DE102010015682B4 - Device for detecting the process radiation during laser material processing - Google Patents

Abstract

Device for detecting process radiation (P) when machining a workpiece (W) by means of a laser beam (LA), in which an intensity of the process radiation (P) emitted by the workpiece (W), which is generated by the outermost edge of a lens ( L) in the direction of the laser is detected by a scraper mirror (SC) arranged behind it and deflected perpendicular to the laser beam (LA) and there falls on a focusing element (ZP), which absorbs the high intensity and undesirable laser radiation (LA) from the workpiece (W ) is reflected and also deflected by the scraper mirror, separates from the process radiation (P) and breaks down the process radiation (P) into its spectral components and focused on one or more detectors (D), characterized in that the focusing element (ZP) is a ring grating is formed, the spacing of the rings of the ring grating becoming narrower towards the outside.

Description

In laser material processing, it is necessary and sensible to measure the electromagnetic radiation emitted by the processing point for process control purposes and to determine parameters which provide information about the quality of the processing and can be used to control the processing process.

Examples include laser welding monitoring, power control of the piercing process and determination of piercing timing prior to cutting, monitoring and control of the cutting process itself, detection of plasma formation, and more.

Such devices are in the publications EP 1 886 757 A1 . DE 10 2004 020 704 A1 . DE 44 33 675 A1 . DE 196 44 101 C1 . DE 10 2008 015 133 B4 . DE 101 60 623 A1 . DE 40 06 622 A1 and US 6,596,961 B2 described.

By way of illustration is in 1 such a device is shown, which corresponds to the prior art. The laser beam LA is focused on the workpiece W by a lens L. From the workpiece W, the process radiation P is emitted, which is detected annularly by means of a scraper mirror SC and laterally deflected out of the machining head, where it is focused onto the detector D with a mirror S. In front of the detector are the filters F1 and F2. The process radiation from the workpiece is broadband, its wave spectrum depends on the temperature of the processing site. The laser radiation LA is monochromatic and in intensity many times stronger than the process radiation.

Since a portion of the laser radiation from the processing point is reflected, the filter F1 has the task not to let the laser radiation through, otherwise this would destroy the detector D. A second narrow-band filter F2 can optionally be used to measure only a discrete wavelength of the process beam. Such devices are also used with multiple detectors that measure at different wavelengths.

Laser radiation also processes reflective materials. Examples are the machining of aluminum, brass or copper. If an error occurs during processing - eg the focus position is wrong, d. H. the power density on the workpiece is too low - so the laser radiation is not adsorbed in the workpiece, but partly completely reflected from the workpiece and reflected in parallel through the lens back into the resonator. It forms an extended resonator from the laser rearview mirror to the workpiece. In the resonator is pumped on, but the radiation can not escape, so peak powers occur in the megawatt range, which destroy the optical elements.

The filter F1, which intercepts the laser radiation from the detector during normal operation, often does not stand up to this enormous energy load. It is destroyed and with it the detector and the underlying electronics, as happens in practical operation.

• 1. Zu teuer in der Herstellung.
• 2. Bei Rückreflektion des Laserstrahls kann die aufgedampfte Schicht und letztlich der Strahlteiler zerstört werden. Er ist deshalb für hohe Strahlintensitäten nur begrenzt einsetzbar. Des Weiteren besteht das fokussierende Element für die Prozessstrahlung (24) aus einer Linse, die ebenfalls bei Rückreflektion des Laserstrahls zerstört wird.

The DE 101 60 623 A1 represents the closest prior art. In this document is in the beam path, a beam splitter ( 22 ) arranged. The beam splitter has two disadvantages:

• 1. Too expensive in manufacturing.
• 2. With back reflection of the laser beam, the vapor deposited layer and ultimately the beam splitter can be destroyed. It is therefore limited for high beam intensities. Furthermore, the focusing element for the process radiation ( 24 ) from a lens, which is also destroyed in back reflection of the laser beam.

The publication EP 1 886 757 A1 is an improvement over the DE 101 60 623 A1 with a scraper mirror ( 13 ) located outside the incoming laser beam ( 4 ) is located. It is made of reflective metal and therefore withstands the strong laser radiation in back reflection, but the focusing element ( 6 ) is a lens that is destroyed by back reflection. Should a suitable lens largely pass the back-reflected laser and process radiation, the narrowband filter ( 10 ) is heated and destroyed, because it lets only the process radiation through, but absorbs the laser radiation.

In the DE 10 2004 020 704 A1 is the element that decouples the process radiation from a beam splitter, as in the DE 101 60 623 A1 With the disadvantages described above or alternatively from a focusing scraper mirror ( 14 ), which also uses the process radiation to reflect the laser radiation reflected back from the workpiece onto the detector ( 19 ) and destroy it. In addition, a spectral separation of the process radiation for 2 detectors ( 41 ) by a divider plate ( 4 ), which is expensive, and which can also be destroyed by the back-reflected laser radiation.

In the devices described in the above patents, the focusing element S - see 1 - From a concave mirror or a lens, z. T. Scraper SC is also designed as a concave mirror. The disadvantage is that these elements also focus the laser radiation on the detector since they do not ( Concave mirror) or only small (lens) dispersion. It is therefore an object of the invention to prevent damage to the detector - by the laser radiation reflected from the workpiece with much higher intensity.

The solution of the problem

The invention described in claim 1 solves the problem in that the focusing element S of the device focuses only the process radiation P to be measured on the detector, but not the laser radiation, which is achieved in that the focusing element is formed as a ring grid.

A Fresnel zone plate is a plate on which concentric rings are applied ( 2 ). The zones differ in alternating reflecting and absorbing zones. The radiation is diffracted and amplified by constructive interference at focal point F. The radii of the zones are calculated as a first approximation for parallel incident radiation R (n) = √ n · λ · f with n = 1, 2, 3, ..., λ = wavelength and F = focal length of the zone plate.

The distance X (n) of the nth ring from the focal point f is X (n) = √ f 2 + 2nλf , According to the invention, it is now provided to use a ring grid instead of a zone plate according to Fresnel, since this can be produced more easily mechanically.

The monochromatic laser radiation is focused at a focal point F1 and then runs apart again. The laser radiation is clearly separated from the beam path of the process radiation, as in 3 using the example of a CO2 laser with a wavelength of 10.6 μm.

The laser radiation does not fall on the detectors and can not damage them.

It can be seen from the formula that each wavelength λ of the radiation has a different focal length f. For the broadband process radiation and the laser radiation, the ring grid acts like a spectrometer whose focal points lie on the central axis. Please refer 3 ,

This results in yet another advantage of the invention. Since the wavelengths at which the process radiation is to be measured are focused at different locations of the central axis, a spectral separation of the process radiation is achieved with the invention. If measurements are to be carried out at different wavelengths, the detectors must be mounted at corresponding locations on the central axis - a complex separation of the process radiation into different wavelength ranges by means of filters and partly transparent mirrors is dispensed with.

embodiment

3 shows a practical example of a 2-color pyrometer for measuring the temperature of process radiation when working with a CO2 laser. From the Scraperspiegel (not shown) falls an annular, parallel beam of process radiation P (broadband) and the laser radiation LA (monochromatic) on the focusing element ZP. It acts like a spectrometer and focuses the spectral components of the incident radiation on the central axis, after which they diverge again.

The radiation of the CO2 laser passes through the focal point F1 and then diverge and is absorbed by an absorber on the wall of the device (not shown). It does not apply to the detectors. As an example, at the focal point D1, a Ga-As detector is mounted, which measures the radiation at 1.6 μm, at the focal point D2 a silicon detector, which measures the radiation at 950 nm. The signals of the detectors are analyzed and evaluated with electronics (not shown). As in 3 shown, can be accommodated in skilful design even the scraper mirror in the sensor housing between D1 and D2, without crossing the different beam paths or affect, which allows a more compact design.

Further embodiment of the invention

Since the reflected or scattered laser radiation in the material processing is always present in the sensor with considerable power, it is recommended to make the focusing element of reflective, polished metal, which reflects the radiation largely and tolerate some heating.

In a Fresnel zone plate, the width of the zones R (n) -R (n-1) changes less and less toward the outside. Since the Scraperspiegel SC images only a narrow ring on the outer edge of a zone plate, you can also leave the zones the same width and according to the invention instead of a zone plate use a ring grid, which can be produced mechanically easier.

In the case of a ring grid, the focal spot has the width of the beam d incident from the scraper mirror, but this is justifiable given the size of the detector.

The effect of the ring grid can be intensified in intensity by using a reflection phase grating with preferred reflection, also called "blazed grating", whose grooves have a sawtooth profile. In this case, the angle of the reflective surfaces is arranged so that the normal points to the reflective surface to the detector.

When designing the focusing element ZP, it should be noted that not only diffraction 1st order, as in 3 simplified, but also second, third, ... etc. However, the higher order diffraction wavelengths are shorter than the first order diffraction at the same location. They are usually outside the spectral sensitivity of the detector and do not interfere, as shown in the following example:
If the focusing element ZP is calculated such that the 1st order diffraction of a silicon detector for the wavelength 950 nm has a focal point at 200 mm, then the 2nd order diffraction there becomes at the wavelength 475 nm, that of the 3rd order Focused at 318 nm. A silicon detector has a spectral sensitivity in the range 1050-850 nm. The radiation of the 2nd, 3rd, ..., etc. order does not receive the detector because it is outside the spectral sensitivity.

The device can also be designed for higher-order diffraction, for example, when working only with a detector and the device is compact, d. h., The focal length of the focusing element ZP should be short. For example, in 3rd order diffraction, the zones become wider by a factor of 3 and are easier to manufacture mechanically. The low intensity at higher order diffraction can be absorbed by a Balzegitter according to the invention. The angle of the blazed grating is designed so that the beam diffracted at the grating and a beam reflected at the stage are deflected in the same direction. One speaks then of autocollimation.

For ease of manufacture, the grid can also be made with a spiral structure.

Claims (6)

Device for detecting process radiation (P) during the machining of a workpiece (W) by means of a laser beam (LA), in which an intensity of the process radiation (P) emitted by the workpiece (W) passing through the outermost edge of a lens arranged in the machining head ( L) is detected in the direction of the laser by a scraper mirror (SC) arranged behind it and deflected perpendicularly to the laser beam (LA), where it falls onto a focusing element (ZP) emitting the high intensity and unwanted laser radiation (LA) emitted by the workpiece (W ) and is also deflected by the scraper mirror, separated from the process radiation (P) and the process radiation (P) decomposed into their spectral components and focused on one or more detectors (D), characterized in that the focusing element (ZP) as a ring grid is formed, wherein the distance between the rings of the ring grid is getting narrower towards the outside. Apparatus according to claim 1, characterized in that the ring grid is designed with a reflective surface of polished metal. Apparatus according to claim 1, characterized in that the ring grid is designed as a reflection-phase grating with preferred reflection. Device according to claim 3, characterized in that the reflection phase grating is a blazed grating. Apparatus according to claim 4, characterized in that the ring grid is not designed for diffraction of the first order but 2nd, 3rd, ... order. Apparatus according to claim 1 to 5, characterized in that the ring grid is produced in a spiral structure.

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