Laser equipment for materials processing

Abstract

A laser device for materials processing is provided. A laser device for laser treatment of the human eye includes a source of a pulsed laser beam, a detector system for optical detection of multiple partial beams generated from the laser beam by extracting radiation energy and providing corresponding detection signals, and a control unit coupled to the detector system for evaluating the detection signals. A first detection signal is provided by a first detection element of the detector system, and this signal is based on single-photon absorption. A second detection signal is provided by a second detection element of the detector system, and this signal is based on two-photon absorption. The control unit ratios the measured signal intensities of the two detection signals to each other.

Description

[Technical field]

[0001] The present disclosure relates to laser devices for materials processing. [Background technology]

[0002] Ultrashort pulsed lasers are becoming increasingly important for processing a wide range of materials. In particular, ultrashort pulsed laser irradiation can be used in transparent materials (transparent to the irradiation wavelength) to create incisions deep inside the material, i.e. below the surface. For the short pulse widths in question here, in the picosecond, femtosecond or attosecond range, the nonlinear interaction processes of the laser radiation with the material, responsible for the material separation (destruction), are mainly concentrated in the area of ​​the beam focus, so that high cutting precision can be achieved simultaneously with limited zones of undesired collateral damage. Particularly in medical applications, for example in eye surgery with lasers, the relatively low energy density with the ultrashort pulse widths necessary to achieve the desired interaction of the radiation with the tissue is a great advantage.

[0003] Ultrashort pulse lasers, especially those supplied by laser manufacturers to customers as "turnkey" systems, are highly complex systems that usually require constant monitoring and adjustment of pulse parameters to maintain the desired level of efficiency in material processing. In laser applications using ultrashort pulse radiation, the interaction of the radiation with the material is mainly based on nonlinear processes, which require a certain intensity and energy density. This can be achieved by tight focusing of the radiation pulse, which results in very high peak intensities (e.g. 10 TW / cm) in the material to be processed. 2 above) and energy density (e.g. 500J / cm 3 (super) can occur in an extremely small volume.

[0004] Once an optimal set of pulse parameters has been determined for a particular material to be processed, it is often necessary to maintain the pulse characteristics within a certain range (neither too low nor too high) for uniform processing, in order to properly balance the desired primary interaction effects with any undesired and potentially harmful secondary effects. When monitoring the beam quality of a laser beam, it can generally be classified into two approaches, namely sequential and simultaneous. In the sequential approach, the tests are performed in a time sequence in a separate upstream test step before the actual material processing, for example, where a specimen is processed for testing purposes or where certain beam parameters are measured directly at the beam focus. On the other hand, in the simultaneous approach, the beam test is performed during the actual material processing. The two methods can be described as offline (sequential) and inline (simultaneous). In material processing by ultrashort pulsed laser irradiation, a stable peak intensity between pulses at the target area is a key parameter for reliable and effective processing. Therefore, inline monitoring of the peak intensity is desirable for this type of application.

[0005] An in-line method for monitoring the quality of a laser beam is known from WO 2011 / 060707 A1. In it, a portion of the laser beam used for material processing is taken off and focused onto a nonlinear crystal, where the second harmonic of the fundamental wavelength of the laser beam is created by non-resonant frequency mixing. The conversion efficiency of the frequency doubling is directly proportional to the peak intensity of the laser pulse applied to the crystal and, consequently, depends on the peak intensity of the pulse of the (main) laser beam. As long as other influencing factors can be excluded, power fluctuations of the second harmonic can be traced back accordingly to fluctuations in the beam quality and / or the pulse duration. Summary of the Invention [Means for solving the problem]

[0006] On the other hand, the present disclosure facilitates in-line monitoring of the laser beam, not by creating harmonics from the fundamental wavelength of the laser beam, but by power measurements based on single-photon and two-photon absorption. Thus, according to certain embodiments, a laser device for material processing is provided, comprising a source of a pulsed laser beam, a detector system for optical detection of a plurality of partial beams generated from the laser beam by radiation energy extraction and providing corresponding detection signals, and an evaluation unit coupled to the detector system for evaluation of the detection signals, where a first detection signal of the detection signals is based on single-photon absorption and a second detection signal of the detection signals is based on two-photon absorption. Two-photon absorption is based on a process in which a molecule passes from a ground state to an excited state when the molecule absorbs two photons simultaneously. Detectors operating on the principle of two-photon absorption have an appropriately selected band gap between the ground state and the excited state, as a result of which the detector emits a detection signal for a given wavelength of laser radiation only if a two-photon absorption event occurs. In contrast to two-photon absorption, in single-photon absorption, a single photon is sufficient to cause a transition of the molecule to an excited state.

[0007] The first detection signal (i.e., the detection signal based on single-photon absorption) is time-averaged and is expressed as P ave represents the average power, and f rep represents the pulse repetition rate, and E Puls represents the pulse energy, Puls =P ave / f rep On the other hand, the second detected signal (based on two-photon absorption), when time-averaged, is proportional to the product of the average power and the peak intensity at the detector surface, which for its part is proportional to the pulse repetition rate f rep and pulse width τ Puls and the spot size, and therefore the size of the laser beam on the detector surface, A Spot The power is proportional to the quotient of the product of the power and the average power. S Zpd / S Epd =c*(Pave ) / (A Spot *τ Puls *f rep ) (Here, S Zpd represents the temporal average value of the second detection signal based on the detection of a two-photon absorption event, and S Epd represents the time average value of the first detection signal based on the detection of a single photon absorption event, and c represents a proportionality constant) applies to the quotient of the second detection signal and the first detection signal. ave ) / (A Spot *τ Puls *f rep ) is proportional to the peak intensity, so S Zpd and S. Epd For the quotient of f and f, a proportional relationship to the peak intensity of the laser irradiation is obtained. rep and constant average power P ave Assuming that, S Zpd and S. Epd The variation of the quotient of and accordingly changes the pulse width τ Puls or / and spot size A Spot This can be traced back to the fluctuation of the spot size A Spot represents a measure of convergence.

[0008] Taking these relationships into account, in a particular embodiment of the laser device according to the invention, the evaluation unit is configured to ratio the measured values ​​of the first and second detection signals to one another, in particular to divide the measured value of the second detection signal by the measured value of the first detection signal.

[0009] In particular for in-line monitoring of the beam quality it is desirable to be able to quickly introduce measures if it is ascertained that the beam quality does not comply with the desired requirements.Thus, in a particular embodiment it is provided that the evaluation unit is part of a control unit which is configured to bring about at least one predefined reaction depending on the fact that the actual state, as determined by the measurements of the first and second detection signals, deviates in a predefined manner from the target state.

[0010] The predetermined reaction can be, for example, deactivation of the source or / and output of a message. Alternatively or additionally, the predetermined reaction can include control by the control unit of components of the laser device that affect the pulse width of the laser beam, in particular a controllable pulse compressor. Another possible reaction consists of control by the control unit of components of the laser device that affect the wavefront of the laser beam, in particular a controllable waveplate arrangement.

[0011] In a particular embodiment, the laser device further comprises a focusing optics for focusing the laser beam on the object to be treated and a focus control device for spatial control of the focus position of the laser beam, the focus control device comprising at least one lateral control element for spatial lateral control of the focus position, which is arranged in the beam path of the laser beam between the source and the focusing optics. An extraction point, at which the radiation energy is extracted from the laser beam for at least one of the partial beams, is in this case arranged in the beam path of the (main) laser beam between the source and the at least one lateral control element.

[0012] According to a particular embodiment, the beam expansion optics can be arranged in the beam path of the laser beam between the source and the at least one lateral control element. The take-off point can then be arranged in the beam path of the laser beam between the source and the beam expansion optics.

[0013] Due to the high sensitivity of the detector element of a detector system operating according to the principle of two-photon absorption, the use of a focusing lens for focusing one of the partial beams on this detector element is recommended.

[0014] The invention will now be described in more detail with reference to the accompanying single drawing. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 shows a schematic diagram of a practical example of a laser device for material processing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A preferred embodiment is described in more detail below with reference to Fig. 1. Fig. 1 shows an apparatus, generally designated 10, for laser-assisted material processing. In the illustrated example, the laser apparatus 10 is used, for example, for laser processing of a human eye 12 and for creating intracorneal tissue slices of the eye 12. In principle, however, the present disclosure is not limited to the use of the laser apparatus 10 in laser-assisted eye surgery. Instead of use in eye surgery, the laser apparatus 10 can also be used for laser processing to cut other living tissues and non-living materials.

[0017] The laser device 10 comprises a laser source 14, which generates a pulsed laser beam 16 with a pulse width in the picosecond, femtosecond or attosecond range. The laser device 10 also comprises a focusing optics 18, for example formed by an f-theta lens. The focusing optics 18 focuses the laser beam 16 on the object to be treated, here the eye 12. Also arranged in the beam path between the laser source 14 and the focusing optics 18 are a beam expansion optics (beam expander) 20 and a scanner 22. The beam expansion optics 20 expands the beam cross section of the laser beam 16, for example by means of a lens arrangement in the style of a Galilean telescope. The scanner 22 is used for lateral and longitudinal control of the focal position of the laser beam 16. For lateral deflection, the scanner 22 can comprise, for example, a pair of tilting mirrors controlled based on a galvanometer or an electrically controlled deflection crystal. For the longitudinal focus control, the scanner 22 can comprise, for example, optical elements that affect the divergence of the laser beam 16, for example, lenses that are longitudinally movable in the beam propagation direction or liquid lenses with variable refractive power or deformable mirrors. Even if the scanner 22 in FIG. 1 is represented as a single functional block, it should be understood that the components of the scanner 22 responsible for the lateral and longitudinal focus control of the laser beam 16 can be located at different points along the beam path of the laser beam 16. For example, the longitudinal control elements of the scanner 22 can be formed by lenses included in the beam expansion optics 20, while one or more lateral control elements (for example, scanner mirrors) can be located in the beam path after the beam expansion optics 20. Thus, the scanner 22 can be formed by a distributed arrangement of various scanning components.

[0018] For controlling the laser source 14 and the scanner 22, a processor-based control unit 24 is provided, which operates according to a control program stored in a memory 26. The control program contains appropriate control parameters (e.g. in the form of coordinates for the individual firing positions of the laser pulses) which determine the incision shape to be made.

[0019] In order to create fine and precise incisions with the laser beam 16, a high spatial and temporal beam quality of the laser beam 16 is desirable. For real-time monitoring (in-line monitoring) of the beam quality of the laser beam 16, the laser device 10 has means for generating two partial beams 16', 16'' from the laser beam 16. For this purpose, the laser device 10 comprises means for extracting a part of the irradiation of the laser beam 16. These means comprise two semi-transparent splitter mirrors 28, 30 arranged in the beam propagation direction in front of the components of the scanner 22 responsible for the lateral focus control and in the example shown in front of the beam expansion optics 20 and arranged behind each other in the beam path of the laser beam 16 in the example of FIG. 1. Each of the splitter mirrors 28, 30 extracts one of the partial beams 16', 16'' respectively from the laser beam 16. In a variant embodiment, only a single partial beam is initially extracted from the laser 16 and then split into the two partial beams 16', 16''.

[0020] A first detector element 32 is arranged in the propagation path of the first partial beam 16' and a second detector element 34 is arranged in the propagation path of the second partial beam 16''. The detector element 32 operates according to the principle of single-photon absorption, whereas the detector element 34 operates according to the principle of two-photon absorption. The detection signal emitted by the first detector element 32 is a measure for the average power of the partial beam 16' calculated as the product of the pulse repetition rate and the pulse energy of the irradiation pulse of the partial beam 16' and thus this detection signal is also proportional to the average power of the (main) laser beam 16. On the other hand, the detection signal provided by the second detector element 34 is a measure for the product of the average power and the peak intensity of the second partial beam 16'' and thus also for the corresponding product of the (main) laser beam 16. To increase the probability of a two-photon absorption event, a focusing lens 36 with a short focal length (e.g. focal length ≈20 mm) is for example arranged in the beam path of the second partial beam 16″ in front of the detection element 34, which lens focuses the partial beam 16″ onto the detection surface of the detection element 34. It is not necessary to specially explain that the focusing lens 36 can be formed by a lens group instead of a single lens.

[0021] The detection signals of the two detection elements 32, 34 are summed together and evaluated in the control unit 24. In particular, the control unit 24 determines the following mathematical relationship: S Zpd / S Epd =c*(P ave ) / (A Spot *τ Puls *f rep ) (Here, S Zpd represents the time average value of the detection signal provided by the detection element 34, and S Epd represents the time average value of the detection signal provided by the detection element 32, and P ave represents the average irradiation power of the laser beam 16, and A Spot represents the spot size of the partial beam 16″ on the detection surface of the detection element 34, and τ Puls represents the pulse width, and f repThe quotient of the detection signals of the two detection elements 32, 34 is calculated according to a constant pulse repetition rate f rep and constant average power P ave Assuming that the ratio S Zpd / S Epd The change in the pulse width τ Puls or / and spot size A Spot can be traced back to changes in

[0022] S Epd S for Zpd The ratio of is calculated by the control unit 24 at least once, preferably repeatedly, for example at regular time intervals or substantially continuously, during the emission of the laser beam 16. Such emission is performed as part of an actual treatment procedure, in which the eye 12 is treated by the laser beam 16. The emission of the laser beam 16 can also be performed in a preceding test procedure, in which the laser device 10 is put into operation for test purposes. Zpd and S. EpdThe actual state of the laser device 10, represented by the quotient of x and y, is compared with the target state by the control unit 24. As soon as the control unit 24 sees that the actual state differs in a certain manner from the target state, it initiates a predefined reaction. This reaction can, for example, include the stopping of the laser source 14, as a consequence of which the emission of the laser beam 16 is interrupted. Alternatively or additionally, the reaction can include the output of an optical and / or acoustic message. The message can, for example, be displayed in text or graphics on a monitor or can include a signal light. An acoustic alarm is also considered as part of the message. Instead of interrupting the operation of the laser source 14, it is considered that the control unit 24 initiates appropriate corrective measures, which in the context of the control loop can again bring the actual state closer to the target state. Possible corrective measures consist of the control of suitable controllable components (by the control unit 24), thereby influencing the pulse width of the irradiation pulses of the laser beam 16. Such a component is, for example, a pulse compressor 38, which can be included in the laser source 14 as part of an amplifier downstream of the laser resonator. To generate high pulse intensities, amplifiers are often used that operate according to the principle of chirped pulse amplification. In this case, the pulses generated by the resonator are first spatially stretched before being compressed again after passing through an amplifying medium. By appropriate control of the pulse compressor used for this compression, the control unit 24 can determine the S Zpd and S. Epd One may attempt to reduce the difference between the measured ratio of 0.1 to 0.2 and the desired target value or range accordingly.

[0023] Another possible corrective measure that can be used instead of or in addition to influencing the described pulse width consists of the control (by the control unit 24) of a component suitable for influencing the wavefront of the laser beam 16. This component can be, for example, a waveplate or an arrangement of such waveplates, which can be inserted in the beam path at the laser source 14 inside or outside the laser resonator. By modifying the wavefront of the laser beam 16, the focusing and thus the part of the detection surface of the detection element 34 illuminated by the focused partial beam 16″ (i.e. the spot size A Spot ) can be influenced.

[0024] The goal state is, for example, the quotient S Zpd / S Epd This reference value is stored in the memory 26 and is used to calculate the quotient S Zpd / S Epd If the detection signal of the detector element 34 (two-photon detector) depends quadratically with sufficient accuracy on the average power of the partial beam 16″ and thus of the laser beam 16, then for a given pulse repetition rate f rep About S Zpd and S. Epd It may be sufficient to store in memory 26 a single reference value of the proportionality coefficient between Zpd / S Epd As soon as the current value of S deviates from a defined tolerance range around the stored reference value, the control unit 24 initiates at least one of the described reactions. In particular, in cases where the detection signal of the detection element 34 does not exhibit a quadratic dependence on the average laser beam power with the desired accuracy, it is considered to include a reference curve over the entire power range of the laser arrangement 10 instead of a single reference value applied to all power values ​​and to store this reference curve in the memory 26. This curve is calculated by dividing the quotient S by the quotient S for various values ​​of the average irradiating power of the laser arrangement 10. Zpd / S EpdA reference value associated with each is identified. The curves can be, for example, created in tabular form or expressed by a mathematical formula (e.g., a Taylor series with at least a power of three). In the emitting operation of the laser device 10, compliance with the reference curves is continuously monitored based on a stored table (by interpolation between pairs of values ​​contained in the table, if applicable) or based on a formula, which depends on the format in which the curves are stored in the memory 26. A tolerance range can also be determined when using a reference curve as an indicator of the target state, which range can have equal magnitude over the entire curve or, if applicable, different magnitudes in different parts of the curve.

[0025] The measured values ​​or / and quotients S of the detection signals of the detection elements 32, 34 Zpd / S Epd are continuously stored in memory 26 in a particular embodiment of control unit 24. Alternatively or additionally, the measurements and calculations are continuously displayed on a computer monitor (not shown in FIG. 1 ) so that the operator receives direct information regarding the beam quality of laser beam 16 during the laser procedure.

[0026] According to a possible further development, at least one of the detector elements 32, 34 can be formed as a position-sensitive detector, whose detection signal is composed of a plurality of partial signals, on the basis of which the control unit can calculate the position of the associated partial beam 16' or 16'' on the detection surface of the detector. Such sensors, which can register one- or two-dimensional positions of light points, are generally well known to the specialist and therefore a more detailed description can be omitted. By determining the position of the partial beam 16' or 16'', the control unit 24 can recognize possible changes in the beam direction of the laser beam 16 and initiate appropriate reactions if the detected changes exceed a certain permissible change range. In particular, appropriate measures are possible as reactions to reduce directional changes of the laser beam 16. In this respect, it is considered that the control unit 24 provides a component of the scanner 22 (e.g. a pair of tilted mirrors) which is responsible for the lateral position control of the beam focus of the laser beam 16 with correspondingly corrected control values, so that directional changes of the laser beam 16 observable in front of the beam expansion optics 20 are reduced or even almost nonexistent after the scanner 22.

[0027] Alternatively or in addition to correcting the directional changes of the laser beam 16, the control unit may display the detected directional changes on a computer monitor in a manner understandable to a user of the laser device 10 and / or record the directional changes in memory 26.

Claims

1. a source of a pulsed laser beam; A detector including a plurality of detection elements, Optically detecting a plurality of partial beams generated from the laser beam by extracting the irradiated energy; and a detector configured to provide corresponding detection signals, where a first detection element provides a first detection signal based on single-photon absorption and a second detection element provides a second detection signal based on two-photon absorption; a processor-aided computer linked to said detector system for evaluation of said detection signals, The quotient S of the detection signals of the detection elements Ｚｐｄ / S Ｅｐｄ where S Ｚｐｄ represents the time average value of the second detection signal of the second detection element, and S Ｅｐｄ represents the time average value of the first detection signal of the first detection element, and and a processor-aided computer, the control unit configured to attempt to reduce a difference between the quotient S Zpd / S Epd and a desired target value or range.

2. The computer is a Ｚｐｄ / S Ｅｐｄ 13. The laser device of claim 1, configured to provide at least one predetermined response in response to a change in

3. The laser device of claim 2 , wherein the at least one predetermined reaction includes shutting down the source or outputting a message.

4. The laser device of claim 2 , wherein the at least one predetermined response comprises control of a component of the laser device that affects a pulse width of the laser beam.

5. The laser device of claim 2 , wherein the at least one predetermined response comprises control of a component of the laser device that affects a wavefront of the laser beam.

6. focusing optics for focusing the laser beam onto an object to be treated; a focus control device for spatial control of a focus position of the laser beam, the focus control device comprising at least one lateral control element for spatial lateral control of the focus position, the focus control device being arranged in a beam path of the laser beam between the source and the focusing optics; 2. The laser device according to claim 1, wherein a take-off point, at which radiation energy is taken off from the laser beam for at least one of the partial beams, is arranged in the beam path of the laser beam between the source and the at least one lateral control element.

7. 7. The laser device of claim 6, further comprising a beam expansion optics disposed in the beam path of the laser beam between the source and the at least one lateral control element, the extraction point being disposed in the beam path of the laser beam between the source and the beam expansion optics.

8. 2. The laser device according to claim 1, further comprising a focusing lens for focusing one of the partial beams onto the detector element of the detector system working according to the principle of two-photon absorption.

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