Laser system and method for generating secondary radiation through interaction of a primary laser beam with a target material

Abstract

The invention relates to a laser system for generating secondary radiation (108) through interaction of a focused primary laser beam (104) with a target material (106), comprising a laser beam source (102) for providing a raw laser beam (110) which has ultra-short laser pulses, a target region (114) for arranging the target material (106), a beam guidance device (112) for forming the focused primary laser beam (104) from the raw laser beam (110), wherein the focused primary laser beam (104) is directed to the one target region (114) in order to interact with a target material (106) arranged in the target region (114), wherein the beam guidance device (112) has a beam focusing device (126) which is configured to form the primary laser beam (104) by focusing a laser beam (129) entering the beam focusing device (126), wherein the laser beam (129) entering the beam focusing device (126) is based on the raw laser beam (110) or corresponds to the raw laser beam (110) and wherein the beam focusing device (126) has at least two spherical mirror elements (154, 156; 176, 180) spaced apart from one another. According to the invention, the beam focusing device has a numerical aperture of between 0.001 and 0.01 as long as the primary laser beam (104) propagates in a medium with a refractive index of less than 1.01.

Description

[0001] Laser system and method for generating secondary radiation by interaction of a primary laser beam with a target material

[0002] The invention relates to a laser system for generating secondary radiation by interaction of a focused primary laser beam with a target material, comprising a laser beam source for providing a raw laser beam having ultrashort laser pulses, a beam guiding device for forming the focused primary laser beam from the raw laser beam, wherein the focused primary laser beam is directed towards a target area in order to interact with a target material arranged in the target area, wherein the beam guiding device has a beam focusing device which is configured to form the primary laser beam by focusing a laser beam entering the beam focusing device,wherein the laser beam entering the beam focusing device is based on the raw laser beam or corresponds to the raw laser beam and wherein the beam focusing device has at least two spaced-apart spherical mirror elements.,

[0003] Furthermore, the invention relates to a method for generating secondary radiation by interaction of a focused primary laser beam with a target material, in which the target material is arranged or is arranged in a target area, a raw laser beam is provided by means of a laser beam source, which has ultrashort laser pulses, the focused primary laser beam is formed from the raw laser beam by means of a beam guiding device, wherein the focused primary laser beam is directed onto the target area and interacts with the target material arranged in the target area, wherein the beam guiding device has a beam focusing device which forms the primary laser beam by focusing a laser beam entering the beam focusing device,The laser beam entering the beam focusing device is based on or corresponds to the raw laser beam, and the beam focusing device comprises at least two spaced-apart spherical mirror elements. The development of ultrahigh-intensity lasers in the scientific field has made great progress in recent decades, for example, through the invention of chirp pulse amplification. Numerous laser devices worldwide now achieve peak intensities of up to 10, 22 W / cm 2 , see for example the scientific publication Danson et al., "Petawatt and exawatt class lasers worldwide.", High Power Laser Science and Engineering 7 (2019).

[0004] One potential application area for these very high-intensity lasers is secondary sources (see, for example, Albert et al., "2020 roadmap on plasma accelerators.", New Journal of Physics 23.3 (2021): 031101). These are so-called secondary beam sources, defined as electromagnetic radiation or particle radiation generated by the interaction of a primary beam source (primary laser beam) and matter (target material). Depending on the type of primary and secondary radiation, the interaction mechanism ranges from nuclear reactions to beam scattering.

[0005] The first industrial application of laser-induced secondary radiation was demonstrated with the generation of EUV light for lithography. In this process, tin droplets are ionized by a CO2 laser pulse. Extreme ultraviolet (EUV) light is emitted from the plasma by inverse bremsstrahlung.

[0006] High-intensity lasers can generate the following secondary radiations: electron beams, photon / X-rays, proton / ion radiation, neutron radiation, and high harmonics. These radiation forms enable numerous applications in science, medicine, defense and security technology, as well as industrial applications, such as tomography and metrology in the semiconductor industry. Furthermore, there are applications in the fields of energy generation and the energy industry, such as nuclear fusion and the denuclearization of fission waste. Examples of relativistic electron or neutron radiation can be found in Albert et al., "Laser wakefield accelerator-based light sources: potential applications and requirements.", Plasma Physics and Controlled Fusion 56.8 (2014): 084015, and Anderson et al., "Research opportunities with compact accelerator-driven neutron sources.", Physics Reports 654 (2016): 1-58.For successful industrialization of these applications, the primary laser beam at the location of the target material must have certain light field parameters, whereby a sufficiently high intensity of the primary laser beam and a scaling of its average power are particularly essential.

[0007] Regarding the generation of secondary radiation with ultra-high-intensity lasers, the current state of the art remains the industrialization of beam guidance and focusing, which is required to provide a primary laser beam with appropriate light field parameters while simultaneously enabling industrial-grade throughput. In the current state of the art, the raw laser beam provided by a laser beam source is usually adjusted in diameter using a transmissive telescope and then focused onto the appropriate target for secondary beam generation using off-axis parabolic mirrors. Transmissive optical elements limit the peak intensity of the laser radiation due to absorption. Furthermore, transmissive optical elements limit the scaling of the repetition rate due to difficult thermal management.Furthermore, in the case of high peak intensities, when a laser beam passes through transmissive optical elements, local phase changes can occur due to the Kerr effect, which influence the wavefront of the laser beam and degrade its beam quality.

[0008] WO 2014 / 044392 A1 discloses an EUV beam generation device comprising a vacuum chamber in which a target material can be arranged at a target position to generate EUV radiation, a beam guidance chamber for guiding a laser beam from a driver laser device toward the target position, an intermediate chamber arranged between the vacuum chamber and the beam guidance chamber, a first window, which can be sealed gas-tight in the intermediate chamber, for the entry of the laser beam from the beam guidance chamber, and a second window, which can be sealed gas-tight in the intermediate chamber, for the exit of the laser beam into the vacuum chamber. DE 10 2009 044 426 A1 discloses an EUV light source device that generates EUV light by irradiating a target material with a pulsed driver laser beam.

[0009] Another EUV light source device is known from US 2022 / 0206397 Al.

[0010] The invention is based on the object of providing a laser system and method as mentioned above, which enable the generation of secondary radiation with industrially suitable throughput, wherein the primary laser beam has the largest possible effective volume for interaction with the target material.

[0011] This object is achieved according to the invention in that the beam focusing device has a numerical aperture between 0.001 and 0.01, provided that the primary laser beam (104) propagates in a medium with a refractive index of less than 1.01.

[0012] A beam focusing device with a numerical aperture in the specified range has proven particularly advantageous in the laser system mentioned above for generating secondary radiation. Such numerical apertures enable the focusing of an incoming laser beam into a focus with a relatively large focus diameter of, for example, approximately

[0013] 150 pm. This allows the primary laser beam to be delivered with a large effective volume, i.e., a large spatial area in which the primary laser beam interacts with the target material to generate secondary radiation. This enables efficient generation of secondary radiation with high throughput.

[0014] The large focus diameter of the primary laser beam enables, in particular, efficient generation of high harmonics. Numerous higher harmonics of the laser frequency are observed through the interaction of a laser beam at the focus. In particular, the laser system according to the invention is suitable or configured to generate secondary radiation in the form of high harmonics.

[0015] In particular, the beam focusing device has a numerical aperture in the above-mentioned range, provided that the primary laser beam propagates in a medium with a refractive index between exactly 1 and less than 1.01 or between exactly 1 and the refractive index of air under normal conditions.

[0016] The numerical aperture of the beam focusing device is proportional to the sine of an angle between a longitudinal central axis and / or a principal ray of the primary laser beam and the peripheral rays of the primary laser beam. The numerical aperture corresponds to the product of the sine of the specified angle and the refractive index of the medium in which the primary laser beam propagates when the specified angle is measured.

[0017] In the present case, the fact that a laser beam is based on another laser beam is to be understood in particular to mean that the laser beam results from or is formed from the other laser beam by beam shaping and / or beam guidance.

[0018] For example, the fact that the laser beam entering the beam focusing device is based on the raw laser beam means that the raw laser beam has already passed through one or more other components of the beam guiding device before entering the beam focusing device, such as a beam adjustment device and / or a beam correction device.

[0019] The beam focusing device is arranged in particular after a beam adaptation device and / or after a beam correction device of the beam guiding device.

[0020] For example, the raw laser beam is a collimated laser beam and / or a Gaussian laser beam. In particular, the primary laser beam formed from the raw laser beam may comprise ultrashort laser pulses.

[0021] The primary laser beam is in particular a Gaussian laser beam.

[0022] The mirror elements of the beam focusing device and / or the mirror elements of a beam adjustment device of the laser system each have, in particular, a reflective surface. The mirror elements, in particular, have a highly reflective coating, such as a dielectric coating or an "enhanced gold" coating, to form the reflective surface. For example, the mirror elements can be designed as glass mirrors or metal mirrors, each with a dielectric coating, or as metal mirrors with an "enhanced gold" coating. It is also possible to design the mirror elements as glass mirrors with a metallic coating, such as an "enhanced gold" coating.

[0023] In particular, the mirror elements are not metal mirrors without a coating.

[0024] The beam guiding device is designed in particular for beam guiding and / or beam shaping of the raw laser beam in order to form the primary laser beam from the raw laser beam.

[0025] In this case, the mirror elements of the beam focusing device are to be understood as meaning in particular the mirror elements of the beam focusing device which contribute to adjusting the diameter of the raw laser beam.

[0026] In particular, the laser beam entering the beam focusing device strikes the mirror elements of the beam focusing device one after the other to form the primary laser beam.

[0027] In particular, it can be provided that the primary laser beam has a focus positioned on the target material and / or in the target material and / or in the region of the target material. The focus of the primary laser beam can provide a sufficiently high radiation intensity for interaction with the target material.

[0028] It can be advantageous if the primary laser beam focus has a diameter between 100 μm and 500 μm. This allows the focus to be formed with a large Rayleigh length, resulting in a large effective volume of the primary laser beam for interaction with the target material.

[0029] It can be advantageous if a beam path within the beam focusing device has no focus. The term beam path is understood to mean, in particular, a beam path within the beam focusing device that is assigned to the incoming laser beam and the primary laser beam. This makes it possible to avoid particularly high intensities of laser radiation that can occur in a focus of the beam path. This makes it possible to reduce or avoid, in particular, the occurrence of nonlinearities in the beam path that can be present at the focus due to the high intensities. These nonlinearities can influence the wavefront of the laser beam and impair its beam quality, whereby the primary laser beam can be focused less effectively and the maximum achievable intensity of the laser radiation available for interaction with the target material is reduced.Because the beam path has no focus, the efficiency of generating secondary radiation can be increased. Furthermore, increased thermal stress on laser system components can occur near a focus of the beam path, which can also be avoided.

[0030] It can be advantageous if the beam focusing device is designed as a reflective optical system. This particularly means that the beam focusing device is implemented using a reflective and / or reflection-based optical concept. This enables the focusing of laser beams whose laser pulses have high peak intensities.

[0031] The axis of symmetry of the beam focusing device runs, for example, parallel to the incoming laser beam and parallel to an axis of symmetry of a first spherical mirror element of the beam focusing device, which is struck by the laser beam entering it.

[0032] It can be advantageous if the diameter of the laser beam entering the beam focusing device is between 15 mm and 100 mm. In particular, the diameter is between 20 mm and 30 mm. This allows the primary laser beam formed from this laser beam to be focused into a focus with a relatively large focus diameter, providing a large effective cross-section for interaction with the target material. Furthermore, a laser beam with this diameter enables a reduction in nonlinear effects that can occur, for example, at a passage element when the laser beam is coupled into a gas-tight chamber if the beam focusing device is arranged in the chamber.

[0033] It may therefore be advantageous for the beam guiding device to have a beam adjustment device configured to adjust the diameter of the laser beam entering the beam focusing device by changing the diameter of the raw laser beam entering the beam guiding device. This allows the laser beam entering the beam focusing device to be provided with a diameter within the advantageous range specified above. The diameter of the laser beam entering the beam focusing device can thus be adjusted to an optimal diameter for the beam focusing device, in particular to form a focus with the largest possible focus diameter.

[0034] For example, the raw laser beam entering the beam guidance device has a diameter between 10 mm and 20 mm. To create a focus with the largest possible diameter, it may be useful to increase the diameter of the beam before entering the beam focusing device.

[0035] The beam adjustment device is particularly designed as a reflective optical system. It may be advantageous if the beam focusing device has a first mirror element, which is impinged upon by the laser beam entering it, and the beam focusing device has a further mirror element, from which the focused primary laser beam emanates, with at least one intermediate laser beam extending between the first mirror element and the further mirror element. This allows the primary laser beam to be formed with a large focus diameter.

[0036] The first mirror element of the beam focusing device is in particular a very first mirror element onto which the laser beam entering the beam focusing device strikes.

[0037] The further mirror element of the beam focusing device is in particular a last mirror element of the beam focusing device, from which the primary laser beam emanates and / or is emitted.

[0038] In particular, it can be provided that a longitudinal center axis of the laser beam entering the beam focusing device and / or a longitudinal center axis of the primary laser beam and / or a longitudinal center axis of the at least one intermediate laser beam lie in the same geometric plane.

[0039] In particular, the at least one intermediate laser beam has no focus. This results in the aforementioned advantages.

[0040] It is fundamentally possible for several intermediate laser beams to be present between a first mirror element of the beam focusing device, which is impinged by the incoming laser beam, and another mirror element of the beam focusing device, from which the primary laser beam emanates. For example, these intermediate laser beams can be convergent or divergent laser beams.

[0041] In particular, it can be provided that the first mirror element, which is impinged upon by the laser beam entering the beam focusing device, is a spherical mirror element. Spherical mirror elements can be manufactured in a technically simple manner with a good surface quality. For the same reason, it can be advantageous if the additional mirror element, from which the primary laser beam emanates, is a spherical mirror element.

[0042] In particular, it can be provided that the first mirror element of the beam focusing device, onto which the laser beam entering the beam focusing device strikes, is concave.

[0043] In particular, it can be provided that the further mirror element of the beam focusing device, onto which the laser beam entering the beam focusing device strikes, is convex.

[0044] In particular, the beam focusing device can be provided with exactly two spherical mirror elements. This results in a simple design of the beam focusing device with the smallest possible number of mirror elements, a design also known as the Schwarzschild configuration. It enables good compensation of imaging errors of the respective mirror elements.

[0045] In one embodiment, the beam focusing device comprises a total of exactly two mirror elements. These two mirror elements are the two spherical mirror elements. This allows the beam focusing device to be designed compactly.

[0046] In particular, it can be provided that the laser pulses of the raw laser beam and / or the primary laser beam have a pulse duration between 10 fs and 300 fs.

[0047] In particular, it can be provided that the laser pulses of the raw laser beam and / or the primary laser beam have a pulse energy between 1 mJ and 20 mJ.

[0048] The average power of the raw laser beam and / or the primary laser beam is, for example, between 0.5 kW and 5.0 kW, for example 1.0 kW. The intensity of the primary laser beam at the focus is, for example, between 10 13 W / cm 2 and 10 15 W / cm 2 and especially between l*10 14 W / cm2 and 9*10 14 W / cm 2 .

[0049] A wavelength of the raw laser beam and / or the primary laser beam is, for example, between 500 nm and 2500 nm and preferably between 900 nm and 1100 nm, for example 1030 nm.

[0050] The parameters mentioned enable the generation of high harmonics in a technically advantageous manner.

[0051] In particular, it can be provided that the laser system comprises the target area for arranging the target material and / or the target material.

[0052] In particular, it can be provided that a negative pressure, in particular a vacuum, is formed in the target area. For example, the target area lies within a gas-tight area of ​​the laser system.

[0053] The laser system may comprise a gas-tight chamber in which the target area for arranging the target material is positioned. The chamber may comprise a passage element for coupling a laser beam into the chamber. The primary laser beam is based on or corresponds to this laser beam. A negative pressure chamber, in particular a vacuum, is formed in the chamber.

[0054] In particular, the passage element has an anti-reflective coating, which is preferably designed as a nanotexturing and / or as a moth-eye structure.

[0055] In particular, it can be provided that the beam focusing device is at least partially arranged within the chamber. This means in particular that at least one component of the beam focusing device, such as a mirror element of the beam focusing device, is positioned within the chamber. This can also mean that the beam focusing device is arranged completely within the chamber. It can be advantageous if the laser system has a shielding element which is arranged between the passage element and the target material, wherein the shielding element is designed for the spatial shielding of an optical component of the laser system. In particular, the shielding element is designed for the spatial shielding of the passage element from the target material. By means of the shielding element, contamination of the optical component orof the passage element by scattered target material during operation of the laser system.

[0056] An optical component of the laser system is understood to be, for example, a transmissive or reflective optical element of the laser system, such as a mirror element of the beam focusing device or a beam expanding device.

[0057] The shielding element can be designed, for example, as a diaphragm element or as a bevel element, or can be realized by means of a fluid flow.

[0058] The target material arranged in the target area is in particular in a gaseous state.

[0059] For example, the target material is or comprises a noble gas such as xenon, argon, helium or krypton.

[0060] It may be advantageous for the beam guidance device to have a beam correction device for forming a corrected laser beam from a laser beam entering the beam correction device, wherein the beam position is stabilized by means of the beam correction device in order to provide the corrected laser beam with a corrected and / or stabilized beam position. This enables stable positioning of a focus of the primary laser beam, which in turn enables stable and low-interference provision of secondary radiation. In particular, it can be provided that the beam correction device is used to perform a wavefront correction of the laser beam entering it in order to provide the corrected laser beam with a corrected wavefront. This allows the primary laser beam to be provided with increased beam quality.

[0061] In particular, the laser beam entering the beam correction device is based on the raw laser beam and / or on an adjusted laser beam formed by a beam adjustment device of the beam guidance device. In particular, the laser beam entering the beam correction device corresponds to this adjusted laser beam.

[0062] In particular, the laser beam entering the beam focusing device corresponds to or is based on the corrected laser beam.

[0063] The laser system may be provided with a secondary beam guidance device for guiding and / or shaping the generated secondary radiation. For example, the generated secondary radiation is guided by the secondary beam guidance device to a location where the secondary radiation is intended for use.

[0064] According to the invention, in the method mentioned at the outset, it is provided that the beam focusing device has a numerical aperture between 0.001 and 0.01, provided that the primary laser beam (104) propagates in a medium with a refractive index of less than 1.01.

[0065] The method according to the invention has, in particular, one or more further features and / or advantages of the laser system according to the invention. Advantageous embodiments have already been explained in connection with the laser system according to the invention.

[0066] The method according to the invention can be carried out in particular by means of the laser system according to the invention. In particular, the method according to the invention is carried out by means of the laser system according to the invention.

[0067] In the context of the present application documents, diameters of laser beams and / or focus diameters are generally defined using the method of second moments according to ISO 11146-3.

[0068] The fact that a first device and / or a first element of the laser system is arranged downstream of a second device and / or a second element of the laser system is to be understood in the present case as meaning that the laser beams guided in the laser system, such as the raw laser beam and / or the laser beams based on the raw laser beam, and / or the primary laser beam, first strike the second device and / or the second element and then the first device and / or the first element. The second device and / or the second element is then arranged upstream of the first device and / or the first element. This information always refers to the main propagation direction of the respective laser beams.

[0069] The following description of preferred embodiments, taken in conjunction with the drawings, serves to further explain the invention. They show:

[0070] Fig. 1 is a schematic representation of an embodiment of a laser system;

[0071] Fig. 2 shows an embodiment of a beam adjustment device of the laser system in a cross-section parallel to the main propagation direction of laser beams guided through the beam adjustment device;

[0072] Fig. 3 is a schematic representation of a parabolic mirror element with an associated paraboloid in a cross-section parallel to the rotation axis of the paraboloid; and Fig. 4 is an embodiment of a beam focusing device of the laser system in a cross-section parallel to the main propagation direction of laser beams guided by the beam focusing device.

[0073] Identical or functionally equivalent elements are provided with the same reference numerals in all figures.

[0074] An embodiment of a laser system for generating secondary radiation is shown in Fig. 1 and designated by 100. The laser system 100 comprises a laser beam source 102, wherein a primary laser beam 104 is formed by beam shaping a laser beam provided by the source. This primary laser beam 104 is directed onto a target material 106, so that secondary radiation 108 is generated by the interaction of the primary laser beam 104 with the target material 106.

[0075] In the example shown in Fig. 1, during operation of the laser system 100, a raw laser beam 110 emerges from the laser beam source 102. The laser system 100 has a beam guiding device 112 configured to form the primary laser beam 104 from the raw laser beam 110 and to direct it onto the target material 106 and / or to focus it into the target material 106. The target material 106 is arranged in a predetermined target area 114 of the laser system 100.

[0076] For example, the target material 106 is or comprises a gaseous material, such as xenon, argon, helium, or krypton. In particular, the target material 106 is continuously introduced into the target area 114 in the form of a material stream and / or fluid stream by means of a suitable conveying device (not shown), so that during operation of the laser system 100, the target material 106 is continuously available there for interaction with the primary laser beam 104.

[0077] It can be provided that a negative pressure, i.e., a pressure reduced compared to ambient pressure, and in particular a vacuum, is formed in the target region 114. The target region 114 is located, for example, within a gas-tight region of the laser system 100. For example, the target region 114 is positioned within a gas-tight chamber 118 of the laser system 100, in which the gas-tight region and / or the negative pressure is formed.

[0078] For example, a gas with a defined pressure and defined composition can be arranged in the target area 114 and / or the gas-tight chamber 118.

[0079] In the example shown in Fig. 1, the chamber 118 includes a passage element 119 through which the primary laser beam 104 enters the chamber 118. The passage element 119 is made of a material transparent to a wavelength of the primary laser beam 104. For example, the passage element 119 is designed as a vacuum window.

[0080] In particular, the penetration element has an anti-reflective coating, such as nanotexturing or a moth-eye structure. Such anti-reflective coatings are known, for example, from the scientific publication "Nanotextured optical surfaces advance laser power and reliability" by Nole et al., Laser Focus World 50.6 (2014): 38-43.

[0081] It may be provided that the beam focusing device 126 is arranged at least partially within the chamber 118 and / or within the gas-tight region.

[0082] In one embodiment, a shielding element 117 is arranged within the chamber 118, which is positioned between the passage element 119 and the target material 106, wherein the primary laser beam 104 passes through the shielding element 117. The shielding element 117 has a transmission region and / or an opening for the passage of the primary laser beam 104.

[0083] The shielding element 117 is configured to shield the passage element 119 from the target material 106. For example, the passage element 119 is shielded by the shielding element 117 from target material 106, which is scattered and / or distributed toward the passage element 119 during operation of the laser system 100.

[0084] For example, the shielding element 117 is designed as a diaphragm element, in particular as a pinhole. Alternatively, the shielding element 117 can be designed as a bevel element or can be implemented by means of a fluid flow, in particular a gas flow.

[0085] The raw laser beam 110 is a pulsed laser beam comprising ultrashort laser pulses (ultrashort pulse laser beam). The laser pulses preferably have a pulse duration between 10 fs and 300 fs and / or a pulse energy between 1 mJ and 20 mJ. The wavelength of the raw laser beam 110 is preferably between 500 nm and 2500 nm, and the raw laser beam 110 preferably has an average power in the range of 0.5 kW to 5 kW.

[0086] For example, the laser beam source 102 is a solid-state ultrashort pulse laser beam source, which has, for example, a Ti:Sa or an ytterbium-doped YAG amplifier.

[0087] The raw laser beam 110 emerging from the laser beam source 102 is in particular a collimated laser beam and / or Gaussian laser beam.

[0088] The beam guiding device 112 has a beam adjustment device 120, by means of which an adjusted laser beam 122 is formed by changing the diameter of a laser beam 121 entering it. In the embodiment according to Fig. 1, the raw laser beam 110 coupled out of the laser beam source 102 is coupled into the beam adjustment device 120 of the beam guiding device 112. The incoming laser beam 121 thus corresponds to the raw laser beam 110 in the example shown.

[0089] In the example shown, the beam guiding device 112 further comprises a beam correction device 124 and a beam focusing device 126, through which the adjusted laser beam 122 passes. In this embodiment, the laser beam emerging from the beam focusing device 126 corresponds to the primary laser beam 104 intended for interaction with the target material 106.

[0090] By means of the beam correction device 124, a laser beam 123 entering it, which in the example shown corresponds to the adjusted laser beam 122, is corrected, forming a corrected laser beam 128. The beam correction device 124 is configured to perform beam position stabilization in order to provide the corrected laser beam 128 with a corrected and / or stabilized beam position. The beam position stabilization particularly comprises correcting and / or stabilizing a spatial position of the corrected laser beam 128 in the near field and an angle of the corrected laser beam 128 in the far field.

[0091] For example, the beam correction device 124 has one or more piezo mirrors which are controlled by means of four-quadrant photodiodes in order to stabilize the spatial position and / or the angle of the corrected laser beam 128.

[0092] Additionally, the beam correction device 124 can be configured to perform a wavefront correction of the incoming laser beam 123 in order to provide the corrected laser beam 128 with a wavefront that is as flat as possible. Methods and devices for wavefront correction for laser beams with high intensities are known, for example, from S. Fourmaux et al., "Laser beam wavefront correction for ultra-high intensities with the 200 TW laser system at the Advanced Laser Light Source," Opt. Express 16, pp. 11987-11994 (2008).

[0093] The beam focusing device 126 is configured for beam shaping and / or focusing an incoming laser beam 129, which in the example shown corresponds to the corrected laser beam 128. By means of the beam focusing device 126, the focused primary laser beam 104 directed onto the target material 106 is formed from the incoming laser beam 129. The primary laser beam 104 has a focus 131 arranged on the target material 106 with a defined focus diameter, which is, for example, approximately 150 pm. The focus diameter is understood to mean a transverse diameter and / or an extension of the primary laser beam 104 in the transverse direction.

[0094] By focusing the primary laser beam 104 onto and / or into the target material 106, the secondary radiation 108 is formed during operation of the laser system 100. It may be provided that the laser system 100 has a secondary beam guiding device 130, which is configured to guide and / or shape the beam of the formed secondary radiation 108.

[0095] The primary laser beam 104 and the above-mentioned laser beams 121, 122, 123, 128, 129 are each based on the raw laser beam 110 and / or are formed by beam shaping the raw laser beam 110.

[0096] The raw laser beam 110 and the laser beams based on the raw laser beam 110 each propagate with a main propagation direction 132. The main propagation direction 132 is a local property of a specific laser beam and is defined, in particular, by the direction of a Poynting vector or averaged Poynting vector associated with the laser beam. In the case of a collimated laser beam, the main propagation direction 132 is oriented, for example, parallel to a longitudinal center axis of the laser beam.

[0097] In the embodiment according to Fig. 1, the beam adjustment device 120 is arranged upstream of the beam correction device 124 and / or upstream of the beam focusing device 126. The beam correction device 124 is arranged, for example, upstream of the beam focusing device 126 and / or between the beam adjustment device 120 and the beam focusing device 126.

[0098] A laser beam, such as the raw laser beam 110, the primary laser beam 104, and / or the laser beams 121, 122, 123, 128, 129, is generally understood here to be a bundle of rays comprising a plurality of partial beams. These can, for example, be convergent, divergent, or, in the case of a collimated laser beam, parallel to one another. The laser beams each have a transverse extension and / or a transverse beam cross-section, i.e., an extension or a cross-section in a direction perpendicular to the main propagation direction 132.

[0099] An embodiment of the beam adjustment device 120 is shown in Fig. 2. This device has a first mirror element 134 and a second mirror element 136 spaced apart from the first mirror element 134. The mirror elements 134, 136 are configured to expand the raw laser beam 110 or the incoming laser beam 121, i.e., to increase a diameter of the transverse beam cross-section of the raw laser beam 110 or the incoming laser beam 121. The laser beam 122 adjusted by means of the mirror elements 134, 136 exits the beam adjustment device 120 and, in the example shown, is coupled into the beam correction device 124.

[0100] The incoming laser beam 121 and the adjusted laser beam 122 are each present as collimated laser beams, with the incoming laser beam 121 having a diameter di and the adjusted laser beam 122 having a diameter d2 (for example, a beam waist of the laser beams 121, 122 is indicated in Fig. 2). The diameter d2 is larger than the diameter di. The diameters di and d2 are understood to be a transverse diameter and / or a diameter of the transverse beam cross-section of the laser beams 121, 122.

[0101] In particular, the incoming laser beam 121 and the adjusted laser beam 122 each have a rotationally symmetric and, in particular, circular transverse beam cross-section. In particular, the diameter of this beam cross-section is the same in every spatial direction that lies in a cross-sectional plane associated with the beam cross-section.

[0102] The mirror elements 134, 136 of the beam adjustment device 120 are each designed as off-axis parabolic mirrors. They each have curved and reflective surfaces (see Fig. 3). In an off-axis parabolic mirror, the reflective surfaces do not contain the rotation axis of the respective paraboloid (and thus also not the vertex of the paraboloid).

[0103] The mirror elements 134, 136 preferably have a highly reflective coating, such as a dielectric coating or an "enhanced gold" coating, to form the reflective surfaces. For example, the mirror elements 134, 136 can be designed as glass mirrors or metal mirrors, each with a dielectric coating, or as metal mirrors with an "enhanced gold" coating. It can also be provided that the mirror elements 134, 136 are designed as glass mirrors with a metallic coating, such as an "enhanced gold" coating.

[0104] In particular, the mirror elements 134, 136 are not metal mirrors without a coating.

[0105] The incoming laser beam 121 strikes the first mirror element 134 and is reflected therefrom, forming a reflected intermediate laser beam 138 that extends between the first mirror element 134 and the second mirror element 136. The intermediate laser beam 138 strikes the second mirror element 136 and is reflected therefrom, forming the adjusted laser beam 122 exiting the beam adjustment device 120.

[0106] The path of the incoming laser beam 121, the adjusted laser beam 122 and the intermediate laser beam 138 in the beam adjustment device 120 is referred to herein as beam path 139.

[0107] The first mirror element 134 is convexly curved and the second mirror element 136 is concavely curved (with respect to a respective incident direction of the incoming laser beam 121 and the intermediate laser beam 138).

[0108] Due to the convex curvature of the first mirror element 134, the

[0109] Intermediate laser beam 138 is formed as a divergent beam, i.e., its beam diameter increases in the main propagation direction 132. It extends continuously as a divergent beam between the first mirror element 134 and the second mirror element 136. In particular, the intermediate laser beam 138 has no intermediate focus and / or no converging beam sections and / or no converging partial beams or beam portions.

[0110] By means of the concave second mirror element 136, the divergent intermediate laser beam 138 incident thereon is transformed into the collimated, adjusted laser beam 132.

[0111] The incoming laser beam 121 has a longitudinal central axis 140, the intermediate laser beam 138 has a longitudinal central axis 142, and the adjusted laser beam 122 has a longitudinal central axis 144. A path of the respective longitudinal central axis 140, 142, 144 corresponds to a path of a central partial beam (also referred to as the main beam or "chief ray") of the associated laser beam 121, 138, 122. Furthermore, the longitudinal central axis 140, 142, 144 is oriented parallel to the local main propagation direction 132 of the associated laser beam 121, 138, 122.

[0112] The longitudinal central axis 140 of the incoming laser beam 121 and the longitudinal central axis 144 of the adjusted laser beam 122 are oriented parallel to each other, for example. The longitudinal central axis 142 of the intermediate laser beam 138 is oriented perpendicular to the longitudinal central axis 140 of the incoming laser beam 121 and / or to the longitudinal central axis 144 of the adjusted laser beam 122, for example.

[0113] For example, the diameter di has a value of 10.0 mm and the diameter d2 has a value of 25.0 mm.

[0114] A distance ds between the first mirror element 134 and the second mirror element 136 is, for example, 500 mm. A distance ds between the first mirror element 134 and the second mirror element 136 is, for example, 1500 mm. The distance ds corresponds to a distance between a point of intersection of the longitudinal central axis 142 of the intermediate laser beam 138 with the surface of the first mirror element 134 and a point of intersection of the longitudinal central axis 142 with the surface of the second mirror element 136. Consequently, the distance ds corresponds to a path length of the central partial beam (chief ray) of the intermediate laser beam 138 lying along the longitudinal central axis 142 between the first mirror element 134 and the second mirror element 136.

[0115] The parameters relevant for describing a mirror element designed as an off-axis parabolic mirror are illustrated in Fig. 3, with the situation prevailing at the first mirror element 134 being shown as an example. Parabolic mirror elements can be generally described using the parameters introduced below.

[0116] The incoming laser beam 121 strikes the first mirror element 134, whereby the intermediate laser beam 138 oriented perpendicular to the incoming laser beam 121 is formed by reflection at the surface of the first mirror element 134.

[0117] The reflecting surface of the first mirror element 134 lies in a section of a paraboloid 146, which is obtained by rotating a parabola 148 around a rotation axis 150. The parabola 148 is defined by the function z(x) = c / 2*x 2described. In Fig. 3, the paraboloid 146 is shown in a cross-section lying along the rotation axis 150.

[0118] In the context of the application documents, a mirror element designed as an off-axis parabolic mirror is to be understood as a mirror element whose reflective surface corresponds to a surface which can be obtained by rotating the function z(x) = c / 2*x 2 , whereby deviations of up to 1% from this function are permissible. In this case, an area of ​​the mirror element is to be considered in which at least 99% of the power of the laser beam incident on this mirror element is present. The paraboloid 146 has an original focal length f par (also referred to as "parent focal length") and a reflective focal length fref ("reflective focal length"). The reflective focal length fref is understood in particular as an effective focal length with respect to the incoming laser beam 121.

[0119] In this case, the incoming laser beam 121 and the intermediate laser beam 138 are oriented perpendicular to each other. In this case, f par = 1 / c and fref = l / (2c). Furthermore, a distance d4 between a vertex 152 of the paraboloid 146 and the longitudinal center axis 140 of the incoming laser beam 121 corresponds to the reflective focal length fref. This distance d4 is also referred to as the "decenter" distance.

[0120] In the example shown in Fig. 2, the first mirror element 134 and the second mirror element 136 each have different reflective focal lengths. For example, a reflective focal length fref-1 of the first mirror element has a value of 500 mm, and a reflective focal length fref-2 of the second mirror element 136 has a value of 1250 mm.

[0121] A magnification factor M of the beam adjustment device 120 generally corresponds to a quotient of the diameter d2 of the adjusted laser beam 122 and the diameter di of the incoming laser beam 121. In the example described, this results in M ​​= 25.0 mm / 10.0 mm = 2.5. Furthermore, in the example shown, M = fref-2 / fref-1 = 1250 mm / 500 mm = 2.5.

[0122] In the embodiment shown in Fig. 2, the beam adjustment device 120 has two reflective mirror elements 134, 136. It is fundamentally possible for the beam adjustment device to have more than two reflective mirror elements.

[0123] It is evident that the beam adjustment device 120 can also be configured or used to reduce the diameter di of the incoming laser beam 121. For this purpose, in the example shown in Fig. 2, the first mirror element 134 and the second mirror element 136 would have to be swapped so that the incoming laser beam 121 first strikes the second mirror element 136 and the then formed intermediate laser beam 138 strikes the first mirror element 134. In this case, the magnification factor M <1, or the beam adjustment device 120 has a reduction factor defined as 1 / M.

[0124] An embodiment of the beam focusing device 126 is shown in Fig. 4. This device comprises a first mirror element 154 and a second mirror element 156 spaced apart from the first mirror element 154. The mirror elements 154, 156 are configured to focus the incoming laser beam 129 to form the focused primary laser beam 104 directed onto the target material 106.

[0125] The incoming laser beam 129 is in particular a collimated laser beam and has a diameter ds which corresponds, for example, to the diameter d2 of the adjusted laser beam 122 emerging from the beam adjustment device 120.

[0126] The diameter ds is understood to be a transverse diameter and / or a diameter of the transverse beam cross-section of the incoming laser beam 129. In particular, the incoming laser beam 129 has a rotationally symmetric and, in particular, circular transverse beam cross-section. In particular, a diameter of this beam cross-section is the same in every spatial direction that lies in a cross-sectional plane associated with the beam cross-section.

[0127] The primary laser beam 104 emerging from the beam focusing device 126 is present as a convergent and / or focused laser beam. It is focused into the focus 131.

[0128] The incoming laser beam 129 has a longitudinal central axis 158, and the primary laser beam 104 has a longitudinal central axis 160, with the longitudinal central axis 158 and the longitudinal central axis 160 being oriented transversely to each other. In particular, the longitudinal central axes 158 and 160 form a non-zero angle. The mirror elements 154, 156 of the beam focusing device 126 are each designed as spherical mirrors. They each comprise curved and reflective surfaces that have the shape of a spherical segment.

[0129] The mirror elements 154, 156 preferably have a highly reflective coating, such as a dielectric coating or an "enhanced gold" coating, to form the reflective surfaces. For example, the mirror elements 154, 156 can be designed as glass mirrors or metal mirrors, each with a dielectric coating, or as metal mirrors with an "enhanced gold" coating. It can also be provided that the mirror elements 154, 156 are designed as glass mirrors with a metallic coating, such as an "enhanced gold" coating. In particular, the mirror elements 154, 156 are not metal mirrors without a coating.

[0130] The incoming laser beam 129 strikes the first mirror element 154 and is reflected therefrom, forming a reflected intermediate laser beam 162 that extends between the first mirror element 154 and the second mirror element 156. The intermediate laser beam 162 strikes the second mirror element 156 and is reflected therefrom, forming the focused primary laser beam 104 exiting the beam focusing device 126.

[0131] A longitudinal central axis 163 of the intermediate laser beam 162 is oriented transversely to the longitudinal central axis 158 of the incoming laser beam 129 and / or to the longitudinal central axis 160 of the primary laser beam 104.

[0132] A path of the respective longitudinal central axis 158, 160, 163 corresponds to a path of a central partial beam (chief ray) of the respective associated laser beam 129, 104, 162. Furthermore, the longitudinal central axis 158, 160, 163 is oriented parallel to the local main propagation direction 132 of the associated laser beam 129, 104, 162. The path of the incoming laser beam 129, the primary laser beam 104, and the intermediate laser beam 162 in the beam focusing device 126 is referred to herein as beam path 164.

[0133] The first mirror element 154 is concavely curved and the second mirror element 156 is convexly curved (with respect to a respective direction of incidence of the incoming laser beam 129 and the intermediate laser beam 162).

[0134] Due to the concave curvature of the first mirror element 154, the intermediate laser beam 162 is formed as a convergent beam. It runs continuously as a convergent beam between the first mirror element 154 and the second mirror element 156. In particular, the intermediate laser beam 162 has no intermediate focus and / or no diverging beam sections and / or no diverging beam components.

[0135] By means of the convex second mirror element 156, the convergent intermediate laser beam 162 impinging thereon is transformed and / or deflected into the convergent primary laser beam 104.

[0136] The reflective surface of the first mirror element 154 lies in a portion of a spherical surface having an axis of symmetry 166 parallel to the main propagation direction 132 and / or the longitudinal central axis 158 of the incident laser beam 129. This axis of symmetry 166 is referred to below as the axis of symmetry of the beam focusing device 126.

[0137] The axis of symmetry 166 runs through a center point (not shown) of a geometric sphere, which is assigned to the section of the spherical surface in which the surface of the first mirror element 154 lies. An extension of this spherical surface assigned to the first mirror element 154 intersects the axis of symmetry 166 at a first point 168. In particular, the focus 131 is positioned at a distance from the axis of symmetry 166 and / or does not lie on the axis of symmetry 166. The reflective surface of the second mirror element 156 lies in a section of a spherical surface, which intersects the axis of symmetry 166 at a second point 170. A normal 172 of this spherical surface at the point 170 encloses a non-zero angle α with the axis of symmetry 166, wherein the normal 172 and the axis of symmetry 166 lie in particular in the same geometric plane.

[0138] The angle o is in particular oriented such that the normal 172 intersects the extension of the spherical surface assigned to the first mirror element 154 in a section between the first point 168 and the longitudinal central axis 160 of the primary laser beam 104.

[0139] For example, the diameter ds of the incoming laser beam 129 has a value of 25.0 mm.

[0140] For example, the first mirror element 154 has a radius of curvature of 537.9 mm and the second mirror element 156 has a radius of curvature of 39.0 mm.

[0141] The angle o, with which the first mirror element 154 and the second mirror element 156 are tilted relative to each other, is, for example, 9.2°.

[0142] A distance de between the longitudinal center axis 158 of the incoming laser beam 129 and the axis of symmetry 166 is, for example, 30 mm.

[0143] For example, a distance d? parallel to the axis of symmetry 166 between the first point 168 and the second point 170 is 250 mm.

[0144] The intermediate laser beam 162 and the primary laser beam 104 each have edge rays 174, which are defined by a maximum geometric extent of the reflective surface of the first mirror element 154. These edge rays 174 each emanate from outer edges of the surface of the first mirror element 154. The edge rays 174 are, in particular, those rays of the intermediate laser beam 162 or primary laser beam 104 that are at the greatest distance from the longitudinal center axis 163 or 160 at a specific point on the longitudinal center axis 163 or 160, respectively, in a distance direction oriented perpendicular to the longitudinal center axis 163 or 160.

[0145] The edge rays 174 of the primary laser beam 104 form an angle θ with its longitudinal central axis 160. A numerical aperture NA of the primary laser beam 104 is given by NA = n*sin(0), where n is a refractive index of a medium in which the primary laser beam 104 propagates. For example, the numerical aperture in a vacuum (n « 1.0) is NA = 0.002.

[0146] Real laser beams have a Gaussian-like envelope, so the effectively utilized numerical aperture can be smaller than the theoretically maximum usable numerical aperture NA, which is given by NA = n*sin(0). Typically, the effectively utilized numerical aperture is at least a factor of 2 smaller than the maximum usable numerical aperture.

[0147] For example, an angle 0' between the longitudinal center axis 160 of the primary laser beam 104 and the symmetry axis 166 is 18.6°.

[0148] The Laser System 100 works as follows:

[0149] During operation of the laser system 100, the raw laser beam 110 emerges from the laser beam source 102 and is coupled into the beam guiding device 112. The raw laser beam 110 passes successively through the beam adjustment device 120, the beam correction device 124, and the beam focusing device 126. The raw laser beam 110 is adjusted by the beam adjustment device 120, corrected and / or stabilized by the beam correction device 124, and shaped and / or focused by the beam focusing device 126, forming the primary laser beam 104.

[0150] The focused primary laser beam 104 is directed onto the target material 106, causing an interaction between the target material 106 and the primary laser beam 104. Due to this interaction, secondary radiation 108 is generated. For example, the generated secondary radiation is electromagnetic radiation with a wavelength of approximately 13.5 nm.

[0151] The formed secondary radiation 108 is shaped by means of the secondary beam guiding device 130 and guided to a target at which use of the secondary radiation 108 is intended.

[0152] List of reference symbols o Angle e Angle

[0153] 0' Angle di Diameter d2 Diameter d3 Distance d4 Distance ds Diameter de Distance d? Distance fpar Original focal length fref Reflective focal length fref-1 Reflective focal length fref- 2 Reflective focal length

[0154] 100 laser system

[0155] 102 Laser beam source

[0156] 104 Primary laser beam

[0157] 106 Target material

[0158] 108 Secondary radiation

[0159] 110 raw laser beam

[0160] 112 Beam guidance device

[0161] 114 Target area

[0162] 117 Shielding element

[0163] 118 Chamber

[0164] 119 Passage element

[0165] 120 beam adjustment device

[0166] 121 incoming laser beam

[0167] 122 customized laser beam

[0168] 123 incoming laser beam

[0169] 124 Beam correction device

[0170] 126 Beam focusing device

[0171] 128 corrected laser beam incoming laser beam

[0172] Secondary beam guidance device

[0173] focus

[0174] Main propagation direction first mirror element second mirror element

[0175] Intermediate laser beam

[0176] Beam path

[0177] Longitudinal center axis

[0178] Longitudinal center axis

[0179] Longitudinal center axis

[0180] Paraboloid

[0181] parabola

[0182] axis of rotation

[0183] Vertex first mirror element second mirror element

[0184] Longitudinal center axis

[0185] Longitudinal center axis

[0186] Intermediate laser beam

[0187] Longitudinal center axis

[0188] Beam path

[0189] axis of symmetry first point second point

[0190] Normal

[0191] marginal ray

Claims

Patent claims 1. Laser system for generating secondary radiation (108) by interaction of a focused primary laser beam (104) with a target material (106), comprising - a laser beam source (102) for providing a raw laser beam (110) comprising ultrashort laser pulses, - a beam guiding device (112) for forming the focused primary laser beam (104) from the raw laser beam (110), wherein the focused primary laser beam (104) is directed onto a target area (114) in order to interact with a target material (106) arranged in the target area (114), wherein - the beam guiding device (112) has a beam focusing device (126) which is designed to form the primary laser beam (104) by focusing a laser beam (129) entering the beam focusing device (126), wherein the laser beam (129) entering the beam focusing device (126) is based on the raw laser beam (110) or corresponds to the raw laser beam (110) and wherein - the beam focusing device (126) has at least two spherical mirror elements (154, 156; 176, 180) spaced apart from one another, characterized in that the beam focusing device (126) has a numerical aperture between 0.001 and 0.01, provided that the primary laser beam (104) propagates in a medium with a refractive index of less than 1.

01.

2. Laser system according to claim 1, characterized in that a focus (131) of the primary laser beam (104) has a focus diameter of at least 100 pm and / or at most 500 pm.

3. Laser system according to claim 1 or 2, characterized in that a beam path (164) within the beam focusing device (126) has no focus.

4. Laser system according to one of the preceding claims, characterized in that a diameter (ds) of the laser beam (129) entering the beam focusing device (126) is between 15 mm and 100 mm and in particular between 20 mm and 30 mm.

5. Laser system according to one of the preceding claims, characterized in that the beam guiding device (112) has a beam adjusting device (120) which is designed to adjust the diameter (ds) of the laser beam (129) entering the beam focusing device (126) by changing a diameter (di) of the raw laser beam (110) entering the beam guiding device (112).

6. Laser system according to one of the preceding claims, characterized in that the beam focusing device (126) has a first mirror element (154; 176) onto which the laser beam (129) entering it strikes, and in that the beam focusing device (126) has a further mirror element (156; 182) from which the focused primary laser beam (104) emanates, wherein at least one intermediate laser beam (162; 184, 186, 188) runs between the first mirror element (154; 176) and the further mirror element (156; 182).

7. Laser system according to claim 6, characterized in that the first mirror element (154; 176) onto which the laser beam (129) entering the beam focusing device (126) strikes is a spherical mirror element, and / or that the further mirror element (156; 182) from which the primary laser beam (104) emanates is a spherical mirror element.

8. Laser system according to claim 6 or 7, characterized in that the first mirror element (154; 176) onto which the laser beam (129) entering the beam focusing device (126) strikes, is concave and / or that the further mirror element (156; 182) from which the primary laser beam (104) emanates is convex.

9. Laser system according to one of the preceding claims, characterized in that the laser pulses of the raw laser beam (110) have a pulse duration between 10 fs and 300 fs, and / or that the laser pulses of the raw laser beam (110) have a pulse energy between 1 mJ and 20 mJ.

10. Laser system according to one of the preceding claims, characterized in that a wavelength of the raw laser beam (110) is between 500 nm and 2500 nm and preferably between 900 nm and 1100 nm.

11. Laser system according to one of the preceding claims, characterized by a gas-tight chamber (118) in which the target area (114) for arranging the target material (106) is positioned, wherein the chamber (118) has a passage element (119) for coupling a laser beam into the chamber (118), on which the primary laser beam (104) is based or which corresponds to the primary laser beam (104), and wherein a negative pressure and in particular a vacuum is formed in the chamber (118).

12. Laser system according to claim 11, characterized in that the passage element (119) has an anti-reflective coating, which is designed in particular as a nanotexturing and / or as a moth-eye structure.

13. Laser system according to claim 11 or 12, characterized by a shielding element (117) which is arranged between the passage element (119) and the target material (106), wherein the shielding element (117) is used for spatially shielding an optical component of the laser system and in particular for shielding the passage element (119) is arranged in front of the target material (106).

14. Laser system according to one of the preceding claims, characterized in that the beam guiding device (112) has a beam correction device (124) for forming a corrected laser beam (128) from a laser beam (123) entering the beam correction device (124), wherein a beam position stabilization is carried out by means of the beam correction device (124) in order to provide the corrected laser beam (128) with a corrected and / or stabilized beam position, and in particular characterized in that a wavefront correction of the laser beam (123) entering it is carried out by means of the beam correction device (124) in order to provide the corrected laser beam (128) with a corrected wavefront.

15. A method for generating secondary radiation (108) by interaction of a focused primary laser beam (104) with a target material (106), in which - the target material (106) is or is arranged in a target area (114), - a raw laser beam (110) is provided by means of a laser beam source (102), which has ultrashort laser pulses, - the focused primary laser beam (104) is formed from the raw laser beam (110) by means of a beam guiding device (112), wherein the focused primary laser beam (104) is directed onto the target area (114) and interacts with the target material (106) arranged in the target area (114), wherein - the beam guiding device (112) has a beam focusing device (126) which forms the primary laser beam (104) by focusing a laser beam (129) entering the beam focusing device (126), wherein the laser beam (129) entering the beam focusing device (126) is based on the raw laser beam (110) or corresponds to the raw laser beam (110) and wherein - the beam focusing device (126) has at least two spaced-apart spherical mirror elements (154, 156; 176, 180), characterized in that the beam focusing device (126) has a numerical aperture between 0.001 and 0.01, provided that the primary laser beam (104) propagates in a medium with a refractive index of less than 1.01.

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