Fabry-Perot optical sensing device for measuring a physical parameter

An optical sensing method for measuring a physical parameter, comprising steps of:

• a) generating a multiple frequency light signal having predetermined spectral characteristics;
• b) passing the light signal in a Fabry-Perot interferometer including two semi-reflective mirrors substantially parallel to one another and spaced by a given distance so as to define a Fabry-Perot cavity having transmittance or reflectance properties which are affected by said physical parameter and which cause said spectral characteristics of the light signal to vary in response to said physical parameter, the light signal being transmitted into said Fabry-Perot cavity with at least one optical fiber, and at least a portion of the light signal outgoing from said Fabry-Perot cavity being collected with said at least one optical fiber;
• c) focusing said at least a portion of the light signal;
• and
• d) passing the focused light signal through a Fizeau interferometer for converting the focused light signal into a spatially-spread light signal indicative of said transmittance or reflectance properties of said Fabry-Perot interferometer;
• whereby said physical parameter can be determined by means of said spatially-spread light signal.

A thermally self-compensated Fabry-Perot strain gage for measuring a deformation of a body, comprising:

• a Fabry-Perot interferometer through which a light signal having predetermined spectral characteristics is passed, said Fabry-Perot interferometer including two semi-reflective mirrors substantially parallel to one another and spaced by a given distance so as to define a Fabry-Perot cavity having reflectance properties which are affected by a change in said distance and which cause said spectral characteristics of the light signal to vary in response to said change, said Fabry-Perot interferometer being provided with at least one optical fiber for transmitting the light signal into said Fabry-Perot cavity and for collecting at least a portion of the light signal outgoing thereof;
• a microcapillary having a longitudinal bore in which said mirrors of the Fabry-Perot interferometer are mounted, said at least one optical fiber having a tip connected to a corresponding one of said mirrors, a portion extending outside said bore, and a weld with the microcapillary;
• and
• a thin wire made of a thermally expanding material, said thin wire having a diameter approximately equal to a diameter of said at least one optical fiber, said thin wire having a tip connected to the other one of said mirrors, a portion extending therefrom in said bore, and a weld with said microcapillary causing said portion to move said other one of said mirrors relative to said corresponding one of said mirrors upon thermal expansion of said material;
• whereby a change in said distance between said mirrors produced by a thermal expansion of said body is compensated by a thermal expansion of said portion of said thin wire within said bore.