INTERFEROMETRIC FIBER OPTIC SENSOR, FIBER OPTIC PROBE AND METHOD OF DETECTION OF CHEMICAL SUBSTANCE

Fiber optic measuring probe for detecting a chemical substance comprising an optical fiber segment (4), with a measuring element (5) constituting a resonant cavity, having a face (6) adapted to come into contact with the chemical substance to be detected. The optical fiber segment (4) is at least along part of its length contained within a capillary (11), and first ending part of the capillary (11) is joined with the measuring element (5) while other portion of the capillary (11) is clenched on the optical fiber segment (4). The capillary (11) has length of at least 5 mm. The optical fiber segment (4) and the measuring element (5) are squeezed together so that there is only a residual cavity (7) between them. A reflection coefficient R1 from a first surface of the optical fiber segment (4) confining the residual cavity (7) from the first side and a reflection coefficient R2 from a second surface of a measuring element (5) confining the residual cavity (7) from the opposite side meet condition R1R2≤0,5.