Photo-thermal sensor including an expansion lens in a light beam path through a sample for determining the concentration of a compound in the sample

A photo-thermal sensor for determining a concentration of a compound in a sample, comprising:

• an excitation light source for generating a first light beam extending along a first light beam path through said sample and having a wave length at which light is well absorbed by the sample compound to be determined, said sample being disposed at a predetermined location,
• a modulator disposed in said first light beam path,
• an optical lens system arranged so as to be disposed in said first light beam path after said modulator such that said first light beam is constricted where it passes through of said sample,
• a probe light source for generating a second light beam of a coherent light extending along a second path, said probe light source being so arranged that said second light beam extends at a right angle to said first light beam and intersects said first light beam at a predetermined point,
• a lens arranged in said second light beam path for expanding said second light beam to such a degree that said second light beam has a radius at the sample location which is at least five times as large as the radius of said first light beam,
• a beam divider having opposite sides and being arranged at the point of intersection of said first and second light beams such that the first light beam emitted from the excitation light source, after passing through the modulator and the optical lens system, is deflected at one side of the beam divider and the second beam emitted from the probe light source reaches the opposite side of said beam divider and is transmitted through said beam divider whereby a first part of said first light beam as well as part of said second light beam coincide on said sample,
• a first photo-sensitive detector arranged in the first light beam path behind said sample for receiving a second part of said first light beam after its passage through said sample,
• a diaphragm arranged in the first beam path immediately after said sample and a second beam divider arranged in said first beam path after said diaphragm for partially reflecting the first light beam,
• a second photo sensitive detector arranged in a part of said first beam path after said first beam divider for receiving said part of said first light beam, which passes through said first beam divider and
• an evaluation unit in communication with said first and second photo-sensitive detectors for determining and indicating the concentration of the compound to be determined in said sample.