Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-05-24
2001-09-11
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S347000
Reexamination Certificate
active
06288384
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of German Application No. 198 22 924.0, filed May 22, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and system for measuring the distribution of the energy field density of a laser beam, as well as to a system for implementing the method as used in the laser technology field which is widely known.
To measure a laser beam requires the measurement of a large number of time-related and site-related beaming data. The measurability as well as the informational value of these data depends largely on the beam source which is used. With respect to the laser, a differentiation must be made between pulsed operation (p) and continuous-wave operation (c
w
). Furthermore, in the case of pulsed operation, the pulse duration t
p
and the pulse period T or the pulse frequency f
p
=1/T must be considered to be largely independent values. In contrast to pulses, which are generated by acousto-optical modulators (Q-switch), in the case of lamp-pulsed systems, the above-mentioned values are mutually dependent to a high degree.
The distribution of the energy field density or intensity in the plane perpendicular to the propagation direction of the laser beam I (x, y) is an important measurable variable during the lasering of material. With respect to its lateral distribution, this measurable variable is responsible for the symmetry of the ablated workpiece volume (particularly important during the laser drilling and removal process). The energy flux density, which in a first approximation is responsible for the removed material volume, can be calculated as the product of the intensity and the pulse duration.
It should be noted that measuring such an intensity distribution should best take place in focus or close to focus (I
focus
(x, y)) There are two important reasons for this: (1) possible additional disturbance variables (lens, deviation mirror, thermal refractive effects, phase front deformations . . . ) are included in the measurement; and (2) this selection of the measuring site also makes it possible to directly measure geometrical beaming data (focus diameter d
f
, beaming quality, beam position, . . . )
Currently, measuring instruments are available on the market for only a few measuring tasks (CO
2
-laser, cw-operation) Particularly in the case of fast and short-pulsed lasers, virtually no adequate measuring instrument is available. Mainly within the wavelength range of 1,064 nm and 532 nm (Nd:YAG), CCD-cameras are used as site-resolving sensors. For measurements in focus or close to focus, systems are used which are based on the principle of a rotating needle or rapidly moving screens. However, the above-described measurable variables can be sensed by means of such systems only in a few exceptions. In particular, the transversal intensity distribution of Nd:YAG lasers in focus or close to focus: I
focus
(x, y), cannot be measured by the offered instruments.
The only systems which can manage the high power densities of the lasering focus (rotating needle or similar devices) have, among other things, an insufficient site resolution for the wavelength of the Nd:YAG laser. In addition, this method is basically unsuitable for pulsed lasers because the measurement of an intensity profile is composed of many partial measurements of different pulses.
For measuring laser pulse groups (hereinafter a laser pulse group is defined as a number of several directly successive laser pulses) and individual laser pulses, CCD cameras or similar site-resolving sensors are particularly suitable. This is because these can record an intensity distribution in a measuring operation. However, these systems, which are based on CCD-sensors, so far have no suitable triggering possibility for recording individual laser pulses. In addition, with the exception of very costly high-speed cameras, they are unable to record successive laser pulses. CCD cameras, as they are partly used in beam diagnostic modules, operate at 25-60 Hz. Based on normal frequencies of laser pulses (percussion drilling 10 Hz-10 kHz), an integration takes place in the extreme case by way of a laser pulse group of approximately 150 to 300 individual laser pulses. In addition, none of the CCD diagnostic systems can be used in focus or at least behind the lasering lens system.
It is an object of the present invention to develop a method and system by which the above-mentioned disadvantages are, at a minimum, reduced.
This and other objects are achieved by a method for measuring the distribution of the energy field density of a preferably pulsed laser beam, characterized in that a measuring beam which is correlated with the laser beam particularly in a constant manner is extracted from the laser beam. The measuring beam is imaged at different points in time in a defined and locally mutually separated manner. The distribution of the energy field density of the time-resolved partial beams of the measuring beam, which are imaged in a locally defined manner, is measured. A system to implement the method has a laser source and a detector which at least partially detects the distribution of the energy field density of the laser beam. The system has an extraction device (beam splitter) for the at least partial intensity-side extracting of a measuring beam preferably constantly correlated with the laser beam, from the laser beam. The extraction device is arranged within the beam path of the laser beam. The system has a deviating device which is arranged in the beam path of the measuring beam. The deviating device has at least one manipulator which, as a function of the time, deflects the measuring beam into defined directions in space and divides it into various partial beams, for example, a mirror which is controlled by means of a rotary step motor. The detector is arranged in the area of partial beams of the measuring beam which are deflected by the deviating device.
In addition to achieving the above object, the present invention also has the following advantages:
(1) It characterizes the laser beam sources as well as time-related beaming characteristics;
(2) By means of the improved time-resolved measurement of a pulsed laser beam, informationally valuable data can be obtained for the process simulation (laser drilling/laser removal);
(3) A process monitoring (on-line) is possible for quality control during manufacturing operations with pulsed beam sources, particularly in the case of laser ablation, laser drilling and/or laser welding processes; and
(4) Systematic and/or statistical machining defects can be diagnosed and analyzed more precisely because, by using an informative beam diagnosis, it can clearly be decided whether the defects originate from fluctuations/defects of the material or from fluctuations asymmetries of the beaming source.
With respect to all fields of laser machining, it is generally applicable that the method according to the invention and the system according to the invention can be used in a simple manner, preferably in manufacturing and production. In particular, the beam diagnostic possibilities according to the invention are easy to operate and can easily be adapted. It is also a special advantage that, during a measurement in or behind the machining focus, no intervention is required in the beam guiding of the machining equipment.
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patent: 4745280 (1988-05-01), Gi et al.
patent: 4807992 (1989-02-01), Noguchi et al.
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F & M 102 (1994) 10 entitled “X/Y-Scankopf fuer die Laserstrahlpositionierung” by M. Muth et al., pp. 521-524.
Bahnmueller Jochen
Beck Markus
Giering Axel
Allen Stephone B.
Crowell & Moring LLP
Daimler-Chrysler AG
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