Thermal measuring and testing – Calorimetry – Total radiant energy or power measurement
Patent
1991-02-12
1993-05-11
Cuchlinski, Jr., William A.
Thermal measuring and testing
Calorimetry
Total radiant energy or power measurement
2503381, G01K 1700
Patent
active
052095673
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The invention relates to an apparatus for measuring the radiation power of lasers, particularly infrared lasers, with a thermal detector emitting a power-caused measurement signal.
2. Prior Art
The precise knowledge of the time dependence of the power of a laser beam over a maximum wide frequency range or a large band width of the power changes is important for numerous machining processes. It is known to couple out a few permille of the radiation power in the resonator by an end mirror or reflector and to measure same with a water-cooled beam trap, which essentially comprises a laser radiation-absorbing, cooled, black casing, to which thermocouples are fitted in such a way that they supply a voltage proportional to the power of the incident laser radiation. Said voltage can be used via an amplifier for indicating the power. However, the known apparatus has a relatively high thermal capacity. Therefore the voltage supplied can only very slowly follow time changes in the radiation power. The time constant is more than 10 seconds. The known apparatus is consequently unsuitable for the measurement of rapid power changes.
DESCRIPTION OF THE INVENTION
The problem of the present invention is to so improve an apparatus of the aforementioned type, that it is suitable for measuring both the time average and also rapid power changes of laser radiation and in particular for infrared lasers, such as carbon dioxide lasers.
This problem is solved in that a device is provided which locally intergrates the laser radiation and to which is connected, apart from the thermal detector, at least one further detector making it possible to detect radiation power changes with a larger band width than that detectable by the thermal detector, and in that there is a circuit combining the measurement signals of the detectors.
In order to be able to detect power changes n a wide frequency range, it is important according to the invention to use detectors having different band widths for the radiation power changes to be detected. However, in the case of several detectors it must be ensured that they are proportionately identically irradiated. This is not very easy to achieve, because over the beam cross-section, the laser radiation has inhomogeneous intensity distributions. However, it is achieved in the case of the device provided according to the invention, which locally integrates the laser radiation and which influences the latter.
In order to bring about the local homogenization of the laser-radiation, the apparatus is constructed in such a way that the device locally integrating the laser radiation has an optics focussing the laser radiation into a small window of a hollow sphere and that the detectors are located in an area of the sphere interior which is only irradiated by reflected radiation, or the integrating device essentially comprises diffusing screen. The laser radiation focussed in the hollow sphere undergoes a multiple diffuse reflection, so that all the surface elements of the inner face of the sphere, to the extent that they are not directly subject to the effect of the laser radiation, are irradiated with the same intensity. Thus, the detectors are uniformly loaded with respect to one another, so that the measurement signals emitted by them are also in a corresponding constant relationship to one another. The further possibility of constructing a local integrating device as a diffusing screen has the advantage of greater constructional simplicity, but suffers from increased absorption losses when the radiation passes through the diffusing screen, which is in particular disadvantageous for those detectors, which require a comparatively high measuring power in order to be able to produce an evaluatable measurement signal.
Advantageously the thermal detector is a thermopile comprising a plurality of thermocuples and the further detector is a pyroelectric detector and, if necessary, a high or low-pass filter is provided. The thermocolumn comprises a series connection of miniaturized th
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Journal of Physics E: Scientific Instruments, vol. 6, No. 2, Feb. 1973. Gunn: "Calorimetric measurements of laser energy and power,", pp. 105-109.
Sensors and Actuators, vol. 5, No. 3, May, 1984. Shaulvov: "Broad Band Infrared Thermal Detector" pp. 207-215.
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Drenker Alexander
Loosen Peter
Sturm Volker
Bennett G. Bradley
Cuchlinski Jr. William A.
Fraunhofer Gesellschaft zur Forderung der angewandten Forschung
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