. The plasma ignition device is a pulsed laser system that generates first and second laser beams that are focused within a chamber. The first laser beam provides an optical breakdown, after which the second laser beam ignites a plasma, of sufficient volume and density to sustain a stable plasma by the CW laser after the second laser pulse ends. Preferably, the first laser beam is generated in a Q-switch mode and the second laser beam is generated in a free-running mode. The technical result includes ensuring a high reliability of igniting the plasma, on the basis of which an electrodeless high-brightness broadband light source with high spatial and power stability is created, and providing the ability to collect broadband plasma radiation over spatial angles greater than 9 sr.

Description

Laser pumping plasma light source and plasma ignition method

Technical Field

The present invention relates to an electrodeless laser pumped plasma light source for generating high brightness light in the Ultraviolet (UV), visible and Near Infrared (NIR) spectral bands and a method of initiating plasma ignition.

Background

A Continuous Optical Discharge (COD) is a stable gas discharge sustained by laser radiation in a relatively dense plasma generated beforehand. COD sustained by a focused beam of Continuous Wave (CW) laser is achieved in various gases, particularly Xe at High pressures of 10-200atm (Carlhoff et al, "Continuous Optical Discharges at Very High Pressure", physica 103C,1981, pages 439-447). Due to the high plasma temperature of about 20,000k (rainier, "Optical Discharges," sov. Phys. Usp.23 (11), 11 months 1980, pages 789-806), the COD-based light source is the highest brightness continuous light source over a broad spectral range between about 0.1 μm and 1 μm. Such laser pumped plasma light sources not only have higher brightness but also have longer lifetimes than arc lamps, which makes these light sources preferable for various applications.

One of the challenges associated with designing a high brightness laser pumped plasma light source involves generating an initial plasma that provides reliable ignition of the COD.

For example, it is known from U.S.9368337, issued on 14.6.2016, that in a laser-pumped plasma light source, two needle electrodes, located on the axis of a transparent chamber, are used to initiate plasma ignition, creating a short-time arc discharge between the two needle electrodes. The CW laser beam is focused in the center of the chamber, in the gap between the two electrodes. The light source has the characteristics of high brightness and convenient use. The ease of use is largely due to the fact that quartz cells or bulbs with two electrodes containing gases, particularly high pressure Xe (10 atm or higher), are commercially available products.

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

The high brightness broadband light source known from patent U.S.9357627 issued 5/31/2016 overcomes this drawback to a large extent. In an embodiment thereof, after COD ignition, the laser beam focusing region and the corresponding radiating plasma region move from the gap between the ignition electrodes towards the chamber walls. By selecting the relative positions of the laser beam, the chamber axis and the radiating plasma region, high spatial and power stability of the broadband laser pumped plasma light source is provided.

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

The electrodeless laser pumped plasma light source in patent application JPS61193358, published as 27/8/1986, in which the laser is used to initiate plasma ignition and COD maintenance, does not suffer from the above-mentioned disadvantages.

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

Patent u.s.10057973, 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 μm. The patent teaches that COD ignition and maintenance is provided by a sharp focus of a CW laser beam having a focal region cross dimension of less than 1-15 microns and a focal region length of 6 microns or less.

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

These drawbacks are overcome by the light source known from patent FR2554302, published on 3.5.1985, in which a focused pulsed laser beam for initial plasma ignition or optical breakdown is used as the means for plasma ignition and a CW laser is used to maintain COD. The method eliminates the problem of laser pumping plasma light source life.

However, both plasma ignition and ensuring high brightness of laser pumped plasma light sources require sharp focusing of the laser beam. Therefore, extremely precise adjustment of the pulse and CW laser focal regions is required. This leads to a complexity and poor reliability of laser ignition, making stable COD ignition problematic in high brightness light sources.

The light source known in patent u.s.10244613, published 5, 25, 2017, overcomes these drawbacks. In one embodiment of the invention, the beam of one or several ignition lasers and the beam of one or several CW lasers for maintaining COD are introduced into an optical fiber for transmitting the radiation of said lasers to a condensing or focusing optical system. In the apparatus, if the wavelengths of the lasers are similar, a focal region in which a pulse laser and a CW laser are superimposed is realized.

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

Disclosure of Invention

The technical problem to be solved by the present invention relates to a method and a device for creating a highly reliable laser ignition for continuous light discharge and on this basis to develop a high brightness highly stable laser pumped plasma light source.

The technical result of the invention consists in ensuring a high reliability of the plasma ignition sustained by the CW laser and on this basis creating an electrodeless high-brightness broadband light source with high spatial and power stability.

This object is achieved by the proposed laser pumped plasma light source comprising: a high pressure plenum chamber, at least a portion of which is optically transparent; a radiating plasma region maintained within the chamber by a focused beam of a Continuous Wave (CW) laser; at least one plasma radiation output beam (which may also be referred to as a utility beam) exiting the chamber; and a plasma ignition device.

The light source is characterized in that the plasma ignition device is a pulsed laser system generating a first and a second laser beam focused within the chamber, whereas said first laser beam is arranged for optical breakdown of the gas and said second laser beam is arranged for plasma ignition after optical breakdown.

In one embodiment of the present invention, the first laser beam has a peak radiation power greater than 104 watts and a pulse length less than 0.1 microseconds.

In one embodiment of the invention, the second laser beam has at least three times higher laser pulse energy and at least one order of magnitude lower laser peak power than the first laser beam.

In a preferred embodiment of the invention, the volume of the plasma that is multiply ignited by the second laser beam exceeds the volume of the plasma generated by the first laser during optical breakdown by an order of magnitude or more.

In a preferred embodiment of the invention, the volume and density of the plasma ignited by the second laser beam is sufficient to stably sustain the plasma by the focused beam of the CW laser.

In one embodiment of the invention, the second laser beam provides a plasma size (measured as FWHM of free electron density or FWHM of the brightness profile of the light-emitting plasma region) of up to about 1mm and up to 10 ¹⁸ cm ^-3 Or higher plasma density (measured as free electrons per unit volume).

In one embodiment of the present invention, the output power of the CW laser does not exceed 300 watts.

In one embodiment of the invention, the radiation pulse of the second laser beam ends no earlier than 50 μ s after the end of the radiation pulse of the first laser beam.

In a preferred embodiment of the invention, the focal areas of the first and second laser beams at least partially overlap or overlap.

In a preferred embodiment of the invention, the pulsed laser system comprises two lasers with a common cavity mirror, and the first and second laser beams are parallel and introduced into the chamber through a common focusing optical system.

In a preferred embodiment of the invention, the pulsed laser system is a solid state laser system.

In a preferred embodiment of the present invention, the pulsed laser system generates the first laser beam in a Q-switched mode or a giant pulse generation mode.

In a preferred embodiment of the invention, the pulsed laser system generates the second laser beam in a free-running mode.

In a preferred embodiment of the invention, only the CW laser has a fiber output.

In one embodiment of the invention, the wavelength of the CW laser is different from the wavelength of the radiation of the first and second laser beams.

In a preferred embodiment of the invention, the focused beam of the CW laser is directed vertically upward or near vertical.

In one embodiment of the invention, the outer and inner surfaces of the transparent portion of the chamber are shaped as concentric spheres or portions thereof, and the region from which the plasma is radiated is located in the center of the concentric spheres.

In a preferred embodiment of the invention, the output beam of plasma radiation exits the chamber at all azimuthal angles.

In one embodiment of the invention, the output beam of plasma radiation leaves the chamber at a solid angle of not less than 9 sr.

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

In another aspect, the present invention relates to a method for igniting a plasma in a laser pumped plasma light source, comprising: directing the focused beam of the CW laser into a chamber having a high pressure gas, initiating plasma ignition and stably sustaining a radiating plasma through the focused beam of the CW laser.

The method is characterized in that plasma ignition is provided by a pulsed laser system that generates first and second laser beams focused within the chamber, with the first laser beam being used to provide optical breakdown, after which the second laser beam is used to ignite a plasma with a volume and density sufficient to maintain a stable plasma by the focused beam of the CW laser.

In a preferred embodiment of the invention, the pulsed laser system is a solid state laser system that generates the first laser beam in a Q-switched mode and the second laser beam in a free running mode.

Designing the light source in the proposed manner allows a reliable ignition of the COD by selecting a suitable energy, duration and pulse power of the first and second laser beams for the following reasons. The first laser beam provides a reliable optical breakdown. However, COD ignition using only one laser beam is unstable and problematic. One of the reasons is that it is difficult to overlap the CW laser focusing region with the optical breakdown region, which is generally very small in size and does not exceed a value of about 50 μm. Even if the focal regions of the pulsed and CW laser beams overlap, COD ignition using only one laser beam remains challenging. This is because the optical breakdown generated by the laser radiation is explosive. The explosion process (especially shock waves) can result in suppression of the optical discharge sustained by a CW laser of low power (typically no more than 300 watts). According to the present invention, this problem is solved by providing plasma ignition after optical breakdown using a second pulsed laser beam. In this case, the pulsed light discharge maintained by the second laser beam is itself free of explosion phenomena, and the plasma ignited by the second laser beam is resistant to interference caused by optical breakdown. At the same time, the second laser beam ensures that the plasma volume and density are sufficient to maintain a reliable stable plasma by a focused beam of a CW laser with a small output power. This achieves reliable COD ignition.

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

Drawings

The nature of the invention is explained by the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a laser pumped plasma light source with a pulsed laser system for plasma ignition according to the present invention;

FIG. 2 is a graph of the radiation power of a laser beam according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of a laser pumped plasma light source with a solid state laser system for plasma ignition;

FIG. 4 is a schematic diagram of a light source with a three channel output for useful plasma radiation;

fig. 5 is a graph of the radiation power of a laser beam according to an embodiment of the present invention.

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

These drawings do not cover and limit the full scope of options for implementing the solution, but are merely illustrative examples of specific cases of its implementation.

Detailed Description

This description is intended to explain how to implement the invention rather than to show its scope.

According to the embodiment of the invention shown in fig. 1, the laser pumped plasma light source comprises a high pressure gas filled chamber 1, typically 10atm or higher. At least a portion of the chamber 1 is optically transparent. Fig. 1 shows an embodiment with a completely transparent chamber made of an optically transparent material (e.g., fused silica). The chamber 1 contains a radiating plasma region 2 maintained in the chamber by a focused beam 3 of a CW laser 4. At least one output beam (or useful beam) directed to the condenser 6 and intended for the plasma radiation 5 used subsequently leaves the chamber 1. The condenser 6 forms a radiation beam 7 which is transmitted, for example via optical fibres and/or mirror systems, to one or more optical consumer systems 8 which use the broadband radiation emitted by the plasma.

Concentrators are described in more detail in U.S. patent No. 9357627, published 5/31/2016, which is incorporated herein by reference in its entirety.

The light source further comprises a plasma ignition device. The light source is characterized in that the plasma ignition means is a pulsed laser system 9 which generates a first laser beam 10 and a second laser beam 11 focused in the chamber 1, i.e. into the region for sustaining the radiating plasma 2. The first laser beam 10 is used to initiate plasma ignition or optical breakdown in the chamber 1. The second laser beam 11 is used for plasma ignition after the optical breakdown provided by the first laser beam 10.

By selecting appropriate energies, durations and corresponding pulse powers of the two laser beams, designing the light source in the proposed manner achieves reliable ignition of the continuous light discharge. This allows the creation of electrodeless high brightness broadband laser pumped plasma light sources characterized by the highest possible spatial and energy stability.

The absence of electrodes simplifies the high pressure chamber design, improves chamber strength and reliability, and in a preferred embodiment of the invention ensures that the output beam 5 of plasma radiation leaves the chamber at a planar angle of 360 ° or at all azimuthal angles, see fig. 1. This means that in an azimuthal plane perpendicular to the axis of the beam 3 of the CW laser and passing through the region where the plasma 2 is radiated, the output beam of plasma radiation exits the chamber at all azimuthal angles from 0 ° to 360 °. Furthermore, in a preferred embodiment of the invention, the output beam 5 of the plasma radiation has an opening angle (flat angle with respect to the plane of the drawing in fig. 1) of not less than 90 °, and accordingly the plasma radiation is condensed by the condenser 6 over a solid angle of 9sr or more. Preferably, a low-cost near-infrared diode laser having an optical fiber output is used as the CW laser 4. In this case, at the exit of the optical fiber 12, the expanded laser beam is guided to a collimator 13, for example, in the form of a condenser lens. After the collimator 13, the expanded parallel beam 14 of CW laser light is directed to a focusing optical element 15, for example in the form of an aspherical condenser lens. The focusing optics 15 ensure a clear focusing of the beam 3 of the CW laser 4 required to achieve high brightness of the light source.

In an embodiment of the invention the power of the CW laser 4 does not exceed 300 watts, which is sufficient for a wide range of applications, but not sufficient to ignite a continuous light discharge without a special plasma ignition device.

In an embodiment of the invention, the pulsed laser system 9 comprises a first laser 16 for generating the first laser beam 10 and a second laser 17 for generating the second laser beam 11, see fig. 1. For example, an optical element in the form of a condenser lens may be used to focus the first and second laser beams, but is not limited to this option.

The characteristic time dependence of the radiation power in the first and

second laser beams

10, 11 and the beam of the CW laser 4 is schematically shown on a logarithmic scale in fig. 2.

Preferably, in order to ensure reliable initiation of plasma ignition or optical breakdown, the first laser beam 10 is characterized by a height (at least 10 a) ⁴ W) pulse radiation power. In this case, it is sufficient that the full width of the laser pulse at half maximum does not exceed 0.1. Mu.s.

According to the invention, the pulse power of the second laser beam is many times lower than that of the first laser beam, e.g. 10 ³ Watts, laser pulse length and energy are many times higher. This allows the second laser beam to be used to generate a plasma volume many times (an order of magnitude or more) larger than the plasma volume generated by the first laser beam after exposure to the first laser beam. At the same time, the radiation power in the second laser beam is more than an order of magnitude higher than the CW laser power, see fig. 2.

The second laser beam is used to generate a plasma of sufficient volume and density for stable plasma maintenance by the focused beam of the CW laser.

In an embodiment of the invention, the generation of the second laser beam starts before the generation of the first laser beam and ends no earlier than 50 μ s after the end of the first laser pulse, see fig. 2. On the one hand, the first and second laser beams are made easier to synchronize, and on the other hand, sufficient time is provided for plasma evolution under the influence of the second laser beam. As a result, large plasma volumes of up to about 1mm and up to 10 are provided ¹⁸ cm ^-3 Is sufficient for reliable stable plasma sustaining by the focused CW laser beam. 10 ¹⁸ cm ^-3 Corresponds to a gas temperature of 18,000k, with 10% ionization in the radiating plasma region at an initial gas pressure in the chamber of about 16 atm.

In one embodiment thereof, the laser pumped plasma light source operates as follows. Directing a focused beam 3 of CW laser light 4 at least partiallyA transparent high-pressure gas-filled chamber 1, see fig. 1. Xenon, other inert gases, and mixtures thereof, including metal vapors (e.g., mercury) and/or various gas mixtures, including gas halides, may be contained in the chamber as a high efficiency plasma fuel. The focused second laser beam 11 of the second laser 17 is directed into the region for sustaining the radiating plasma 2. In an example of embodiment of the present invention, the maximum radiation power in the second laser beam 11 may have about 10 ³ The value of Watt, and the laser pulse length may be about 10 ^-4 And s. During the radiation pulses of the second laser beam 11, a first laser beam 10 is generated, the focal area of which at least partially overlaps the focal area of the second laser beam. Short, less than 0.1 mus, high power, approximately 10 of first laser 16 ⁴ A radiation pulse of watts or more (with an energy of about a few mJ) is used to provide initial local gas ionization for optical breakdown in a small volume with a feature size of 50 to 100 μm. The energy and laser pulse length of the second laser beam 11, which is used at the laser radiation power (about 10), are many times higher than those of the first laser beam 10 ³ W or more) which is many times higher than the radiation power in the beam 3 of the CW laser. In the case where the light discharge is sustained by the second laser beam 11 having a pulse length of about 100 μ s or more, the plasma volume increases due to its movement along the focal divergence toward the laser beam 11 and its radial expansion. Thus, plasma sizes up to 1mm can be achieved. Due to the sufficiently high (about 0.1J/pulse or higher) radiation pulse energy of second laser beam 11, in a larger plasma volume, an electron density level is provided that is sufficient to reliably sustain a radiating plasma through focused beam 3 of CW laser 4 having a relatively small power of no more than 300 watts. Thus, the second laser beam provides a plasma density above a value of about 10 ¹⁸ Electron/cm ³ Or higher threshold plasma density of a continuous light discharge. In the stable mode, broadband radiation is output from the radiating plasma region 2 by at least one output beam 5 of plasma radiation which exits through an optically transparent portion of the chamber 1 and is used for subsequent use.

The light source is designed as described above such that a reliable ignition of the continuous light discharge can be achieved without the use of an ignition electrode. This allows to significantly improve the design of the chamber, increasing the reliability and lifetime of the light source, by simplifying its shape and eliminating mechanical stresses in the point where the metal seal is introduced into the chamber. The design simplification allows the use of chamber shapes that reduce aberrations introduced into the output beam of plasma radiation exiting the chamber, thereby increasing light source brightness. Furthermore, the possibility of using chamber materials with a higher transparency in the UV spectral range is provided. Electromagnetic noise when starting the light source is reduced. Because the metallization of its optically transparent portions is eliminated, the chamber life is increased. Furthermore, the absence of electrodes allows to significantly increase the spatial angle of the radiation output and to increase the power of the output beam of the plasma radiation. At the same time, the elimination of the ignition electrode significantly reduces convective turbulence within the chamber, thereby significantly increasing the spatial and power stability of the laser pumped plasma light source. A further improvement of the stability is achieved due to the possibility of optimizing the size of the electrodeless chamber. Generally, an increase in the brightness and stability of the light source is achieved, the possibility of increasing its light output in the UV range is achieved, the reliability and lifetime are increased, the convenience of its operation is improved, and the operating costs are reduced.

The above possibilities are most easily implemented in light sources where the pulsed laser system 9 is solid-state, see fig. 3. In this embodiment of the invention the pulsed laser system 9 comprises two optically pumped

solid state lasers

16, 17. For example,

flash lamps

18, 19 with reflectors may be used as optical pumping sources. The lamps are turned on with a delay that is optimized with respect to each other. Rods made of a transparent substrate, for example, yttrium Aluminum Garnet (YAG) doped with metal ions such as neodymium (Nd), may be used as the

active elements

20, 21. The first and

second laser beams

10, 11 are preferably parallel and are introduced into the chamber 1 via one common focusing optical system 22 (e.g. in the form of an aspherical condenser lens). To ensure that the

laser beams

10, 11 are parallel, the first and second solid-

state lasers

16, 17 preferably have a

common cavity mirror

23, 24. This provides the overlap of the focal areas of the first and

second laser beams

10, 11 required for plasma ignition.

In a preferred embodiment of the present invention, the pulsed laser system 9 generates the first laser beam 10 in a Q-switched mode or a giant pulse generation mode and the second laser beam 11 in a free-running mode. To achieve the Q-switch mode, the first laser is equipped with a Q-switch 25, e.g. a passive Q-switch made of a phototropic material. In another embodiment of the invention, an active Q-switch may be used.

During the generation of the giant pulse, an excessively high radiation power of the pulsed laser system 9 does not allow the use of an optical fiber for transmitting its radiation, since the optical fiber may be damaged. Thus, in an embodiment of the invention, only the CW laser is provided with a fiber output, see fig. 1, 3.

Preferably, the first and

second lasers

10, 11 have the same wavelength of radiation, e.g., λ ₁ ＝λ ₂ =1.064 μm, different from CW laser λ _CW Of wavelength, e.g. λ _CW =0.808 μm or 0.976 μm: lambda [ alpha ] _CW ≠λ ₁ ＝λ ₂ . This allows the expanded beam 14 of CW laser light to be directed to the chamber using a dichroic mirror 26, see fig. 3.

In order to facilitate optical alignment and to improve the light source configuration, an additional deflection mirror 27 (see fig. 3) or several such mirrors may be used therein.

In embodiments of the present invention, additional optical elements (not shown) may be installed in the path of the CW laser beam 14, or in the pulsed laser system 9, to counteract chromatic aberration and more accurately align the focal regions of the CW and pulsed laser beams. In pulsed laser systems, in particular, within the chamber formed by the

mirrors

23, 24, additional optical elements, for example polarizers, filters, diaphragms, may be installed to control the parameters of the first and second laser beams.

In the preferred embodiment of the present invention, the axis of the CW laser focused beam 3 is vertically up, i.e., against gravity 28, see fig. 3, or nearly vertical. The proposed design achieves the highest stability of the radiation power of the light source. This is because the area irradiating the plasma 2 is usually moved slightly from the focal point towards the focused beam 3 of the CW laser until the intensity of the focused beam 3 of the CW laser is still sufficient to maintain the focused laser beam cross-section irradiating the plasma area 2. When the focused beam 3 of the CW laser is directed from the bottom upwards, the radiating plasma region 2 containing the hottest plasma with the lowest mass density tends to float under the influence of buoyancy. The rising region of the radiating plasma 2 ends up in the position closest to the focal point, where the cross-section of the focused beam 3 of CW laser light is smaller and the intensity of the laser radiation is higher. On the one hand, this increases the plasma radiance and, on the other hand, this equalizes the forces acting on the radiating plasma region, which ensures a high stability of the radiation power of the high-brightness laser pumped plasma light source.

To achieve these positive effects, the chamber 1 must preferably be axisymmetric and the axis of the focused beam 3 of the CW laser must be aligned with the symmetry axis of the chamber.

In addition to providing highly stable output parameters, the present invention also enables the possibility of achieving the highest brightness of laser pumped broadband light sources, in particular by optimizing the shape and size of the electrodeless chamber. Accordingly, in a preferred embodiment of the invention, the outer and inner surfaces of the chamber or the transparent part thereof are shaped as concentric spheres, the region of the radiating plasma 2 being located in the center of said concentric spheres, see fig. 3. In this embodiment of the invention, aberrations introduced by the chamber walls are eliminated, so that a sharper focusing of the beam 3 of the CW laser light can be achieved and the source brightness is increased. Furthermore, aberrations in the path of the twisted rays in the useful plasma radiation beam 5 are eliminated, increasing its brightness.

Another positive result of the invention is the possibility to minimize the size of the chamber. This increases the clear focusing of the CW laser beam 3 by moving the focusing optical system 22 closer to the region where the plasma 2 is irradiated. Furthermore, the closer the zone of the radiating plasma is to the walls of the chamber 1, in particular to the top wall of the chamber, the smaller the pulses obtained by the gas heated in the zone of the radiating plasma 2, under the effect of buoyancy. Thus, the smaller the velocity and turbulence of the gas convection, the smaller the distance from the plasma to the chamber wall. Thus, the possibility is provided to further increase the brightness and stability of a laser pumped plasma light source designed according to the invention.

In order to ensure a plasma radiation output in a broad spectral range from ultraviolet to near-infrared, the optically transparent part of the chamber is preferably made of: crystalline magnesium fluoride (MgF) ₂ ) Crystalline calcium fluoride (CaF) ₂ ) Crystalline sapphire or leuco sapphire (Al) ₂ O ₃ ) Fused silica or crystalline silica.

In an embodiment of the invention the chamber ensures that the output beam of plasma radiation 5 leaves the chamber at a plane angle of 2 pi radians, but is not limited to the options in fig. 1, 3.

In another embodiment of the invention, the light source may have at least three diverging

output beams

5a, 5b, 5c of plasma radiation, as shown in fig. 4, fig. 4 showing a cross-section of the light source in a horizontal plane through the area where the plasma 2 is radiated. The laser beam used for COD ignition and maintenance in fig. 4 is located below the plane of the figure. In various industrial applications, it is desirable to use several beams (in particular three beams) of plasma radiation from a single light source. In this embodiment of the invention the chamber 1 of the laser pumped plasma light source is mounted in a housing 29, which is equipped with three

light concentrators

6a, 6b, 6c. The

condensers

6a, 6b, 6c form a plasma radiation beam 7a, 7b, 7c which is transmitted, for example via an optical fiber, to an

optical consumer system

8a, 8b, 8c using broadband plasma radiation. This allows one light source to be used for three or more optical consumer systems, resulting in a compact size of the system and the same parameters of broadband radiation in all optical channels.

The plasma ignition method in the laser pumped plasma light source shown in fig. 1, 3 according to the present invention is as follows. A focused beam 3 of CW laser light 4 is directed into a high pressure gas-filled chamber 1, typically 10atm or more. Plasma ignition is provided by a pulsed laser system 9 which generates first and

second laser beams

10, 11 which are focused in the chamber. The first laser beam 10 is arranged to provide an optical breakdown, after which the second laser beam 11 is used to ignite a plasma, of sufficient volume and density to maintain a stable plasma by the focused beam 3 of the CW laser 4.

In a preferred embodiment of the invention, a solid state laser system is used which generates a first laser beam 10 in a Q-switched mode and a second laser beam 11 in a free running mode, see fig. 3. The pulsed laser system 9 preferably comprises two solid-

state lasers

16, 17, for example Nd: YAG lasers with

optical pump sources

18, 19 in the form of flash lamps. The first and

second laser beams

10, 11 are preferably parallel and are introduced into the chamber 1 via a focusing optical system 22. In order to superimpose the focal areas of the first and

second laser beams

10, 11, the solid-

state lasers

16, 17 preferably have a

common mirror

23, 24 of the cavity. The first laser 16 is provided with a Q-switch 25.

In an example of embodiment of the present invention, the Xe gas pressure within the chamber is 30atm. The first laser 16 emits in Q-switched mode a pulse energy of 3mJ, a pulse duration of 20ns and a laser wavelength of λ ₁ =1.064 μm. The optical breakdown plasma has a feature size of 50 to 100 μm. The optical breakdown mode does not provide reliable ignition of the optical discharge maintained by the focused beam 3 of the CW laser 4. Thus, after optical breakdown, the second laser beam is used to ignite a plasma, the volume of which (up to 1 mm) ³ ) And density (over 10) ¹⁸ cm ^-3 ) It is sufficient to maintain a stable plasma by the focused beam 3 of the CW laser 4. In an example of embodiment of the present invention, the energy of the second laser beam is 150mJ, the pulse length is 100 μ s, and the laser wavelength is λ ₂ ＝1.064μm。

Preferably, the radiation pulse of the second laser beam ends no earlier than 50 μ s after the end of the radiation pulse of the first laser beam, as shown in fig. 2. A time of at least 50 mus is required to allow for attenuation of interference from optical breakdown and to allow the plasma size and density to develop to a value sufficient to maintain stable plasma by the focused beam of the CW laser.

The generation of the second laser beam may be started before the first laser pulse, see fig. 2. Meanwhile, the present invention is not limited to these examples. As shown by the study, COD ignition is also provided when the second laser beam has a delay of up to ten seconds or more after the generation of the first laser beam, as shown in fig. 5. This mechanism of plasma ignition may be related to the effect of the giant pulse on the chamber walls when long-lived clusters or solid particles are generated due to this effect.

In general, the invention allows to ensure a high reliability of the laser-ignited laser-sustained plasma and on this basis create a high-brightness broadband light source with the highest spatial and power stability.

INDUSTRIAL APPLICABILITY

The high-brightness high-stability laser pumping plasma light source designed according to the invention can be used for various projection systems, and is used for spectrochemical analysis, spectral microanalysis of biological objects in biology and medicine, micro-capillary liquid chromatography, optical lithography process inspection, spectrophotometry and other purposes.

Claims

1. A laser pumped plasma light source, comprising: a gas-filled chamber, at least a portion of which is optically transparent; a radiating plasma region maintained within the chamber by a focused beam of a Continuous Wave (CW) laser; at least one output beam of plasma radiation exiting the chamber; a plasma ignition device is characterized in that,

the plasma ignition device is a pulsed laser system that generates first and second laser beams focused within a chamber, and

the first laser beam is arranged for gas optical breakdown, and

the second laser beam is arranged for plasma ignition after optical breakdown.

2. The light source of claim 1, wherein the first laser beam has a power greater than 10 ⁴ Peak radiant power in watts and pulse length less than 0.1 mus.

3. The light source of claim 1 or 2, wherein the second laser beam has at least three times higher laser pulse energy and at least one order of magnitude lower laser peak power than the first laser beam.

4. A light source as claimed in any one of the preceding claims wherein the volume of plasma multiply ignited by the second laser beam exceeds the volume of plasma generated by the first laser during optical breakdown by an order of magnitude or more.

5. A light source as claimed in any one of the preceding claims wherein the volume and density of the plasma ignited by the second laser beam is sufficient for stable maintenance of the plasma by the focused beam of the CW laser.

6. A light source as claimed in any preceding claim, wherein the second laser beam provides a plasma size of up to about 1mm and up to 10 ¹⁸ cm ^-3 Or higher plasma density.

7. A light source as claimed in any preceding claim wherein the output power of the CW laser does not exceed 300 watts.

8. A light source as claimed in any one of the preceding claims wherein the radiation pulses of the second laser beam end no earlier than 50 μ β after the end of the radiation pulses of the first laser beam.

9. A light source as claimed in any one of the preceding claims wherein the focal regions of the first and second laser beams at least partially overlap.

10. A light source as claimed in any preceding claim, wherein the pulsed laser system comprises two lasers with a common cavity mirror, and wherein the first and second laser beams are parallel and introduced into the chamber through one common focusing optical system.

11. A light source as claimed in any one of the preceding claims, wherein the pulsed laser system is a solid state laser system.

12. A light source according to any one of the preceding claims, wherein the pulsed laser system generates the first laser beam in a Q-switched mode or a giant pulse generation mode.

13. A light source as claimed in any one of the preceding claims, wherein the pulsed laser system generates the second laser beam in a free-running mode.

14. A light source as claimed in any preceding claim wherein only the CW laser has a fibre output.

15. A light source as claimed in any preceding claim, wherein the wavelength of the CW laser is different to the wavelength of the radiation of the first and second laser beams.

16. A light source as claimed in any preceding claim, wherein the axis of the focused beam of the CW laser is vertically up or near vertical.

17. A light source as claimed in any one of the preceding claims, wherein the outer and inner surfaces of the transparent portion of the chamber are shaped as concentric spheres or portions thereof, and the region from which plasma is radiated is located in the centre of the concentric spheres.

18. A light source as claimed in any preceding claim, wherein the output beam of plasma radiation exits the chamber at all azimuthal angles.

19. A light source as claimed in any preceding claim, wherein the output beam of plasma radiation exits the chamber at a solid angle of not less than 9 sr.

20. A light source as claimed in any one of the preceding claims having three or more plasma radiation output beams.

21. A method for igniting a plasma in a laser pumped plasma light source, comprising: directing a focused beam of a CW laser into a chamber having a high pressure gas, plasma igniting and stably sustaining a radiating plasma through the focused beam of the CW laser, wherein,

the plasma ignition is provided by a pulsed laser system that generates first and second laser beams that are focused within a chamber

The first laser beam is used to provide optical breakdown, after which the second laser beam is used to ignite a plasma, with a volume and density sufficient for stable plasma maintenance by the focused beam of the CW laser.

22. The method of claim 21, wherein the pulsed laser system is a solid state laser system that generates a first laser beam in a Q-switched mode and a second laser beam in a free-running mode.