Optical sensor device

An optical sensor device comprising:

• a light source;
• a sensing element;
• a photodetector;
• a fixture;
• and
• an elastic sealing component,
• wherein the sensing element includes:
• a prism including a hypotenuse face and two leg faces;
• and
• a sensing film on the hypotenuse face, the sensing film being configured to sense a target substance,
• wherein the sensing film is exposed to a gastight space to include the target substance,
• wherein a confining wall confining the gastight space to include the target substance has an opening and the opening is covered with the sensing element,
• wherein the elastic sealing component is located between the sensing element and the opening,
• wherein the sensing element is fixed by being pressed by the fixture in such a strength that the elastic sealing component is elastically deformed,
• wherein the light source and the photodetector are located outside the gastight space to include the target substance,
• wherein light from the light source is configured to pass through one of the two leg faces and hit the sensing film,
• wherein light reflected off the sensing film is configured to go out through the other one of the two leg faces, and
• wherein the photodetector is configured to detect light that is reflected off the sensing film and comes out through the other one of the two leg faces.