WO2016148608A1

WO2016148608A1 - Source of broadband optical radiation with high brightness

Info

Publication number: WO2016148608A1
Application number: PCT/RU2016/000135
Authority: WO
Inventors: Игорь Георгиевич РУДОЙ; Николай Германович СОЛОВЬЕВ; Аркадий Матвеевич СОРОКА; Михаил Юрьевич ЯКИМОВ
Original assignee: Игорь Георгиевич РУДОЙ; Николай Германович СОЛОВЬЕВ; Аркадий Матвеевич СОРОКА; Михаил Юрьевич ЯКИМОВ
Priority date: 2015-03-16
Filing date: 2016-03-11
Publication date: 2016-09-22

Abstract

The invention relates to sources of broadband optical radiation with high spectral brightness, and is of interest for use in the fields of microelectronics, spectroscopy and photochemistry, amongst others. The technical result of the claimed technical solution is an increase in spectral brightness, a decrease in fluctuations in plasma position and a stabilization of the brightness of the plasma radiation, as well as an increase in the working life of the source. This technical result is achieved in that the present source of broadband radiation with high brightness comprises a chamber which is filled with a high-pressure gaseous medium, and two lasers which irradiate the chamber and which have two radiation focusing systems with substantially overlapping focal regions and an angle of at least 60° between the direction of radiation of the lasers, wherein at least one of the lasers is a repetitively pulsed laser.

Description

SOURCE OF A WIDEBAND OPTICAL

HIGH BRIGHT EMISSIONS

The claimed technical solution relates to sources of broadband optical radiation with high spectral brightness and is of interest for applications in microelectronics, spectroscopy, photochemistry, medicine and other fields.

A source of high-brightness broadband optical radiation is known, which is a sealed chamber excited by an arc discharge, filled with high-pressure gas. The chamber is a transparent flask (lamp) made of quartz glass; xenon (a mixture of xenon with mercury) at a pressure of ~ 1 MPa is used as a filling gas. Arc discharge electrodes are placed in a lamp, the interelectrode gap is several millimeters, for special-purpose lamps even 0.5-1.5 m ([1]: G. N. Rokhlin “Discharge light sources.” 2nd ed., Revised. and add. — M.: Energoatom Publishing House, 1991–720 s; section 19.3). Such lamps are serially produced by many manufacturers, in particular, K.K. Hamamatsu Photonics. (Japan), a description of the respective lamps is presented on the company's website (see .. for example [2]: http://www.hamamatsu.com/resources/pdf/etd/Xe-HgXe TLSX 1044E.pdf). Known sources generate radiation with a continuous spectrum in the range from -180-220 nm to> 1 000 nm (the lower boundary of the spectrum is determined by the transparency boundary of the material used for the lamp bulb) at sufficiently high stability (better than 1%) and integrated radiation brightness. However, the continuous operation resource of such sources is limited and is determined by the degradation of the electrodes themselves in a high-current arc discharge, as well as by the deposition of erosion products of the electrodes on the inner surface of the lamp, which reduces its transparency. As a result, the guaranteed source life is typically up to -3000 hours, which is not enough for many applications. In addition, with a high total spectral intensity of light {in units of W / (nm * sr * mm)}, the spectral brightness of a known source {in units of W / (nm * sr * mm2)} is insufficient, in particular for applications in microelectronics, since the illumination of an object is determined namely, the brightness of a unit surface of a radiation source.

A well-known source of broadband optical radiation with high spectral brightness, including filled with a high pressure gas medium

SUBSTITUTE SHEET (RULE 26) a camera irradiating the camera with a laser; a system for focusing laser radiation into a camera ([3]: US Pat. No. 7,435,982, "Laser-driven light source"). In fact, the well-known source is one of the options for realizing the phenomenon of continuous optical discharge, discovered in 1970 in the USSR ([4]: Generalov N.A., Zimakov V.P. et al. “Continuously burning optical discharge.” Letters in JETP 1970, v. 11, pp. 447-449). Sources based on such an optical discharge are produced, in particular, by Energetiq Technology, Inc. (USA), they are described in detail on the website of this company ([5]: htt p: // www. En erge ti q. Com, ⁷ ), namely EQ-99 (in versions X, XFC, CAL), as well as EQ-1500 et al. Well-known sources include a camera in the form of a flask transparent to ultraviolet radiation, filled, as a rule, with xenon with a pressure of up to ~ 3 MPa (pressure is indicated at room temperature), a laser with a power of ~ 20 W to -300 W ( as a rule, a diode laser with a wavelength of ~ 1 μm), a system for focusing laser radiation into a chamber with a gas with a sufficiently large numerical aperture NA up to 0.45-0.50. In the gas-filled chamber, electrodes are additionally placed for preliminary ionization of the gas by an arc discharge or pulsed electrical breakdown, after which the plasma is supported by focused laser radiation even in the absence of electric current and voltage on the electrodes.

Important advantages of the known source are the electrodeless method of supplying energy to the plasma (with the exception of the moment of its initiation), as well as the compactness and rather stable position of the broadband radiation source. The absence of any noticeable erosion of the electrodes can significantly increase the life of the radiation source — up to> 9 thousand hours or more, as indicated in the product specifications of Energetiq Technology, Inc., when the life is apparently determined by the degradation of the transparent walls of the flask under the influence of a short-wavelength laser plasma radiation. Further, a known source has a significantly higher spectral brightness than arc lamps: a gain in brightness compared to a xenon lamp with a comparable power consumption of to 10 times in the far ultraviolet range of 190-250 nm and up to 2-3 times in the spectral range 300-700 nm. However, the spectral brightness of a known source is not maximum, and it is important to note that its brightness increases very slowly as the power of the laser used increases, since together with an increase in the laser power, the volume of the emitting plasma also increases. For example, if the laser power is increased from 20 W (source EQ-99) to 60 W (source EQ-1500), the size of the emitting plasma

SUBSTITUTE SHEET (RULE 26) the level of 50% of the maximum brightness increases from 060 microns x 140 microns to 0125 microns x 300 microns, that is, the plasma volume increases by 9 times. This means that the power of energy release per unit volume of the plasma decreases with increasing laser power, as does the maximum plasma temperature. That is, an increase in the spectral brightness of the source is achieved in an inefficient way — by increasing the optical thickness of the plasma, which is mainly transparent to intrinsic thermal radiation, and the spectral distribution of the radiation corresponds to a lower plasma temperature.

The slow increase in the brightness of laser plasma with increasing laser power in a known technical solution, the authors of the present invention associate with the refraction of laser radiation in a heated gas: with an increase in the power of laser radiation, the heat generation in the focal region also increases. As a result, the size and optical power of the “scattering thermal lens” that arises in the region of the emitting plasma and around this region increases, which worsens the conditions for focusing laser radiation.

Further, in the opinion of the authors of the claimed technical solution, an increase in the plasma size (in particular, due to thermal conductivity) with an increase in the power of the laser supporting the plasma leads to an increase in the absorption of laser radiation at a greater distance from the focal region - this is especially important when using diode lasers, when the absorption of the laser radiation occurs from excited levels of xenon (or another inert gas). Accordingly, an increase in temperature at the periphery of the focal region leads to an increase in the population of the corresponding atomic states and absorption coefficients - as a result, the laser radiation intensity directly in the focal region can even decrease rather than increase with increasing laser power.

Changes in the power of laser radiation in a known source lead to variations not only in the brightness of the plasma radiation per se, but also in the position of the region of the laser plasma with maximum brightness, which further increases the instability of the radiation — both the integral and spectral brightness of the known source, especially for significant time intervals.

In addition, for focusing laser radiation in a known source, complex (and expensive) aspherical optics with a large numerical aperture are used, which partially reduces the effect of refraction. As a result, the size of the laser beam on the surface of the lamp bulb is large enough and aberrations on the inhomogeneities of the walls of the bulb itself further impair the quality of focusing the laser radiation. Further,

SUBSTITUTE SHEET (RULE 26) since the short-focusing focusing system occupies a significant solid angle in the space surrounding the plasma, the fraction of plasma radiation that can be removed and used is reduced in a known source.

Similarly, the minimum increase in plasma brightness (and even a decrease in its maximum temperature) with increasing laser radiation power is manifested in broadband radiation sources based on a laser-supported (laser wavelength ~ 1 μm) power up to 1 kW of compact plasma in lamps produced by high-pressure xenon KLA-Tencor Corporation ([6]: https: // w \ ^ v.research: ate.net/publication/277130938 High Power Laser-Sustained Plasma Light Sources for KLA-Tencor Broadband Inspection Tools). This source also uses a complex focusing system ([6]) and, as one of its simplification options, in the patent ([7]: US 7,705,331: “METHODS AND SYSTEMS FOR PROVIDING ILLUMINATION OF A SPECIMEN FOR A PROCESS PERFORMED ON THE SPECIMEN) )) it is proposed to use laser plasma excitation simultaneously from several directions, for example, from two mutually perpendicular directions (paragraphs 68-69 of the claims, in particular paragraph 68) with substantially coincident focal regions. In this case, each individually focusing system can be made much simpler (see also below). We especially note that in a known source, the maintenance of a plasma by continuous (cw) laser radiation is considered optimal. We also point out that a well-known source [6, 7] uses an electrodeless lamp with a high-pressure gas, and plasma is initiated from the outside with respect to the lamp.

The technical result of the claimed invention is to increase the spectral brightness, reduce fluctuations in the position of the plasma and stabilize the brightness of its radiation, as well as increase the resource of the source of broadband optical radiation.

The technical result is achieved in that the high-brightness broadband radiation source is a chamber filled with a high-pressure gas medium, two irradiating laser cameras with two radiation focusing systems with a substantially coincident focal region and an angle between the laser radiation direction of at least 60 °, and at least at least one laser is a repetitively pulsed laser.

In a preferred embodiment of the proposed technical solution, one

SUBSTITUTE SHEET (RULE 26) The laser is a cw laser, and the second is a pulsed-periodic laser. The authors of this technical solution unexpectedly found that the combination of a cw laser with a power of P i and a pulsed-periodic laser with a power (in pulse) of P2 makes it possible to generate a significantly brighter plasma than when using two continuous lasers with a power of Pi and 2 2 ^or ( ^moreover ) a single cw laser with a power of (Pj + P2), and the increase in pulsed brightness can be multiple. This is probably due to the fact that the time-average energy input to the plasma in the source proposed by the authors is significantly lower, since the average power of a repetitively pulsed laser is a factor of one times smaller than its pulse value (for close-to-rectangular laser pulses), which reduces the negative effect of laser refraction radiation on the plasma and the surrounding hot gas, as well as unwanted absorption of laser radiation at the periphery of the emitting region of the plasma source. As a result, the radiation of a repetitively pulsed laser with greater efficiency reaches the focal region of the focusing system, providing greater brightness of the plasma. This also apparently explains the greater stability of the position of the region of maximum plasma brightness in the technical solution proposed by the authors of this application.

Similarly, when a plasma is excited by focused radiation of two lasers with substantially coincident foci, the region of high brightness of such an optical discharge (for example, at a level of 50% of the maximum brightness) is concentrated near the intersection of the focal regions of each of the rays and can be significantly smaller than the region occupied by the plasma for each of the laser beams individually. As a result, for a sufficiently large angle Θ between the direction of the optical axes of each of the laser beams, namely, for Θ> 60 °, the position stability of the region of the optical discharge with maximum brightness is further increased, the bright region of the “joint” plasma is much smaller than the size of the bright plasma region generated each of the lasers used individually, and the brightness of the radiation of the optical discharge plasma ΙΣ significantly exceeds the arithmetic sum of the plasma brightness I] + 12, where 1 \, 12 is the plasma brightness in the case of bots of only one laser (respectively, the first or second)

In the framework of this technical solution, the angle between the direction of laser radiation Θ is the smaller of the angles between the corresponding optical axes, as

SUBSTITUTE SHEET (RULE 26) indicated in FIG. 1. In a preferred embodiment of the proposed technical solution, the angle θ is about 90 ° —in this case, at a fixed laser power, the brightness of the plasma radiation is maximum, and its position is the most stable.

In a preferred embodiment of the claimed technical solution, a continuous laser maintains an optical discharge at a level near the threshold (in the generally accepted sense of the term — near the threshold for maintaining an optical discharge, the fraction of laser radiation energy absorbed by a plasma is small, see for example [8]: Yu. P. Raiser “Gas Physics” discharge. ”M, Nauka, 1987–592 pp.) with a minimum heat release in a gas, a minimum plasma size and a small power emitted by a plasma, respectively, minimal refractive distortions and minimal n by absorption at the periphery of the plasma bunch and a repetitively pulsed laser generates a repetitively pulsed laser plasma, which at the same time exhibits maximum brightness with a minimum size. In this case, the plasma brightness is determined by the pulsed power of the Pim laser, and the defocusing of the radiation due to refraction and absorption at the plasma periphery are determined by the average power of the repetitively pulsed laser Pav. For the preferred mode of generation of essentially rectangular laser pulses, the ratio between Pav and Pim is determined by their duty cycle G: G = Pim / Pav and to significantly reduce refractive distortions and peripheral absorption losses it is advisable, as established by the authors, to use a pulse-periodic laser with a duty cycle> 2 .

In the common case of the use of diode lasers (similar to known sources), the use of a repetitively pulsed laser according to the claimed technical solution in some cases allows to further increase the pulsed power when using lasers with moderate average power. This is due to the fact that the limiting power of a cw diode laser is determined by the maximum permissible temperature in the generation region, and in the pulse-periodic mode for sufficiently short laser pulses, the temperature in the radiation generation region is determined to a large extent by the average radiation power (heating of the transition during a single laser pulse relatively small), the pulse power in this case can be much higher than average. In particular, at a pulse duty cycle of ~ 5 for the DLM-50 diode laser module manufactured by the NTO IRE-Polyus / 1PC Photonics company (http://www.iitoire-polus.ru/products low dlm.html) with a maximum power of

SUBSTITUTE SHEET (RULE 26) 50 W continuous operation mode, the authors of this technical solution implemented a stable pulse-periodic laser operation mode (and laser plasma generation) with a pulsed radiation power of P OW and a single laser pulse duration of 20 μs (laser pulse repetition rate 10 kHz). In this case, the average laser power was 22 W and the refractive effects in the plasma, as well as the absorption at its periphery, corresponded to approximately such an average power of a repetitively pulsed laser (possibly lower, but not higher in any case). For laser pulses of long duration (usually ~ 100 μs or more, when the laser transition temperature reaches a stationary value during a single pulse), a substantial increase in the pulsed power compared to the laser power in the continuous mode cannot be realized.

The use of a repetitively pulsed laser plasma with a sufficiently high pulse repetition rate instead of a continuous plasma, as in the known technical solutions, does not limit the use of the inventive source in the vast majority of applications, at least when the duration of an individual pulse of broadband radiation in the microsecond range. This is due to the fact that the sensitivity of modern optoelectronic receivers with a time resolution of 200–3 ° C is not (1 μs) no worse than the sensitivity of optoelectronic receivers of continuous (quasicontinuous) radiation. Accordingly, in the analysis of the surface (the most frequent application of the considered light sources in microelectronics) with the required level of light intensity on the surface, one pulse with a duration of ~ 1 μs is sufficient. At the same time, the irradiation of the surface can be turned off for the duration of the movement of the irradiating surface and the recording systems. For a pulse repetition rate of 10 kHz broadband radiation and an area irradiated per pulse of only 350 x 350 microns (with an extremely high brightness of the claimed source, this is a very real size), the surface analysis performance will be ~ 12 cm ² / s and, thus, a plate with a diameter of 300 mm can be examined in ~ 1 minute, while the surface scanning speed is 3.5 m / s.

As discovered and established by the authors of the claimed technical solution, already near the threshold of the optical discharge, when the plasma brightness and heat generation in it are low, the absorption coefficient of the laser radiation is sufficient to heat the plasma core, supported by a continuous near-threshold laser power, to

SUBSTITUTE SHEET (RULE 26) the maximum brightness is produced by the laser pulse rather quickly, during this time the plasma size does not noticeably increase, and the stability of the position of the bright core and the reproducibility of its brightness from pulse to pulse remains high up to a pulse repetition rate of 10 kHz and higher. In this case, the duration of an individual laser pulse of the second (pulse-periodic) laser is preferable to choose slightly more (for a scale time of 0.3-10 μs) of the duration at which the plasma is heated to the maximum brightness of its radiation, since, as mentioned above, the duration " a bright plasma flash of ~ 1 μs (or even less) is sufficient for extremely high sensitivity of optoelectronic registration systems, and further heating of the plasma with a laser pulse only leads to an increase in the average energy on and the plasma and, as a consequence, to increase the absorption and refractive distortions at the periphery of a bright plasma kernel.

Accordingly, in any case, it is preferable that the duration of an individual laser pulse does not exceed the time of formation of a stationary plasma, including its geometry, corresponding to the simultaneous irradiation of a plasma with two continuous lasers with a total power of (P] + P2). The time of formation of a stationary plasma depends on the laser power, gas pressure, and conditions for focusing radiation, usually amounting to 10-200 μs.

It is also important to indicate that the claimed technical solution allows you to vary the repetition rate of ultra-bright pulses of broadband radiation over a wide range from tens of kilohertz to 1 Hz or less without additional initiation of plasma before each individual radiation pulse. Changing the pulse repetition rate allows you to widely vary the average power of the claimed source without changing its spectral brightness (since it is determined by the pulse power), which is impossible in known sources and is useful for a number of applications.

At a sufficiently high repetition rate of laser pulses, as established by the authors, it is possible to maintain a stable pulse-periodic regime of plasma combustion in the case when both lasers used operate in a pulse-periodic mode. In this case, the minimum allowable pulse repetition rate is determined both by the duration of an individual laser pulse and by the time during which the plasma cooled in the absence of laser radiation maintains a sufficiently high absorption coefficient so that the plasma is rapidly heated

SUBSTITUTE SHEET (RULE 26) the next impulse (impulses) could be realized without external initiation. For chambers filled with heavy inert gas, this time is determined by the rate of formation and radiative decay of excimer molecules R2 * (R is an inert gas) at a temperature of the cooling plasma (for xenon 7-10 kK) and in xenon with a pressure of ~ 15 atm at room temperature 100- 200 μs. Thus, for a laser pulse duration of 100 μs, the minimum pulse repetition rate, when both lasers operate in a periodic periodic mode, is xenon with a “cold” pressure of -15 atm 3-5 kHz. With a higher pressure in the lamp, the rate of formation of excimer molecules increases and the pulse repetition rate of the laser pulses must be increased.

The fact that a bright laser plasma is located at the intersection of the focal regions of each of the lasers used, and it is in this (in the optimal case, small) region that a significant part of the laser power is absorbed, allows the use of simple focusing systems with a small numerical aperture, for example, with NA <0 , 2. As an output element of such a focusing system, for example, a telephoto lens with a ratio of the focal length F to the light diameter D F / D> 3 with flat and spherical optical surfaces can be used. This type of lens with NA <0.2 is significantly simpler than short-focus aspherical systems. In this case, for a diode laser with a characteristic beam size on the lens of 6-H O mm, it is possible to use a lens with F ~ 30 mm (or more). Then, when the diameter of the flask with a high-pressure gas medium is ~ 10 mm, the size of the laser beam on the flask wall does not exceed 1.5-2 mm, which minimizes the aberration associated with the flask walls, in particular, defocusing the beam on the inhomogeneities of the flask wall or flask thickness. As a result, when using two simple long-focusing focusing systems that provide a characteristic size of each prefocal region of ~ 0200 μm x 400 μm (when using one laser and significantly exceeding the plasma maintenance threshold, the laser plasma is usually located precisely in the prefocal region to the “point” of focus), for the angle between the direction of the laser beams ~ 90 ° with the appropriate settings, the size of the bright plasma is realized - 0150 microns x 150 microns or less. The use of long-focusing focusing systems makes it possible not only to simplify and reduce the cost of the optical system of the source, but also to increase the solid angle at which laser radiation can be collected and used in applications (the laser focusing system obviously does not allow the use of plasma radiation in

SUBSTITUTE SHEET (RULE 26) corresponding solid angle, which is proportional to NA2).

In one embodiment, in order to increase the stability of the radiation of the broadband radiation source, according to the claimed technical solution, the source includes a feedback element of at least one of the lasers in terms of power or spectral power of the broadband radiation source. The feedback element controls the radiation power of the plasma at one or more wavelengths and, when the signal changes, accordingly adjusts the power of one of the lasers generating a laser plasma, preferably a continuous laser. The authors found that, in contrast to the known sources with plasma supported by a high-power continuous-wave laser, when the continuous-laser power changes over a sufficiently wide range in the region of the plasma maintenance threshold, the position of the bright region of the laser plasma does not change, which makes it possible to maintain stable plasma radiation precisely due to changes in laser power without taking into account the movement of a bright region of the plasma.

In one embodiment of the claimed technical solution, the source further includes a broadband radiation blocker synchronized with a repetitively pulsed laser. In the preferred case, the blocker passes the plasma radiation during the laser pulse or, excluding the initial stage of plasma heating in each pulse, even a slightly shorter time, while the plasma radiation in the rest of the time - in particular, between laser pulses - is blocked. In this case, the studied object irradiated by a broadband radiation source is exposed only to radiation with maximum brightness, which minimizes the possible harmful effects of the source, for example, excessive heating of a biological object or the occurrence of photochemical reactions under the influence of a constant background of plasma radiation. Note that in the “near-threshold” mode of operation of a continuous laser, the ratio of the plasma brightness during the operation of a pulsed laser and during a pause can be 100-500 or more even without using a blocker.

Thus, in a preferred embodiment of the claimed technical solution, the claimed light source is actually a pulse-periodic source of broadband optical radiation with high brightness, the pulse repetition rate of which is determined by the frequency of the pulse-periodic laser (s), and in the intervals between pulses the power of broadband radiation at orders less than maximum using

SUBSTITUTE SHEET (RULE 26) There is no broadband radiation between bright plasma flashes during a laser pulse.

Known pulse-periodic short-arc high-pressure lamps with xenon filling, for example, the same manufacturer Hamamatsu Photonics K.K. ([9]: http://www.hamamatsu.com/us/en/product/category/1001/3024/L9456/index.htmn. These lamps generate light pulses with a duration of 2-4 μs (half-height), which is enough for signal detection by optoelectronic devices with high sensitivity, however, such lamps do not work at frequencies> 500 Hz (usually the pulse repetition rate does not exceed 70-200 Hz), which is not enough for many applications, including in microelectronics. such lamps is limited and amounts to 109 pulses at a frequency of 500 Hz (108 pulses for more powerful flash lamps with a frequency of 50- 70 Hz), which corresponds to a lamp operating time of not more than a month, we also indicate less stability compared to continuous short-arc lamps and, especially, with respect to the claimed technical solution, the position of the brightest discharge region, which moves from pulse to pulse.

The radiation blocker can be made in various ways, including both electro-optical light choppers and mechanical, for example, a rotating disk with slots. The possibility of using this variant of periodic interruption of radiation is associated with the fact that the radiation of a broadband light source is usually transmitted using small diameter optical fibers, for a small plasma, the diameter of the optical fiber can be 100-200 microns. In this case, when the peripheral speed of rotation of the disk is 20 m / s (for example, disk diameter of 16 cm, rotation frequency of 2400 rpm) and the width of a single slot in it is 0.2 mm, the duration of a single light pulse transmitted by the slot will be ~ 10 μs ( for a fiber diameter of 100 μm); at a peripheral rotation speed of 30 m / s, the duration of a single light pulse transmitted by the slot will be ~ 6-7 μs.

Laser plasma can be generated in a high-pressure inert gas (helium, neon, argon, krypton, xenon) or a high-pressure inert gas mixture; at least one component from the group may also be included in the gas mixture: mercury, hydrogen, nitrogen. In a preferred embodiment, the chamber irradiated by focused laser radiation is filled with a heavy inert gas (argon, krypton, xenon) or a mixture of high inert heavy gases of up to several MPa (at

SUBSTITUTE SHEET (RULE 26) room temperature).

Similarly, plasma initiation can be carried out using electrodes placed in the chamber or using a source external to the chamber.

Further, the claimed technical solution is illustrated using examples with which it, however, is not limited, with reference to the accompanying drawings. The drawings show:

FIG. 1: determination of the angle Θ between the radiation directions of the lasers used in the utility model; 1 — optical axis of the radiation of the first laser, 2 — second laser.

FIG. 2: optical diagram of an embodiment of the invention; 3.4 — lasers, 5.6 — focusing systems, 7 — a chamber filled with high-pressure gas, 8 — a plasma radiation collection system.

As a gas chamber, an OSRAM XBO 75W lamp filled with high-pressure xenon with quartz glass walls was used; the outer diameter of the lamp was ~ 10 mm. As lasers, the DLM-30 and PLD-70 diode modules of the NTE IRE-Polyus / 1PC Photonics company were used, the angle between the direction of laser radiation was -90 °. As the output element of the focusing systems, we used lenses with an effective focal length F = \ 6 mm and = 32 mm, the diameter of the laser beams on the lenses was 4 and 8 mm, respectively, which corresponds to the numerical aperture of the focusing system NA ~ 0, 12, and the diameter laser rays on the surface of the lamp did not exceed 1.5 mm, which is less than the characteristic spatial scale of the inhomogeneity of the optical thickness of the quartz lamp shell. In this case, the influence of the lamp shell is reduced mainly to a shift in focus, and not to an increase in the size of the focal region; focus shift can be almost completely compensated for when focusing systems are tuned together.

Preliminary ionization in a gas is created by an arc discharge; after plasma ignition in laser beams, the arc discharge is switched off. The relative position of the focal regions of the two lasers used was preliminarily coincident and then fine-tuned to obtain maximum plasma brightness, while the focal regions of the lasers remained essentially the same.

Using two continuous lasers with a power of 26 W and 37 W (total power 63 W, option corresponding to [8]), the spectral brightness was obtained, which almost exactly coincides with the spectral brightness of the EQ-1500 with a diode laser with a power of about 60 W and much more

SUBSTITUTE SHEET (RULE 26) “Sharp” focusing with NA ~ 0.5. In this case, the summation of the intensities of the plasma radiation generated separately by each of the cw lasers (when the second laser is turned off) with the above power gives a value of ~ 3 times less in the wavelength region 400-600 nm, ~ 5 times less for λ ~ 300 nm and even greater difference for λ <250 nm. When compared with the plasma generated using a single 56 W cw laser with a similar (ΝΑ ~ 0.12) numerical aperture of the focusing system, the brightness when using two lasers with a similar total power (about 60 W) is ~ 2 times higher in the length range 300-600 nm waves, ~ 3 times more for λ ~ 250 nm.

With the simultaneous use according to the claimed technical solution of a continuous laser with a power of 27 W (the threshold for maintaining the plasma was 20-22 W) and a laser operating in a pulsed-periodic mode (pulsed power 63 W, pulse repetition rate 10 kHz, the duration of an individual laser pulse 22 μs, the duration of an individual pulse of plasma radiation 15-17 μs) the pulsed brightness of the plasma is 2.5 times higher than the brightness of the source of the prototype (EQ-1500) in the wavelength range of 350-600 nm and ~ 3 times in the region of λ ~ 300 nm. Moreover, the average total power of the two lasers used was -40 W — 1.5 times less than that of the prototype EQ-1500 with a much simpler laser focusing system. When the pulsed radiation power was increased to 1 10 W (the other parameters of the repetitively pulsed laser, as well as the cw laser power, did not change), the plasma radiation brightness increased by a further 1-7-4 times depending on the spectral range and exceeded the prototype brightness by 3-5 times at close total laser power.

The use of a cw laser along with a pulsed-periodic laser with a power supporting an optical discharge near the threshold of such a discharge makes it possible, on the one hand, to minimize the heat release in the plasma upon absorption of continuous radiation and, on the other hand, as established by the authors, to ensure a sufficient level of pulse-periodic absorption radiation already at the leading edge of the laser pulse. As a result, the laser pulse quickly (within 3-5 μs in the described example) heats the plasma to the maximum possible temperature, ensuring the maximum brightness of the optical discharge radiation. Thus, the claimed technical solution allows the efficient use of a pulsed-periodic laser generating sufficiently short pulses with a sufficiently high duty cycle — for example, as indicated above, with a duration of ~ 20 μs and a duty cycle of ~ 5

SUBSTITUTE SHEET (RULE 26) (Rapid heating of the plasma also makes it possible to reduce the duration of the laser pulse to ~ 10 μs, while increasing the duty cycle of the pulses to -10, etc.). It is this range of combination of parameters that allows a diode laser to obtain a multiple pulse power of laser radiation than the maximum allowable power in continuous operation, and, accordingly, the maximum spectral brightness of a broadband radiation source. In addition, the use of a cw laser supporting an optical discharge allows one to independently and widely vary the pulse duration and repetition rate of a repetitively pulsed laser, that is, to realize the frequency and duration of high-spectral brightness broadband radiation pulses necessary for a particular application. In the absence of cw laser radiation, as established by the authors, it is necessary to use laser pulses of significantly longer duration or frequency (i.e., less duty cycle), which significantly limits the characteristics of the source of broadband optical radiation.

It is also important to note that the authors of the claimed technical solution found that in the proposed embodiment of a bright pulsed-periodic light source, significant pulsed plasma heating during each laser pulse and plasma cooling between successive pulses with the inevitable occurrence of gas-dynamic disturbances, nevertheless, does not (as expected) lead to a decrease in the stability of the core position (the brightest region) of the plasma and the stability of brightness from pulse to pulse, of at least up to the repetition frequency of 10-20 kHz pulse scale that is sufficient for most applications.

The variation of the cw laser radiation power, as established by the authors, allows with a response time of not more than 250-300 μs (this corresponds to the thermal relaxation time of the bright region of the plasma) to widely control the pulsed plasma power (if a pulse-periodic laser is used), which makes it possible to realize feedback, controlling the pulsed plasma power, including according to a given program, and also provide active stabilization of the parameters of the pulsed laser plasma radiation, controlling the intensity s plasma radiation in one or more spectral bands and appropriately varying the power of one of the lasers is preferably continuous when ispoldzuetsya combination of continuous and pulsed-periodic laser.

It is also necessary to indicate that the average plasma radiation power over

SUBSTITUTE SHEET (RULE 26) the claimed technical solution does not exceed (and can be significantly less) the radiation power of the plasma in known sources, and, accordingly, the degradation rate of transmission of the walls of the lamp with compressed gas in the claimed technical solution is not greater than that of analogues. This means that the resource of the claimed source is not less than the resource of the prototype (in many cases, more), while the pulsed brightness of the claimed source is several times greater than the brightness of the prototype.

Thus, the technical result provided by the combination of features provided in the claimed invention is an increase in spectral brightness, a decrease in plasma position fluctuations and stabilization of the brightness of its radiation, as well as an increase in the source resource and the possibility of changing its average power and pulse repetition rate over a wide range.

A comparative analysis of the proposed technical solution and known analogues reveals the presence of significant distinctive features, which ensures that it meets the criteria of "novelty" and "significant differences".

The ability to create the claimed source on the basis of well-known components: diode lasers, focusing systems based on lens optics, lamps with heavy inert high-pressure gases (primarily xenon) with built-in electrodes for initiating plasma by electric discharge, as well as the feasibility of using the inventive broadband radiation source with high brightness in microelectronics, spectroscopy, etc. provides industrial applicability of the claimed technical solution.

In order to satisfy any possible specific requirements, changes in the above-described embodiments of a pulse-periodic high-bandwidth high-brightness optical pulse source and also its alteration without deviating from the provisions protected by the claims can be made obvious to qualified specialists in this field. In particular, another material (not silica glass) can be used for the chamber in which the high-pressure gas mixture is located (for example, to operate at a higher pressure of tens of atmospheres); the chamber may have a window made of a material that is transparent in the far UV and VUV (MgF2, etc.) for better output of short-wave radiation; the composition and pressure of the gas mixture can vary widely. Laser radiation can be focused not only by lens systems, but also by more complex optical elements (for example, off-axis paraboloidal or ellipsoidal

SUBSTITUTE SHEET (RULE 26) mirror) or optical systems with different numerical apertures located at different angles to each other and to the direction of gravity (vertical), the preferred, but not the only possible way, is to illuminate the chamber with the gas medium with a continuous laser from the bottom up. The specific implementation of the optical scheme for the extraction of plasma radiation from a chamber filled with a gaseous medium, including the use of optical fibers, may be different. In the optical scheme for the formation of an optical discharge, it is possible to use blockers (absorbers) of laser radiation that have passed an optical discharge, as well as a system for returning and re-focusing this laser radiation to further increase the contribution of laser energy to the plasma. The latter is technically simplified due to the positive effect of reducing the influence of refraction on repetitively pulsed laser radiation. Preliminary ionization of the gas can be carried out as a source located inside the chamber (similar to the example of the implementation of the proposed method), and an external source - for example, a powerful pulsed laser. Fiber lasers, diode lasers, gas lasers (e.g. CO2 lasers), etc., including two lasers with different radiation wavelengths, can be used to irradiate the gas. Systems based on an oscillating or rotating mirror can be used as a broadband source radiation blocker, the blocker can be implemented on electro-or magneto-optical effects, due to additional spectral devices, a spectrum section of a broadband source important for a particular application can be highlighted, various feedback algorithms can be used and etc.

SUBSTITUTE SHEET (RULE 26)

Claims

CLAIM

1. A source of high-brightness broadband optical radiation, including a chamber filled with a high-pressure gas medium, two laser irradiating cameras with two radiation focusing systems with a substantially coincident focal region and an angle between the laser radiation directions of at least 60 °, and at least one laser is a repetitively pulsed laser.

2. A source of high-brightness broadband optical radiation according to claim 1, characterized in that one laser is a cw laser and the second is a pulsed-periodic laser.

3. A source of high-brightness broadband optical radiation according to claim 2, characterized in that the duty cycle of the radiation of a repetitively pulsed laser is at least 2.

4. The source of broadband radiation with high brightness in p.p. 1 -2, characterized in that the numerical aperture of the radiation focusing systems does not exceed 0.2.

5. A source of high-brightness broadband radiation according to claim 1, characterized in that the source includes a feedback element of at least one of the lasers in terms of power or spectral power of the broadband radiation source.

6. A source of high-brightness broadband radiation according to claim 2, characterized in that the source includes a broadband radiation blocker synchronized with a repetitively pulsed laser.

SUBSTITUTE SHEET (RULE 26)