Method and apparatus for thermowave analysis
AssigneeJenoptik Jena G.m.b.H.
InventorsHans-Dieter Geiler, Matthias Wagner, Peter Kowalski
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Abstract
A method and apparatus for thermowave analysis employs a single laser beam, which has an additive, two-frequency, intensity modulation, and is directed onto the object. The amplitude of the mixed frequency of the discrete modulation frequencies, which is not contained in the exciting beam, is analyzed as response signal. By obtaining a reference signal from the exciting beam and forming the difference between this exciting beam and the measurement signal, noise suppression and a lowering of the limit of sensitivity are achieved in an advantageous manner.
Claims32
In the method of thermowave analysis in which a laser beam is directed onto an object to be analyzed, whereby the intensity of said laser beam is modified by the reaction of the object to the energy of the beam directed thereto, and modification of the beam is detected with a measurement detector by way of a focussing objective, the improvement comprising
- modulating the intensity of the laser beam, before it strikes the object, to have a modulation spectrum that contains two discrete frequencies (f.sub.1 ;
- f.sub.2) and
- measuring the optical response of the object by measuring the amplitude of a mixed frequency which is produced by the reaction of said object to the two modulation frequencies (f.sub.1 ;
- f.sub.2) at the output of the measurement detector, with a frequency-selective device.
In an apparatus for thermowave analysis comprising a source of a laser beam, means for focusing said beam by way of an objective onto an object to be investigated, optical imaging means in the beam path following the interaction of the beam with the object, and a measurement detector for detecting the intensity of energy received from said object via the optical imaging means resulting from the reaction of the object to the energy directed thereto, the improvement wherein
- the laser source comprises means for providing a laser beam with two discrete frequencies (f.sub.1 ;f.sub.2) in the modulation spectrum of the beam, and
- a frequency-selective device connected to exclusively select a mixed frequency of the two modulation frequencies (f.sub.1 ;f.sub.2) produced by said object reaction, said frequency-selective device being coupled to the output of the measurement detector to determine the amplitude of said mixed frequency.
In an apparatus for thermowave analysis in which a laser beam is focused by way of an objective onto an object to be analyzed, whereby the beam interacts with the object, optical imaging means positioned to receive the results of the interaction of the laser beam with the object, and a measurement detector positioned to receive the output of said optical imaging means for detecting the intensity of the energy caused by reaction of the laser beam with the object, the improvement comprising
- a semitransparent mirror positioned in the path of the laser beam before it impinges on said object, said semitransparent mirror producing two equivalent partial beams,
- means for eliminating the ability of the two partial beams to interfere with one another,
- a separate optical element for modulating the intensity of each partial beam, the two modulating optical elements having different modulating frequencies (f.sub.1,f.sub.2),
- means for decoupling the laser beam reflected by the object in both partial beams,
- a beam-splitting prism for combining the two partial beams and disposed before the objective and
- a frequency-selective element for determining the amplitude of a mixed frequency produced by reaction of the object with the laser beam modulated with the modulation frequencies (f.sub.1 ;f.sub.2) , said frequency-selective element being connected in series with the measurement detector.
In an apparatus for thermowave analysis, in which a laser beam is focused by way of an objective onto the object to be analyzed, and optical imaging means are disposed in the path of the beam after its interaction with the object, and a measurement detector is provided for detecting the intensity of the output of the optical imaging means, whereby the beam is modulated by the reaction of the object to the energy directed thereto, the improvement comprising
- means for producing the laser beam from the beams of two separate laser sources, which are intensity modulated differently, each with a different frequency (f.sub.1 ;
- f.sub.2),
- a beam-splitting prism mounted before the objective to combine the beams of the two laser sources and
- a frequency-selective device for determining the amplitude of a mixed frequency of the frequencies (f.sub.1 ;f.sub.2) produced by the object due to a reaction with the laser beam, said frequency-selective device being connected in series with the measurement detector.
Dependencies are inferred from the claim text.