• 专利附录
  • 专利说明
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    • 专利摘要:
    The invention proposes a dielectric mirror (1) which has a high destruction threshold when irradiating with high power ultrashort laser pulses, on the one hand, and a large bandwidth dispersion bandwidth. , on the other hand. A dielectric mirror (1) according to the invention comprises a stack of layers (3) with a succession of layers having different refractive indices, which act as a reflection interference filter, the layers consisting of at least three different materials having different destruction thresholds.
    公开号:FR3025612A1
    申请号:FR1558184
    申请日:2015-09-03
    公开日:2016-03-11
    发明作者:Dirk Apitz
    申请人:Schott AG；
    IPC主号:
    专利说明:

    [0001] The invention relates generally to optical systems for laser lasers. More particularly, the invention relates to mirrors with a high destruction threshold, for the reflection of high power laser pulses. For high-power laser active media, particularly in the field of research, material processing or applications in the field of nuclear fusion, the tendency to increase the power continues. Examples include the ELI, Apollo and Petawatt projects. To reach ever higher powers, up to the range of petawatts, we increase the energies of the pulses and reduces the duration of the pulses. Typical pulse times are now in the range of 10 to 200 femtoseconds. Pulses with this duration can not be directly amplified and must be extended, for example using networks to reach durations of the order of one nanosecond, to be amplified in this form in active laser environments and then to be new compresses. On the other hand, these pulses have no individual wavelength and are therefore not monochromatic, as is known from conventional lasers, for example continuous emission. On the contrary, these pulses consist of an entire spectrum of wavelengths. This results from the Fourier transformation between frequencies (wavelengths) and the pulse duration, respectively of the Heisenberg uncertainty principle. To compress, after amplification, again a pulse of the order of one nanosecond or to cause the interaction of light and matter, all the wavelengths of the pulse must therefore be present at the same time. In the absence of components with the shortest or the longest wavelengths, the compressed pulse will no longer have a duration of a few femtoseconds, but will be much longer. However, the materials have a dispersion, that is to say that the speeds of light with different wavelengths are generally different. As a result, the spectrum diverges in time and space, and the different components of the spectrum can no longer be compressed together to form a pulse of the order of the femtosecond. To conserve as such the femtosecond pulses (to prevent them from diverging), to be able to compress longer pulses and to obtain femtosecond pulses, or to interact with one another. femtosecond pulses with matter, at a precise location, the optical components that transmit these laser pulses must have as little dispersion as possible over a sufficiently wide wavelength range, that is, that is, the components of the light pulse must be transmitted at the same speed. For example, short wavelength components should not pass through a dispersive medium at a slower rate than long wavelength components. It is therefore an object of the invention to provide a mirror which, on the one hand, best meets the above-mentioned requirements for a low dispersion of group delay (Group Delay Dispersion - GDD, hereinafter simply referred to as "dispersion"). "), over the wide range of wavelengths, and which is also adapted to reflect pulses of high intensity and maximum intensity. This object is achieved by a dielectric mirror which comprises a stack of layers with a succession of layers having different refractive indices, which act as a reflection interference filter, the layers being made up of at least three different materials having thresholds. of destruction, the destruction threshold being the energy per area of a laser pulse directed at the material, from which the material is destroyed, a first material among the three materials having the lowest refractive index and the highest destruction threshold, and the second material having a destruction threshold higher than the third material, knowing that at least once inside the stack of layers, a layer is formed with the second and third material, and knowing that the composition of the layer varies in the direction perpendicular to its interfaces, whereas on an interface of the layer there is the second material and on the opposite interface there is the third material, the variation of the composition being chosen such that the luminous intensity of the standing wave of a laser pulse passing through the stack of layers and reflected by the dielectric mirror is higher in the second material than in the third material. According to an advantageous embodiment of the invention, the layer with the second and the third material comprises two layers, one layer of which is a layer of the second material and the other layer is a layer of the third material. According to another advantageous embodiment, it is provided that in the layer with the second and the third material, layers of the second and third materials succeed each other, the succession of layers being chosen such that the light intensity of the wave Stationary of a laser pulse passing through the stack of layers and reflected by the dielectric mirror is higher in the layer of the second material than in the layer of the third material. According to another advantageous embodiment, the layer with the second and the third material comprises at least one additional layer of another material, the succession of layers being preferably chosen such that along a direction, the thresholds of laser destruction of the layers materials increase successively. Preferably, the second and third materials are chosen so that the destruction threshold of the second material is at least 1.5, preferably at least a factor 2, greater than the destruction threshold of the second material. third material. According to another aspect of the invention, the dielectric mirror has at least one of the following characteristics: the first material comprises at least one of the following materials: SiO 2, MgF, AlF 3, BaF 2, CaF 2 YF 3, YbF 3, CeF 3 , DyF3, GdF3, LaF3, ThF4, Na3AlF6, Al2O3, or is a mixture which contains at least one of these materials, the second material comprises at least one of the following materials: HfO2, Sc203, Ta2O5, Y2O3, ZrO2 , MgO, ZnO, the second material contains as components the first and the third material, the third material comprises at least one of the following materials: TiO 2, Nb 2 O 5, Ta 2 O 5, ZrO 2, or a mixture which contains at least least one of these matters. According to another advantageous embodiment, the stack of layers furthermore has an alternating succession of layers of the first and third materials.
    [0002] According to still another embodiment, the thicknesses of the layers of the stack of layers are designed such that the extreme maximum of the luminous intensity of a laser pulse inside the stack of layers is located in a layer of the first material, between the interfaces of this layer.
    [0003] An improvement of the invention provides that the layer in which the extreme maximum of the light intensity of a laser pulse is located has a higher layer thickness than the other layers of the first material. According to another advantageous embodiment, the layers 25 of the stack of layers are constituted in such a way that for the two upper successions of the layers of the second and third materials, the ratios between the destruction threshold of the second and the third material third respective material and the maximum light intensity in the layers differ by up to 25% between them.
    [0004] According to another advantageous embodiment, the succession of layers and the layer thicknesses of the stack of layers are chosen such that at the surface of the dielectric mirror, the light intensity has a minimum or that the light intensity 3025612 does not exceed 10% of the extreme maximum of the luminous intensity at the surface. According to another advantageous embodiment, there is provided in the stack of layers at least once a layer of the second material which is adjacent to a layer of the first material but does not abut a layer of the third material. Another advantageous embodiment provides that inside the layer with the second and the third material, the second and third materials are mixed, the ratio of the portions of the second and third material being continuously modified at least along a portion of length. The invention and the problem on which the invention is based will be explained in detail hereinafter and with reference to the accompanying drawings, in which: FIG. 1 schematically represents a laser pulse, before and after passing through a FIG. 2 shows the spectral reflectivity of two dielectric mirrors with different layer materials; FIGS. 3 and 4 show examples of successions of 20 layers of dielectric mirrors, as well as the luminous intensity (modulus squared of the IEC 60050 - International Electrotechnical Vocabulary - Details for IEV number 511-21-21 Electro-magnetic field for the incidence inside the mirror layer stacks FIG. 5 represents an example of a stack of layers of a dielectric mirror according to the invention, as well as the distribution of the luminous intensity of a laser pulse in the stack of layers, FIGS. 6 and 7 show graphs of the laser destruction threshold of different materials, as a function of the refractive index, FIG. 8 represents the dispersion as a function of the wavelength of different stacks of layers according to FIGS. 3 to 5, FIG. 9 represents a variant of the embodiment FIG. 10 shows a detail of a stack of layers according to another exemplary embodiment, FIG. 11 represents an exemplary embodiment with a composition gradient, and FIG. the bandwidth of the dispersion as a function of the wavelength of different layer stacks for a compositional gradient system. The graph of FIG. 1 schematically illustrates the influence of dispersive media on broadband ultrashort laser pulses. More particularly, Figure 1 shows a laser pulse 2 on a time scale before and after passing through a dispersive medium. Before the passage (shown on the left in the graph as a function of time), the laser pulse has a certain duration and intensity. After passing (right), the spectral components separate on the time scale. In the example, the red and blue components are designated respectively by "R" and "B" and symbolized by hatched surfaces. In the example shown, the dispersion of the medium is such that the short wavelength spectral components travel the material more slowly than the long wavelength spectral components, so that after the passage, the spectral components blue are behind the red components. Although the integrated pulse energy is substantially conserved with the exception of losses due to absorption and dispersion, the maximum intensity or power decreases due to the time dispersion of the pulse. laser 2. However, it is precisely the pulse power that is generally decisive for high power laser applications.
    [0005] To reflect high power laser pulses, dielectric mirrors are frequently used. Apart from dimensions, flatness (reflected wavefront) and surface roughness, an example of a typical pulse-like mirror specification of the femtosecond range may be as follows: Angle of incidence: 45 ° Light polarization: 5 Wavelength range: 700 to 900 nm Reflectivity:> 99.5% Dispersion (GDD): at most + 50 fs ^ 2 Laser Destruction threshold:> 0.5 J / cm 2 (pulse duration 20 fs, wavelength 800 nm).
    [0006] A very simple dielectric mirror consists of a succession of layers with a high refractive index and a low refractive index, at the interfaces of which a portion of the light is reflected and interferes with itself, the corresponding layer thicknesses. exactly at the optical path length of a quarter of the design wavelength. Thus, a mirror of this type can only have a limited bandwidth because for substantially different wavelengths the condition for the interference is fulfilled differently.
    [0007] To create a mirror with a larger bandwidth, as would be advantageous for the laser pulse reflection of the femtosecond domain, two measures can be taken: i) the difference in refractive index is increased between the two types of layers, i.e., between the high and low index refractive index layers, ii) or a modulated mirror (referring to the frequency change), in which layer thicknesses, and where light of one wavelength is reflected in one area (e.g., upper layers) and light of another wavelength is reflected in another (vertical) area of the mirror (for example deeper layers). Figure 2 illustrates the effect of increasing the difference in refractive index using the spectral reflectivities of two different dielectric mirrors.
    [0008] The dashed line of FIG. 2 represents the reflectivity of a SiO 2 / HfO 2 layer system, and the solid line represents the reflectivity of a SiO 2 / TiO 2 layer system. The difference in refractive index between the TiO 2 layers and the SiO 2 layers is greater than the difference between the HfO 2 and SiO 2 layers. As shown in FIG. 2, the bandwidth of the SiO 2 / TiO 2 layer system is therefore much larger. On the other hand, TiO2 has a much lower laser destruction threshold than HfO2.
    [0009] Therefore, both types of mirrors have considerable disadvantages for use as high power ultrashort pulse mirrors (described in the "dilemmas" section hereinafter). In the layer packet of a dielectric mirror, a standing wave is formed, so that in some places an increased electric field / light intensity is created. These peaks constitute the points intended for breaking with respect to the laser destruction threshold. At an interface, the destruction threshold is generally lower than inside a layer. Multiphoton absorption causes energy absorption of the material and, with sufficient power, destruction of the material. The resistance of a material depends to a large extent on its band gap. In materials with greater bandgap, the destruction threshold is generally higher than in low bandgap materials. Therefore, SiO 2 is more stable than materials with a higher refractive index. For a dielectric mirror well suited to the high-energy pulses of the femtosecond (or nanosecond) domain which must be compressed up to the femtosecond, it is preferable to have a high reflectivity, a high bandwidth of reflectivity; - low dispersion of group delay; - high bandwidth of group delay dispersion; - low maximum values of electric fields; - large forbidden bands of materials; large electric fields found in materials with large bandgap, 5 - large electric fields that are not at the interfaces but inside the layers. Under these conditions, some dilemmas occur which will be illustrated by taking the TiO2 material with a high refractive index as an example: TiO 2 would be advantageous for a high reflectivity (because high refractive index), TiO 2 would therefore also be advantageous for large bandwidths, cf. Figure 2, - TiO2 would be advantageous for low dispersion.
    [0010] A high refractive index, as found in TiO 2, means that a smaller number of layers participate in the reflection and that the difference in path (dispersion) of waves of different lengths becomes smaller. However, TiO 2 has disadvantages because the forbidden band and the destruction threshold are low. - Hf02 or Sc203 would be advantageous because the forbidden band and the destruction threshold are larger, - on the other hand, Hf02 or Sc203 have disadvantages because the refractive index is lower.
    [0011] Therefore, a SiO 2 / TiO 2 mirror does not meet the specification of the laser destruction threshold, and a SiO 2 / H 2 O 2 mirror does not meet the bandwidth specification of the dispersion. FIG. 3 shows in this respect the distribution or variation of the square of the electric field strength (which is proportional to the light intensity) in a dielectric mirror formed of layers of SiO 2 / TiO 2 arranged alternately. The layers are designated correspondingly by 'SiO2' and 'TiO2', and the interfaces between the layers are indicated by dashed lines. Variation 6 of the field strength is shown in Figure 3, and in the following figures for the design wavelength, respectively the wavelength with the maximum intensity of the laser pulse. The variation of the integrated total intensity as a function of the spectrum of the laser pulse does not differ significantly, so that it is sufficient to study the design wavelength. More particularly, Figure 3 shows the eight upper layers of a SiO2 / TiO2 mirror having a large number of layers. The mirror surface is at a zero depth, and the mirror environment is to the right of zero. On the left side, at depths greater than 900 nanometers (and therefore not shown in Figure 3), follow other layers of SiO2 / TiO2 and finally the substrate on which are deposited layers of the dielectric mirror.
    [0012] The layer thicknesses are chosen in such a way that the maximum of the electric field is shifted in the second upper layer of SiO 2. The maximum corresponds to about 75% of the intensity of the incident wave and is at a depth of about 150 nm below the surface. In the TiO 2 layer above, the maximum is on an interface at about 30% (at a depth of about 70 nm). In order to further raise the destruction threshold, the thicknesses of the layers could be adapted so that the maximum intensity is further away from the upper TiO 2 layer, so that the ratio is rather 1: 3 to 1: 6. .
    [0013] By Bellum et al., "Reactive, Ion-Assisted Deposition of E-Beam Evaporated Ti for High Refractive Index TiO2 Layers and Laser Damage Resistant, Broad Bandwidth, High Reflection Coatings", Applied Optics, Vol. 53 (4), A205-A211, it is known to substitute high-refraction TiO2 layers of a dielectric mirror in HfO2 in which the largest electric fields occur. To obtain a destruction threshold higher than that of the mirror shown in FIG. 3, it is therefore possible for example to replace the two or three upper TiO 2 layers, but not all the layers of TiO 2, with layers of HfO 2, so that the electric field is carried by a more resistant material, in the layers where it is still relatively large. FIG. 4 shows in this respect an example of a succession of 5 layers of such a dielectric mirror, as well as the luminous intensity inside the stack of layers. In this example, the maximum values of the electric field are as follows: 90% in the SiO 2 layer (at about 150 nm) and 40% in the HfO 2 layer, on an SiO 2 -HfO 2 interface (at about 70 nm) . The destruction threshold is here already higher than in the example of the layer diagram of FIG. 3, because part of the HfO 2 is used instead of the TiO 2 and the first has a higher destruction threshold. However, this decreases the bandwidth or increases the dispersion.
    [0014] A dielectric mirror according to the invention comprises a stack of layers with a succession of layers having different refractive indices, preferably layers with alternating higher and lower refractive indexes, which act as an interference filter. reflective, the layers consisting of at least three different materials having different destruction thresholds, a first material among the three materials having the lowest refractive index, and the second material having a higher destruction threshold than the third material, knowing that at least once inside the stack of layers, a layer 14 is formed with the second and the third material, and knowing that the composition of the layer 14 (Fig. 5) varies in the direction perpendicular to its interfaces, so that on an interface 140 of layer 14 there is the second material and su the opposite interface 141 of the layer 14 there is the third material, the variation of the composition being chosen such that the luminous intensity of the standing wave of a laser pulse 2 passing through the stack of layers 3 and reflected by the dielectric mirror 1 is higher in the second material than in the third material.
    [0015] A preferred and simple improvement of the invention lies in the fact that the layer 14 with the second and the third material comprises two layers, knowing that one layer is a layer of the second material and the other layer is a layer. layer of the third material. Preferably, the layers of the second and third materials succeed one another and therefore have a common interface. In other words, the variation of the composition of the layer 14 is here a discontinuous variation of the composition during the passage of the layer of the second material to the layer of the third material. The layers of the second and third materials therefore form sub-layers of the high refractive index layer 14. Consequently, the succession of layers is chosen so that the luminous intensity of a standing wave of a laser pulse passing through the stack of layers and reflected by the dielectric mirror is higher in the layer of the second material than in the layer of the third material. By "destruction threshold" is meant the energy per area of a laser pulse sent to the material, from which the respective material is destroyed.
    [0016] As a result, the layers of the second and third materials are pairwise associated. The two layers together can be considered respectively as a layer 14 with high refractive index, knowing that high index and low index layers alternate in the stack of layers.
    [0017] In general, not only is the destruction threshold of the second material higher than that of the third material, but in addition the second material preferably has a lower refractive index than the third material. As explained above, this is typically determined by the band gap of the respective material. A large band gap is often accompanied not only by a higher destruction threshold but also a lower refractive index. The distribution of field strength, or light intensity, which is proportional to the square of the field intensity, also depends somewhat on the wavelength. However, the aforementioned condition is in any case in particular for a laser pulse whose average wavelength corresponds to the design wavelength of the dielectric mirror. If appropriate, the angle of incidence and polarization of the laser pulse should also be taken into account if the dielectric mirror is designed for a precise angle of incidence, for example 45 °, and polarization. In the example shown in FIG. 4, three different materials are also used, but the materials with the lower destruction thresholds do not follow each other. However, the examples of Figures 3 and 4 also have characteristics which are also advantageous for a dielectric mirror according to the invention. This will be explained in detail later.
    [0018] FIG. 5 shows an example of a stack of layers 3 of a dielectric mirror 1 according to the invention, as well as the distribution of the light intensity of a laser pulse in the stack of layers 3. Typically, the dielectric mirror 1 comprises, besides the stack of layers 3, a substrate on which the stack of layers 3 is deposited. Since FIG. 5 only shows part of the stack of layers 3 with its surface at a depth of 0 nm, the substrate is not shown. The exemplary embodiment is based on the fact that the part of the high refractive index layer in which the light intensity exceeds a defined value (for example, in the example shown, a value of about 20%) , consists of HfO2, and the remainder of TiO2. As a result, the maximum values of the electric field are as follows: 80% in the SiO 2 layer, 35% in the HfO 2 layer, on an SiO 2 -HfO 2 interface, and 20% in the TiO 2 layer, on a SiO 2 layer, TiO2-HfO2 interface. Thus, the properties of the two high refractive index materials are exploited optimally. Of course, the invention is not limited to the specific example shown or the materials it uses. Instead of the HfO 2 one can for example use another material with a higher destruction threshold than TiO 2, or mixtures of several of these materials. In this example, the SiO 2 layers thus form the layers 11 of the first material, the HfO 2 layers form the layers 12 of the second material and finally, the TiO 2 layers 5 form the layers 13 of the third material. The layers 12, 13 of the second and third successive materials respectively form a layer 14 whose composition varies in the direction perpendicular to its interface, so that the second material is present on an interface 140 of the layer 14, and the The third material is present on the opposite interface 141 of the layer 14. Due to the succession of the layers 12, 13, the variation of the composition of the layer 14 is chosen such that the luminous intensity of the standing wave of the a laser pulse 2 traveling through the stack of layers 3 and reflected by the dielectric mirror 1 is higher in the second material, that is to say here specifically in the layer 12, than in the third material, That is, the layer 13 is here. In accordance with an improvement of the invention, it is, moreover, in general advantageous that layers of the first and third materials succeed each other at least twice in the course of the invention. the stack of layers. This is advantageous because the field strength of a high power laser pulse generally decreases only to a depth of a few layers to a value below the destruction threshold of the third material.
    [0019] The example shown in FIG. 5 provides exactly twice successive layers 12, 13 which form a layer 14 with the second and the third material. More particularly, the second and third layers, as well as the fifth and sixth layers, counting from the top layer, form such pairs of successive layers, the second and fifth layers being made of the third material, and the third and sixth layers being made of the second material. However, depending on the structure of the stack of layers or the requirement for the pulse intensity to be reflected, more than two of these successions or pairs of layers of the second and third materials can also be used, That is, more than two layers 14. In order that the combination of the layers 12, 13 of the second and third materials can effectively increase the destruction threshold and the bandwidth of the dispersion of the dielectric mirror, it is furthermore, that the destruction thresholds of the material for the layers 12 are as high as possible and the refractive index of the third material for the layers 13 is as high as possible. According to an improvement of the invention, it is then expected that the destruction threshold of the second material is greater than at least a factor of 1.5, preferably at least a factor of 2, at the destruction threshold of the third material. . In the example of FIG. 5, the destruction threshold of HfO 2 as the second material is about a factor 2.3 higher than the TiO 2 destruction threshold. To increase the destruction threshold, it is provided, as described above, to choose the succession of layers 12, 13, consisting of the second and third material, so that the light intensity of the laser pulse passing through the stack of 20 layers, respectively of the standing wave that is formed, or higher in the layers 12 formed of the second material in the layers 13 formed of the third material. In the example shown in FIG. 5, the maximum values of the luminous intensity are therefore closer to the layers 12, whereas the layer 13 adjacent to the layer 12 is located in a region of lower luminous intensity, on the sidewall descending towards the surface of the mirror. For the layers 11, 14, it is possible to provide other materials than the SiO 2, the HfO 2 and the TiO 2 used in the example of FIG. 5. FIG. 6 shows the laser destruction thresholds for various oxidic materials, in particular function of the refractive index. This graph provides criteria for choosing the appropriate materials for the stack of layers. Among the materials shown, SiO 2 has the highest destruction threshold and a low refractive index. This material is therefore particularly suitable as the first material. In addition to SiO 2 as the first material with the lowest refractive index, aluminum oxide (Al 2 O 3) and fluorides such as aluminum fluoride (AlF 3) can also be used, for example. magnesium fluoride (MgF), barium fluoride (BaF2), calcium fluoride (CaF2), yttrium fluoride (YF3), ytterbium fluoride (YbF3), cerium fluoride (CeF3), dysprosium fluoride (DyF3), gadolinium fluoride (GdF3), lanthanum fluoride (LaF3), thorium fluoride (ThF4) and aluminum and sodium double fluoride (Na3A1F6). It is also possible to mix the aforementioned materials or to dope them with other elements or compounds. Very high refractive index materials, which are suitable as the third material, include, in addition to the titanium oxide (TiO 2) mentioned above, niobium oxide (Nb 2 O 5), zirconium oxide (ZrO 2) and tantalum oxide (Ta2O5), as well as zinc sulfide (ZnS), zinc oxide (ZnO) and zinc selenide (ZnSe) or a mixture containing at least one of these materials. Another suitable combination is, for example, SiO 2 as the first material, HfO 2 as the second material and Ta 2 O 5 as the third material. It is also possible to use these materials in pure form or as mixed oxide and / or doping.
    [0020] In addition to the mentioned Hf02, Sc203 is also particularly suitable as a second material. Similarly, tantalum oxide (Ta2O5) can be used. Other suitable materials are magnesium oxide (MgO), yttrium oxide (Y203), zirconium oxide (ZrO2) and zinc oxide (ZnO).
    [0021] A well-suited combination includes SiO 2 as the first material, Ta 2 O 5 as the second material and TiO 2 as the third material. As shown in both of the above examples, there are materials that can be used both as the second and as the third material, depending on the respective other materials. In the two examples above, Ta205 can be used in one case as a second material and in another case as a third material. It is the same for zinc oxide.
    [0022] With an almost identical refractive index, Sc203 has an even higher destruction threshold than HfO 2, but it is a very expensive coating material whose purity is further difficult to control. In accordance with an improvement, there can also be used as a constituent of the second material mixtures with Sc203 and / or HfO2 or a mixed oxide of the two materials, as well as additions of other materials, preferably oxidic materials. To obtain second materials with a higher destruction threshold than the third material, it is also conceivable to use mixtures or compounds of the first and second materials. Thus, the second material may for example be a mixed oxide of the first and third materials or contain a mixed oxide. It is also possible to imagine mixtures of the first material and materials which are suitable for the second layer (for example as a first material combination: Sio2, second material: SiO2: HfO2 and third material: TiO2). According to an improvement of the invention, it is therefore provided that the second material contains as constituent the first material and the third material. Here, it is particularly simple to make the second material as a mixture of the first and third materials. The first and / or third material may in turn be mixtures of two or more components. For example, if a mixture of AlF 3 and SiO 2 was used for the first material, and a mixture of TiO 2 with ZrO 2 for the third material, the second material could be made as a mixture of the first and the third material and would contain in this case the four constituents A1F3, SiO2, TiO2, ZrO2. If necessary, one or more additional constituents may be added to the constituents of the first and third materials.
    [0023] With a mixture of different constituents, the materials can be tailored to the variation in field strength, with respect to the destruction threshold. The graph of Fig. 7 shows this by taking as an example two mixtures, namely a mixture of SiO 2 with Nb 2 O 5 and, as a second example, a mixture of SiO 2 with ZrO 2. In FIG. 7, the destruction thresholds of the SiO 2, ZrO 2 and NbO 2 feed materials are additionally related to the respective refractive indexes. The destruction thresholds of the SiO 2 / ZrO 2 and SiO 2 / Nb 2 O 5 mixtures are given for different mixing ratios. In both mixtures, the refractive index decreases as the SiO 2 fraction increases, while at the same time the laser destruction threshold increases. These variations make it possible to choose a mixing ratio as a function of the place in the stack of layers and of the light intensity, respectively of the field intensity, prevailing at this point. In general, it is advantageous not to use the three materials continuously for the bandwidth. As in the examples shown in FIGS. 3 to 5, the stack of layers can always have an alternation of layers 11, 13 of the first and third materials. This is arranged in particular under the succession of layers of the second and third material, because here the light intensity is lower. In the example of FIG. 5, the succession of layers 11, 13 of the first and third materials starts with the seventh layer under the surface, that is to say below the second layer 12 of the second material. On the other hand, it is advantageous, as shown in the example shown in FIGS. 3 to 5, to provide the thicknesses of the layers 11, 12, 13 of the stack of layers in such a way that the maximum maximum 5, the where appropriate also the second maximum, the luminous intensity inside the stack of layers, in a layer 11 of the first material, lies between the interfaces of this layer. Thus, the location of the maximum field strength is placed in the material with the highest destruction threshold. In the examples shown in FIGS. 3 and 4, this layer is the third layer of the stack of layers, namely an SiO 2 layer, counting from the upper layer which is also an SiO 2 layer. The layer packet is here covered only by way of example with a layer of SiO 2. A mirror of this type can also be terminated directly by a high refractive index layer, for example a layer 13. A layer of SiO 2 to complete the stack of layers is particularly advantageous for protecting the surface of the stack. The design of the example of FIG. 5 is similar in this respect, and here the second high refractive index layer is divided into two partial layers in the form of a succession according to the invention of layers 11, 12 second and third material. Therefore, the layer 11 of the first material with the maximum of the light intensity is here the fourth layer of the top.
    [0024] As can be seen in FIGS. 3 to 5, both the maximum values and the minimum values of the field strength in high refractive index layers are generally on the interfaces thereof. This is advantageous for increasing on the one hand the destruction threshold of the mirror and on the other hand improving the reflectivity and the bandwidth. In order for the extreme maximum of the luminous intensity of a laser pulse to be shifted in a layer 11 of the first material, it is provided for this layer 11, preferably, a greater layer thickness than for the other layers 11 of the first material.
    [0025] According to still another improvement of the invention, which is also implemented in the embodiments of FIGS. 3 to 5, the succession of layers and the layer thicknesses of the stack of layers 3 are chosen such that the The light intensity has a minimum value on the surface of the dielectric mirror. It is not necessary for the minimum to be exactly at the surface, as is the case in the exemplary embodiments shown. However, it is advantageous that the luminous intensity at the surface does not exceed 10% of the extreme maximum of the luminous intensity. This improvement makes it possible to obtain that dirt or defects in the low destruction threshold surface do not cause destruction of the mirror. To design a dielectric mirror according to the invention, the ratios between the destruction threshold and the electric field value can be determined and then the layer thicknesses can be selected so that the electric fields in the respective materials are so large that there is no preference for more points destined for rupture. Ideally, the probability of destruction is the same for all maximum values per material and on all interfaces. If this is not the case, it is not necessary that this condition be fulfilled exactly, and most of the time it is sufficient that the ratios between the destruction threshold of the second and the third material and the maximum light intensity in the layers. differ by not more than 25% between them.
    [0026] For locations within the stack of layers, where the maximum values of the light intensity are below the destruction threshold of the third material, it is no longer necessary for this condition to be fulfilled. In the example shown in FIG. 5, the cited condition applies for the region of the stack of layers 20 in which the two extreme maximums of the light intensity are located. The third maximum already has a field strength which is below the threshold of destruction of the third material, that is to say here of TiO2. Therefore, the stack of layers is thus constructed such that the above-mentioned conditions, with the ratio deviation of at most 25%, apply to the two upper successions of the layers 12, 13 consisting of the second and the third material. According to an improvement of this embodiment, this also applies to the layer 11 made of the first material, in which the maximum light intensity appears. The part of the high refractive index layer which consists respectively of the two layers 12, 13 and in which the electric field is greater than a defined value (approximately 20% in the example of FIG. 5) consists of HfO 2 and the remaining part of 3025612 TiO2. As a result, the maximum values of the electric field according to the example embodiment are 80% in the SiO 2 layer 11, 35% in the HfO 2 layer 12, on an SiO 2 -HfO 2 interface, and 20% in the SiO 2 layer. the TiO 2 layer, on a TiO 2 -HfO 2 interface. Thus, the properties of the two high refractive index materials are exploited optimally. With reference to FIG. 8, the effects of such a configuration of the layers on the dispersion will be described below. For this purpose, FIG. 8 shows the dispersion of the group delay (GDD) as a function of the wavelength for three stacks of different layers. Curve 20 (dashed line) represents the dispersion of a stack of SiO 2 / TiO 2 layers as shown in FIG. 2. Curve 21 (long dashed line) represents the dispersion of a stack of 4 according to FIG. 4, in which, starting from the example shown in FIG. 3, the two upper TiO 2 layers are replaced by HfO 2 layers. Finally, the curve 22 (solid line) represents the dispersion of a dielectric mirror according to the invention with a succession of layers according to FIG.
    [0027] Curve 20 shows a very good bandwidth, but the associated dielectric mirror, comprising a stack of SiO 2 / TiO 2 layers, has a low destruction threshold. Curve 21 represents an optimal bandwidth, if the high refractive index layers consist respectively of a material. (The high refractive index layers consist of either only TiO 2 or, where the electric fields are high, only HfO 2.) Curve 22 shows the dispersion, if high refractive index layers are distributed over the surface. HfO 2 and TiO 2 as intended according to the invention. The destruction threshold is approximately the same as in a mirror according to FIG. 4. However, according to curve 21, the bandwidth of the dispersion is almost as great as for curve 20. With regard to the width of band for a dispersion ranging from 3025612 22 -50 to 50 fs2, in the examples of FIGS. 3 to 5, with the associated dispersion curves 20 to 22, the following values are obtained: Two upper layers with a high refractive index Hf02, otherwise TiO2 (Fig. 4) All high refractive index TiO2 layers (Fig. 3) High refractive index mixed layers in HfO 2 / TiO 2 succession (Fig. 5) of [nm] 732.5 728.3 728 , 3 to [nm] 913.5 921.4 920.1 Width of 181 194.8 191.8 band [nm] The arrangement of the layers of a dielectric mirror 15 according to the invention therefore allows in the example of Figure 5 to gain more than 10 nm bandwidth of the dispersion (in the range of -50 to 50 fs2), while the threshold of destru ction remains the same. FIG. 9 shows a variant of the embodiment shown in FIG. 5. This variant only provides a single layer 14 with a high refractive index, with the second and the third material. As in the example shown in FIG. 5, this layer 14 consists of two successive layers 12, 13 formed from the second and third materials respectively. The upper layer, with a high refractive index, of the stack of layers 3 is not formed here of a layer 14 with the second and the third material but of an individual layer formed of the second material. This material (here also HfO 2) certainly has a lower refractive index than the third material (here TiO 2), but this layer, whose refractive index is slightly lower than that of the third material, has an influence relatively low on dispersion. On the other hand, the structure of the layers is weaker. For this embodiment of the invention, the following rule applies generally, without limitation to the specific embodiment shown: high refractive index and low refractive index layers alternate in the stack of layers, knowing that there too, at least once inside the stack of layers 3, a layer 14 is formed with the second and the third material, so that the composition of the layer 14 varies in the direction perpendicular to its interfaces, so that on an interface 140 of the layer 14 there is the second material and on the opposite interface 141 of the layer 14 there is the third material, and a layer 12 of the second material being present at least once and adjoining a layer 11 of the first material but not adjoining a layer 13 of the third material. This last layer of the second material forms an individual layer with a high refractive index. On the other hand, the invention is not limited to replacing a high refractive index layer with two layers 12, 13 having different laser destruction thresholds. It is also possible to continue the succession of layers of the second and third material. In other words, the succession of the layers 12, 13 of the second and third materials may be part of an additional succession with a layer of a fourth material, if necessary an additional layer of a fifth material, and and so on. It is then advisable that in the succession from the layer of the second material to the layers of other materials, the laser destruction threshold decreases successively, and the refractive index preferably increases, insofar as a value of The maximum intensity of the field intensity is not in the layer 14. FIG. 10 shows in this respect an exemplary embodiment of a stack of layers 3 in which the layer 14 with a high refractive index exhibits not only the layers 12 13 of the second and third materials but additionally a layer 16 of a fourth and a layer 17 of a fifth material. In general, without limitation to the embodiment, an improvement of the invention therefore provides that the layer 14 with the second and third material comprises two layers 12, 13, a layer 12 is a layer the second material and the other layer 13 is a layer of the third material, and the layer 14 with the second and the third material comprises at least one additional layer of additional material. The laser destruction thresholds of all the materials of the layer 14 are preferably different, and the succession of the layers 12, 13 in the layer 14 is preferably chosen such that along a direction, the destruction thresholds at the Laser layer materials increase successively.
    [0028] On the other hand, high refractive index layers can be replaced by varying numbers of high refractive index materials. Thus, the first high refractive index region could be a combination of three high refractive index materials, and the following replacements could be made of only two high refractive index materials. In the embodiments of the invention described so far, the layer 14 was formed of a succession of layers having different laser destruction thresholds. In other words, in this embodiment, the layer 14 with the second and the third material (and possibly other materials) constitutes a stack of elementary layers in the layer stack 3 of the dielectric mirror 1 However, as seen in FIG. 7, it is possible to continuously vary the laser destruction threshold, by combining two materials with different destruction thresholds, depending on the mixing ratio of these two materials, which typically also leads to a continuous variation of the refractive index. Therefore, according to another embodiment of the invention, it is provided that within the layer 14 with the second and the third material, the second and the third material are mixed, the ratio of the parts the second and third material being continuously changed in the direction perpendicular to the interfaces 140, 141 of the layer 14, at least along a portion of length. An example of a corresponding embodiment is shown in FIG.
    [0029] As in the other embodiments, the composition of the layer 14 in the direction perpendicular to its interfaces 140, 141 is modified, so that the second material 7 is on an interface 140 of the layer 14, and the third material 8 is on the opposite interface 141 of the layer 14. But unlike the embodiments described so far, the variation of the composition is here continuous. This is illustrated by the example shown in FIG. 11, by the third material 8 symbolized by hatching, whose density decreases in the direction going from the interface 140 to the interface 141. The third material is here on the interface 141, and the second material on the opposite interface 140. This embodiment can also be combined with the other embodiments described so far, comprising discrete layers 12, 13. Thus, it is for example possible to provide between the layers 12, 13 of the second and third material, a transition zone with a continuous variation of the composition. Finally, Figure 12 shows the bandwidth of the dispersion as a function of the wavelength of different layer stacks for a compositional gradient system.
    [0030] The composition gradient system is as shown in FIG. 11. The dispersion as a function of the wavelength is very similar to the dispersion shown in FIG. 8. The curve 30 (dashed line), the Curve 31 (long dashed line) and Curve 32 (solid line) represent the dispersion of a compositional gradient system with continuous variation of the composition. Curve 30 is a SiO 2 / TiO 2 composition gradient system, curve 31 is a compositional gradient system which has a HfO 2 layer, and curve 32 represents the dispersion of a dielectric mirror in accordance with FIG. invention.
    [0031] 3025612 26 1 List of references 2 Dielectric mirror Laser pulse 5 3 Stack of layers 5 Extreme maximum of light intensity 6 Variation of square of electric field strength 7 Second material 8 Third material 10 11 Layer of first material 12 Layer of a Second Material 13 Layer of a Third Material 14 Layer with a Second and Third Material 16 Layer of a Fourth Material 17 Layer of a Fifth Material 20, 21, 22 Dispersion Curves 30, 31, 32 Dispersion curves
    权利要求:
    Claims (13)
    [0001]
    REVENDICATIONS1. A dielectric mirror (1) which comprises a stack of layers (3) with a succession of layers having different refractive indices, which act as a reflection interference filter, the layers consisting of at least three different materials having thresholds of different destruction, the destruction threshold being the energy per area of a laser pulse directed at the material, from which the material is destroyed, a first material among the three materials having the lowest refractive index and the destruction threshold, and the second material having a destruction threshold higher than the third material, knowing that at least once inside the stack of layers (3), a layer (14) is formed with the second and the third material, and knowing that the composition of the layer (14) varies in the direction perpendicular to its interfaces, so that on an in the surface (140) of the layer (14) there is the second material and on the opposite interface (141) of the layer (14) there is the third material, the variation of the composition being chosen such that the intensity luminous wave of the stationary wave of a laser pulse (2) passing through the stack of layers (3) and reflected by the dielectric mirror (1) is higher in the second material than in the third material.
    [0002]
    2. Dielectric mirror according to the preceding claim, characterized in that the layer (14) with the second and third material comprises two layers (12, 13), one layer (12) is a layer of the second material and the other layer (13) is a layer of the third material.
    [0003]
    3. Dielectric mirror according to the preceding claim, characterized in that in the layer (14) with the second and third material, layers (12, 13) of the second and third material succeed one another, the succession of layers (12, 13) being chosen such that the luminous intensity of the standing wave of a laser pulse (2) passing through the stack of layers (3) and reflected by the dielectric mirror (1) is higher in the layer of second material than in the layer (13) of the third material.
    [0004]
    4. Dielectric mirror according to one of the two preceding claims, characterized in that the layer (14) with the second and the third material comprises at least one layer (16, 17) additional of another material, the succession of wherein the layers (12, 13, 16, 17) are preferably selected such that along a direction, the laser destruction thresholds of the layer materials (12, 13, 16, 17) increase successively. 10
    [0005]
    5. dielectric mirror (1) according to one of the preceding claims, characterized in that the second and the third material are chosen such that the destruction threshold of the second material is greater than at least a factor 1.5, preferably at least a factor of 2, at the destruction threshold of the third material. 15
    [0006]
    6. dielectric mirror (1) according to one of the preceding claims, characterized in that it has at least one of the following properties: - the first material comprises at least one of the following materials: SiO2, MgF, A1F3, BaF2 , CaF2 YF3, YbF3, CeF3, DyF3, GdF3, LaF3, ThF4, Na3A1F6, Al2O3, or is a mixture which contains at least one of these materials, - the second material comprises at least one of the following materials: HfO2, Sc203, Ta205, Y203, ZrO2, MgO, ZnO, the second material contains as components the first and the third material, the third material comprises at least one of the following materials: TiO2, Nb2O5, Ta2O5, ZrO2 or a mixture which contains at least one of these materials.
    [0007]
    7. dielectric mirror (1) according to one of the preceding claims, characterized in that the stack of layers (3) further has an alternating succession of layers (11, 13) of the first and third material.
    [0008]
    8. dielectric mirror (1) according to one of the preceding claims, characterized in that the thicknesses of the layers (11, 12, 3025612 29 13) of the stack of layers are designed so that the maximum extreme (5) the luminous intensity of a laser pulse (2) inside the stack of layers (3) lies in a layer (11) of the first material, between the interfaces of this layer (11).
    [0009]
    9. Dielectric mirror (1) according to the preceding claim, characterized in that the layer (11) in which is located the maximum extreme of the light intensity of a laser pulse (2) has a higher layer thickness than the other layers (11) of the first material.
    [0010]
    10. dielectric mirror according to one of the preceding claims, characterized in that the layers of the stack of layers (3) are formed such that for the two upper successions of the layers (12, 13) of the second and the 15 third material, the ratios between the destruction threshold of the second and third respective material and the maximum light intensity in the layers (12, 13) differ at most by 25% between them.
    [0011]
    11. dielectric mirror (1) according to one of the preceding claims, characterized in that the succession of layers and the layer thicknesses of the stack of layers (3) are chosen such that the surface of the dielectric mirror ( 1), the luminous intensity has a minimum or that the luminous intensity does not exceed 10% of the extreme maximum of the luminous intensity at the surface. 25
    [0012]
    12. dielectric mirror (1) according to one of the preceding claims, characterized in that there is provided in the stack of layers (3) at least once a layer (12) of the second material which is adjacent to a layer (11) of the first material but does not abut a layer (13) of the third material. 30
    [0013]
    13. Dielectric mirror according to one of the preceding claims, characterized in that inside the layer (14) with the second and third material, the second and the third material are mixed, the ratio of the shares of the second and the third material being continuously changed in the direction perpendicular to the interfaces (140,141) of the layer (14), at least along a length portion.
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    同族专利:
    公开号 | 公开日
    FR3025612B1|2018-11-30|
    DE102014113077B4|2019-11-14|
    CN105406336B|2020-05-29|
    US10209415B2|2019-02-19|
    US20160070041A1|2016-03-10|
    DE102014113077A1|2016-03-10|
    CN105406336A|2016-03-16|
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    优先权:
    申请号 | 申请日 | 专利标题
    DE102014113077.4|2014-09-10|
    DE102014113077.4A|DE102014113077B4|2014-09-10|2014-09-10|Dielectric mirror for high-power laser pulses|
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