WO2008125327A1 - Method and system for the reproducible positioning of a target object in the effective volume of a laser beam - Google Patents

Abstract

The invention relates to a method and a system for the reproducible positioning of a target object in the effective volume of a first, particularly pulsed laser beam, which can be focused into the effective volume by means of a focusing lens system (2) following a deflecting lens system (1), which is highly reflective for the wavelength of the first laser beam (I) and is permeable for at least one second wavelength, particularly a second laser beam (II), characterized in that the image of a mask element (5) is projected at the desired position (P) of a target object (4) to be positioned in the effective volume (3) through the deflecting lens system (1) using a second, particularly laser, beam, and the degree of defocusing of the image is determined, particularly minimized, on the target object (4) to be positioned.

Description

 Method and system for the reproducible positioning of a target object in the effective volume of a laser radiation

The invention relates to a method for the reproducible positioning of a target object in the effective volume of a first laser radiation, in particular pulsed laser radiation, which is highly reflective for the wavelength of the first laser radiation and transmissive for at least a second laser radiation by means of a focusing optics in the Effective volume is focusable. The invention further relates to a system having the same features, in particular for carrying out the method.

The invention is based on the fact that it is known in the art with laser pulses, in particular high-intensity laser pulses with powers of terawatts (10 ¹² watts) or more, to illuminate target objects, the so-called targets, and thus to produce physically desired processes, which are largely dependent on depend on the achieved light intensity in the focus.

Thus, it is known that, for example, from light intensities of 10 ¹⁸ watts per square centimeter, the running processes are highly nonlinear, resulting in a variety of novel physical effects. Examples are the acceleration of electrons to several 100 million electron volts and the acceleration of protons to several 10 million electron volts on an acceleration distance of only a few microns. Similarly, very strong particle beams with these properties can be generated in an extremely short time, for example, less than 10 picoseconds (10 ^"12 seconds), which are of interest for medical applications such as cancer radiotherapy or the production of short-lived radiopharmaceuticals , It is also possible with the aid of lasers to produce ultrashort coherent X-ray pulses, which are extremely interesting in addition to their use in medicine, for example in microchip production, ie for lithographic processes.

Quite essential for the above-described or other desired effects is a precise focusing of the laser beam or of the laser pulse on the target object or the target. In this case, beam focusing is usually carried out with mirror optics of the shortest possible focal length, rather than with lenses, in order to maximize the intensity in the focal spot on the material of the target object which is to be used for the production of particles or X-radiation, as well as an extension of the pulse duration by dispersive Avoid effects when passing through the optical material.

In principle, due to the very strong focus, the area of highest intensity is achieved only in a very small effective volume, so that for maximum efficiency the distance between the focusing optics and the surface of the target object must be set very precisely, usually to within a few micrometers.

This is all the more problematic since used focusing optics in these high-intensity pulses can be formed by focusing mirror, parabolic focusing mirror in particular, which may have very high masses of several 10 kilos, whereas the target objects usually form only a few microns large arrangements, such as metal - or plastic films.

It is problematic here that a misadjustment of the distance between these objects acts quadratically in a loss of light intensity.

To prevent this problem, it is known in the art that permanent illumination of the amplifier medium of a used laser, the so-called enhanced spontaneous emission or English Amplified Spontaneous Emission to focus ASE ₁ on the surface of a target object and to observe it with the aid of high-intensity telescopes. There then takes place an adjustment of the distance between the focusing optics, ie in particular a parabolic concave mirror and the target object until the experimenter has gained the impression of having approached an optimal position. This method is carried out for each individual laser pulse to be used to generate the aforementioned particles or X-rays.

It is evident that the aforementioned method is extremely tedious and inaccurate, since it is geared to the personal impression of the experimenter and moreover does not proceed without destruction, since even on sensitive target objects the focused spontaneous emission of the non-Q-switched laser medium can reach such a high intensity, that a sensitive target object, in particular plastics o. Ä., Even by this radiation already affected or even destroyed. For high yield production applications, therefore, this known method is unsuitable.

The object of the invention is to provide a method of the aforementioned generic type and a system with which a reproducible positioning of a target object can be achieved, in particular to achieve high yields or repetition rates and in particular a shot to shot reproducibility. It is a further object of the invention to provide a method and an apparatus with which the focusing of the laser beam on a target object quickly, accurately, quantifiable and beyond even in further delimitation from the prior art destructive or without influence on the target object feasible is.

The object is achieved according to the invention in that to the desired position of a target object to be positioned in the effective volume of the first, in particular pulsed laser radiation, the image of a mask element by the deflection optics is projected, in particular with a second laser radiation, and the degree of defocusing of the image of the mask element on the target object to be positioned is determined, in particular minimized.

An essential core idea of the invention can be seen in the fact that the image of a mask element is projected onto the point within the effective volume of a focused laser beam, which is perceived as optimal for the experiment to be performed, so that this target object is located at a target object positioned in the focus environment of the laser beam serves as a projection surface for the image and thus on the basis of the sharpness or the degree of defocusing of the image of the mask element on the target object can be determined whether the target object is positioned at the optimum position in the effective volume or if there is still a shift of the target object or a change the distance between the focusing optics and the target object is needed to set this optimal position.

Thus, a target object can first be positioned (roughly) within the effective volume and then optimized in its position, which can be done to a highly accurate degree due to the determination of the degree of defocusing and the usually very short focal lengths of the focusing optics. It may be provided to observe the image of the mask object by a corresponding observation optics, so as to determine the degree of defocusing, which can be done both manually and more preferably automated.

It is thereby a particular advantage of the method according to the invention that the projection of the image of the mask element onto the target object, which serves as a projection surface during the adjustment, is performed through the deflection optics which direct an incident laser beam or laser pulse onto the aforementioned focusing optics. Thus, by the method according to the invention or with a system according to the invention the adjustment of the Beam path for focusing the first, in particular pulsed laser radiation completely untouched, so that therefore makes no further impact by carrying out the method according to the invention in experiment setup noticeable.

In order to allow the projection to take place through the deflection optics, it is provided, as mentioned at the outset, that this deflection optics is highly reflective for the first, in particular pulsed, laser radiation used in the experiment, whereas it is transparent for a second wavelength, which is used for imaging the mask element is, in particular for a second laser radiation with a different laser wavelength.

Accordingly, for such deflection optics, for example, dichroic mirrors can be used which have corresponding dielectric coatings. Furthermore, in today's high-power lasers, which generally emit radiation in the infrared wavelength range, the mirrors used highly reflective for this wavelength, however, usually transitive for the visible wavelength range, so that here the proposed method or system no adjustment or change already existing optics required.

In the case of the abovementioned method or system, it can thus be provided that in order to avoid any intervention in the beam path of the first laser radiation to be focused, the focusing optics for the first laser radiation form part of the imaging optics for the mask element. This can be done, for example, such that the imaging optics for the mask element is effected by two focusing optics, wherein one of the optics for imaging in the beam path in front of the deflection optics of the first, in particular pulsed laser radiation is arranged and the second focusing optics formed by the focusing optics of the first laser radiation is, so that in this sense, the imaging optics for the mask element image around the deflection optics in the beam path of the first, in particular pulsed laser radiation is arranged around. In a further embodiment of the invention, it may be provided that the image of the mask element generated on the target object to be positioned, ie the temporary projection surface, is again imaged, namely here opposite the original imaging direction through the deflection optics into a second image plane, so that it is according to the invention may be provided, in this embodiment, not to determine the degree of defocusing directly on the target object, but in the second image plane and in particular to minimize. Thus, in particular by the further imaging, an enlargement of the image of the mask element located on the target object can also be made so as to achieve a further simplified adjustment. This also makes it easier to automate the method since it is possible to position a detector, in particular a camera, in the second image plane and thus to enable an apparatus-supported examination of the degree of defocusing.

Since, in this second image as well, this image is again, but here in the rearward direction, by the deflection optics for the first laser radiation, the beam path of the first laser radiation remains completely untouched even for this imaging measure. Again, the focusing optics for the first laser radiation is preferably used as part of the imaging optics for this second image, in particular as one of two focusing optics.

In one development, it can be provided to tilt the mask element at an angle to the optical axis of the second laser radiation, ie in particular at an angle not equal to 90 degrees, in particular to allow a sharp image of the mask element on a tilted to the optical axis of the first laser beam targets , Thus, a requirement to place certain (eg planar or planar) targets at an angle other than perpendicular (90 degrees) to the direction of incidence of the first laser beam into the effective volume can be taken into account, in particular around back reflections of the first laser beam from the target substrate into the first and / or second laser. As a result, in the first and the second image respectively, a respective tilted image plane is generated, so that it may be provided in this embodiment, the image plane of a detector, in particular a camera tilted at the same angle to the optical axis to order a sharp picture on the to preserve the entire picture plane. As a result of this arrangement, it is possible, in particular, to check or measure the degree of tilting of the target substrate relative to the optical axis of the first laser radiation in addition to the position.

Likewise, it is possible to use more complicated, non-planar mask elements which advantageously take into account the spatial structure or arrangement of the target substrate in the effective volume.

According to the invention, it may be provided for assessing the degree of defocusing to judge the contrast of the image either on the target object serving as a projection surface or preferably in the second image plane and thus on the basis of the detector signal, in particular on the basis of a camera image.

Thus, only when the contrast has been maximized, the target object have reached its optimum position in the effective volume, in particular focus of the first laser beam. For this purpose, it may be provided that a contrast function is formed and evaluated from the detector signal, in particular thus from the camera image, in particular based on the gradient of the contrast function. Thus, with a displacement of the target object and thus taking place focusing or defocusing depending on the direction of displacement a clear maximum in the amount of the gradient can be determined, for example, it may be provided to position the target object where exactly the maximum is reached. On the basis of this consideration, it is therefore possible to construct an electronic control system which evaluates the aforementioned detector signals and, in particular, formed gradients and thus drives an automatic positioning device for changing the distance between the focusing object and the target object. Thus, for example, the optimal position of the target object in the effective volume can also be automatically found by an iteration process, in particular taking into account the modulation transfer function of the entire imaging system.

For the realization of the first image of the mask element on the target object and thus also for the optionally used second image in a second image plane, it may be provided to illuminate the mask element with a second laser radiation and thereby redirect the transmitted light of the mask element by means of a beam splitter and so Mask as shown by the deflection optics of the first laser radiation to the desired position in the effective volume, where as mentioned above, the imaging optics of two focusing optics and in particular the focusing optics of the first laser radiation form as one of these two optics.

By using a beam splitter for deflecting the light passing through the mask, it is possible in the case of the second image, which may be provided according to the invention, into the second image plane for this further imaging to take place in transmission through this beam splitter. This ensures that the image in the second image plane is not returned to the mask itself, but the second image plane is separated from the object plane of the mask element.

In this case, it is found to be particularly advantageous if the mask element is illuminated by means of collimated laser radiation, possibly after a previous widening by a telescope, since this makes it possible to obtain an afocal image of the mask element on the target object, ie it avoids that used to illuminate the mask element Laser beam is also focused on the target, so that in this way the known from the prior art problems influencing or destroying the target object by focused Justagestrahlung is completely avoided.

Thus, it may be provided in a preferred embodiment for the implementation of the method and implementation of the system that the two focusing optics of the imaging optics with the mask element and the image of the mask element in the effective volume forms a 4F optics configuration, in particular in the mask element and the image of the mask element in the effective volume are in mutually conjugate planes.

In the same way, it can therefore also be provided that the two focusing optics of the imaging optics together with the image of the mask element in the effective volume and the image of the mask image in the second image plane, thus thus with the detector, which can be arranged in the second image plane, Forms a 4F optical configuration, in particular in which the image of the mask object in the effective volume and the detector are in mutually conjugate planes.

As mentioned above, it is precisely through the realization of the 4F optical configuration for possibly both images, but at least for the first image of the mask element on the target object, that afocal imaging can be achieved in the effective volume by the telecentric imaging system thus formed, in which the second laser beam both at the location of the mask as well as at the location of the image of the mask element on the target object is expanded, that is not focused and thus any destructive or influencing risks are eliminated.

For the method or system according to the invention, it is furthermore advantageous that a positioning of the mask image in the effective volume at a desired position, ie in particular the optimal position of the focus of first laser radiation can be achieved by a shift of the mask, whereby the object plane of the mask changes in the imaging system and thus the image plane is shifted in the effective volume.

In a further mapping into said second image plane, in which a detector can be arranged, it may then be additionally provided that in the same way with the displacement of the mask and the detector spacing can be changed, which, since the imaging system for both images identical can be done with the same displacement. This can be done for example by an automatic coupling of two displacement units of both the mask element and the detector.

For a basic adjustment of a possible detector, such as a camera in the second image plane, it may be additionally provided behind the beam splitter mentioned above within the two imaging systems and the beam path in front of the deflection optics for the first laser beam to arrange an autocorrelation optics, with the mask image can be imaged directly into the second image plane, so that the optimal distance of the detector to the beam splitter and thus the optimal position in the second image plane can be adjusted by such a correlation optics first. Again, this can be done fully automatically according to the invention.

The aforementioned method or system used in the same way can thus be used particularly advantageously according to the invention for positioning a target object, such as metal or plastic foils in the focus of a laser pulse, for example a pico (10 ¹² ) or femto (10 ¹⁵ ) Seconds of high energy laser pulses that can achieve more than 10 ¹⁸ watts per square centimeter in the focus area.

Since the method and the system with respect to the beam path, which is intended for the deflection and focusing of the laser pulse, is completely non-invasive, there are no changed experimental conditions and a System for carrying out the method can thus be completely externally coupled to this without influencing the experiment.

In particular, there are automatically and in particular objective possibilities for assessing an optimal adjustment position of the target object in the effective volume. In this case, it can also be taken into account that, in the case of a high-intensity laser pulse, the focus may be outside the geometric focal length of the focusing concave mirror used, e.g. is arranged due to noticeable non-linear properties, since the mask image can be adjusted at any desired position and deviating from the geometric focus.

Thus, in particular after a single finding of the optimal position of a target object (target) within the effective volume, the mask image can be placed in exactly this optimum position and used as an adjustment aid for future positioning of target objects, in particular in the context of an automated method.

An embodiment of the invention is explained in the following figure.

The single figure shows in this embodiment of the method according to the invention a first beam path I, which is deflected with respect to the representation from above via a deflecting mirror 1 by approximately 90 degrees to the right. The mirror 1 may be a dichroic mirror which is highly reflective for the wavelength of the laser used in the experiment and transmissive for an alignment laser beam. Instead of a Justagelasers can also be used in general, any other. Also not incoherent light source.

In the beam path 1, following the deflecting mirror, a focusing element 2, which is symbolically represented here and is usually formed in practice as a parabolic concave mirror follows. These concave mirrors can especially with ultrashort pulses in the femtosecond range be specially designed to avoid a temporal pulse extension by dispersion effects of the dielectric coatings.

It is clear here that a focus 3 is generated by the focusing element 2 in particular a parabolic concave mirror in the beam path 1, around the center of which an effective volume is formed, in which a target object 4 must be accurately positioned to carry out an experiment.

In order to mark this position P, it is provided according to the invention, the image of a mask element 5 by an imaging optics, which is formed by a focusing optics 6, for example a first lens or a first concave mirror, and the focusing optics 2.

Thus, it is provided here to illuminate the mask element 5 by means of a telescopically arranged in particular focusing elements 7 and 8, in particular lenses, with a laser beam of a second wavelength for which the deflection mirror 1 is transmissive. By the focusing elements 7 and 8, the laser beam is both expanded and collimated, so that between 4 masking element, focusing element 6, focusing element 2 and the image of the mask element 3 results in a 4F configuration and thus a telecentric afocal image, which means in that an image of the mask 5 is produced at the position P, while the collimation, ie the parallel beam path of the laser beam of the second wavelength is maintained and thereby no destructive increase in intensity of the laser beam of the second wavelength takes place on the target.

Here, it is provided that the light emanating from the mask element 5 by a second beam path Il, which is deflected by a beam splitter 9 and coincides after passing through the deflecting element 1 with the beam path I of the first laser beam. This results in the beam splitter 9 both an excellent adjustment of the beam path Il and the ability to make a backward image of the projected mask image of an inserted into the effective volume target 4 in a second image plane B, in which a detector, such as a camera to be positioned can.

In this second illustration, the light of the mask image backscattered by the target object passes through the beam splitter 9, so that the image does not fall back on itself and thus the second image can be used for evaluation by means of a detector. Here, for the second image, as well as for the first image, the focusing optics 6 and 2 are used as the imaging system, the focusing optics 2 coinciding with the focusing optics in the beam path 1 of the experimental laser beam.

It is clear here that no interference is made in the beam path of the adjusted laser beam I by the construction of the adjustment system.

Thus, an inventive adjustment system can also be used later on an existing experimental arrangement without having to make an intervention in the arrangement. Furthermore, it may be provided here to provide an autocorrelation optics 10, which is not shown here and which reflects back a portion of the light emanating from the mask element 5 which has passed through the beam splitter 9, that is to say by the reflection at the beam splitter 9 an immediate mapping of the mask element into the image plane B can take place.

Thus, by means of this autocorrelation optics, an optimal adjustment of the detector in the image plane B can first be carried out for carrying out the method according to the invention. Sources of error due to misadjustments of the detector within the image plane B, which would affect an unsatisfactory positioning of the target object, can thus be avoided be, because it is thus initially made sure that the adjustment aid system is optimally adjusted in itself.

With regard to all embodiments, it should be noted that the technical features mentioned in connection with one embodiment can be used not only in the specific embodiment, but also in the other embodiments. All disclosed technical features of this invention description are to be classified as essential to the invention and arbitrarily combined or used alone.

Claims

claims 1. A method for reproducible positioning of a target object in the effective volume of a first, in particular pulsed laser radiation, which is highly reflective for the wavelength of the first laser radiation and transmissive for at least a second wavelength, in particular second laser radiation by means of a focusing optics in the Effective volume can be focused, characterized in that at the desired position (P) of a target object to be positioned (4) in the effective volume (3) the image of a mask element (5) through the deflection optics (1) is projected through with a second particular laser radiation and the degree of defocusing of the image on the target object (4) to be positioned is determined, in particular minimized. 2. Method according to claim 1, characterized in that the image of the mask element (5) generated on the target object to be positioned (4) is imaged through the deflection optics (1) into a second image plane (B) in which the degree of defocusing is determined , in particular minimized. 3. The method according to any one of the preceding claims, characterized in that the focusing optics (2) for the first laser radiation forms part of the imaging optics (6,2) for the mask element (5), in particular such that the imaging optics (6,2) around the deflection optics (1) is arranged around. 4. The method according to any one of the preceding claims, characterized in that a mask element (5) with a second laser radiation, in particular after a widening (7) and collimation (8), is illuminated and the transmitted light of the mask element (5) deflected by means of a beam splitter (9) and the mask element (5) through the deflection optics (1) of the first laser radiation through to the desired position (P) in the effective volume (3) is with imaging optics (6.2) of two focusing optics (6.2), which are arranged around the deflection optics (1). 5. The method according to claim 4, characterized in that the image of the mask image from the effective volume (3) out in the second image plane (B) by the beam splitter (1). 6. The method according to any one of the preceding claims 4 or 5, characterized in that in the second image plane (B) a detector, in particular a camera is positioned, in particular their distance from the beam splitter (9) the distance of the mask element (5) to the beam splitter ( 1) corresponds. 7. The method according to any one of the preceding claims 4 to 6, characterized in that a tilting of the first image plane at the desired position (P) in the effective volume of the first laser radiation (3) by a tilting of the mask element (5) is brought about. 8. The method according to any one of the preceding claims 4 to 7, characterized in that the second image plane (B), in particular a camera arranged therein is also tilted at the same angle to the optical axis to a sharp over a surface image of the first image plane at the desired position (P) in the second image plane (B) to reach. 9. The method according to claim 6 or 7, characterized in that an automatic positioning of a target object (4) in the effective volume (3) or the focusing optics (2) takes place by evaluation of the detector signal and dependent thereon control of a positioning mechanism. 10. The method according to claim 9, characterized in that from the detector signal, the gradient of a contrast function is formed and evaluated. 11. The method according to any one of the preceding claims 9 to 10, characterized in that automatically the position (P) of the target object (4) in the effective volume (3) is iterated, in particular taking into account the modulation transfer function of the imaging system (6.2). 12. The method according to any one of the preceding claims, characterized in that the two focusing optics (6.2) of the imaging optics (6.2) together with the mask element (5) and the image of the mask element (5) in the effective volume (3) a Form 4f optic configuration. 13. The method according to any one of the preceding claims, characterized in that the two focusing optics (6.2) of the imaging optics (6.2) together with the image of the mask element (5) in the effective volume (3) and the image of the mask image (5 ) in the second image plane (B) form a 4f optical configuration. 14. The method according to any one of the preceding claims, characterized in that a positioning of the mask image in the effective volume (3) to a desired position (P) by a displacement of the mask (5), in particular with a change in the detector spacing. 15. The method according to any one of the preceding claims, characterized in that behind the beam splitter (9) an autocorrelation optics is arranged, by means of the mask image directly in the second image plane (B) can be imaged, in particular for automatic calibration of the detector. 16. The method according to any one of the preceding claims, characterized in that it is used for positioning a target object (4) in focus (3) a laser pulse with a power of more than one, in particular more than one hundred tera watts. 17. System for the reproducible positioning of a target object in the effective volume of a first, in particular pulsed laser radiation, which can be focused into the effective volume by means of focusing optics after deflecting optics which are highly reflective for the wavelength of the first laser radiation and transmissive for at least one second laser radiation; characterized in that at the desired position (P) of a target object to be positioned (4) in the effective volume (3), the image of a mask element (5) through the deflection optics (1) through with a second laser radiation is projected and the degree of defocusing of the image determinable on the target object (4) to be positioned, in particular for automatic positioning of the target object (4) can be minimized.