Grating-based sensor

A method for conducting optical fiber-based sensing in an environment rich in a selected gas, comprising the steps of:

• A. providing a plurality of sensor gratings inscribed into the one or more segments of optical fiber, each sensor grating having a respective wavelength that shifts in response to a change in temperature, a change in a strain applied across the sensor grating, and a change in the diffusion of a selected gas throughout the sensor grating, wherein each of the plurality of sensor gratings is configured to have the same wavelength shift over time in response to a change in gas diffusion;
• B. feeding an input light into the plurality of sensor gratings;
• and
• C. using wavelength shift data reflected back from the plurality of sensor gratings to determine single, unique values for the respective amount of change in one or more physical quantity from a group of physical quantities including temperature, applied strain, and gas diffusion,
• wherein gas diffusion parameters are excluded in step C.

A method of determining pressure or temperature in an environment rich in a selected gas, the method comprising:

• A. providing a set of three fiber Bragg grating (FBG) elements capable of receiving an optical signal and reflecting light in response to the optical signal, light reflected from each respective FBG element having a wavelength dependent on parameters of the respective FBG element, the determination of pressure or temperature in the environment rich in the selected gas being obtained in relation to the wavelengths of light reflected by the FBG elements in response to the optical signal;
• the first and second FBG elements forming a first pair of FBG elements and the third FBG element being distinct from the first pair of FBG elements,
• wherein the three FBG elements, respectively, have (i) different temperature coefficients but (ii) the same wavelength shift in the presence of diffusion of the selected gas into the respective FBG elements, and wherein strain coefficients of the three FBGs need not be the same, and
• wherein the FBG elements in the sensor, in use, are exposed to an applied temperature and an applied pressure in the environment rich in the selected gas;
• B. providing a mechanism for coupling the applied pressure into the FBG elements in the form of a respective strain component in each respective FBG element;
• C. providing a mechanism for mechanically reinforcing the third FBG element, such that while strain due to the applied pressure will be different as between the third FBG element and the first pair of FBG elements, both the third FBG element and the first pair of FBG elements will be exposed to the same gaseous and temperature conditions in the environment rich in the selected gas;
• and
• D. determining the applied pressure or the applied temperature based on detected wavelengths of light reflected from the first pair of FBG elements and the third FBG element, respectively, and without requiring that the strain coefficients of the three FBG elements be the same.

A method for making a twin-core optical fiber, wherein both fiber cores have the same wavelength shift, within tolerance, in response to a change in gas diffusion, comprising the steps of:

• A. making single-core fibers with different dopants and/or dopant concentrations in the cores;
• B. writing gratings into the single-core fibers;
• C. testing the gratings for gas diffusion characteristics;
• D. manipulating the respective dopant profiles of the single-core fibers with respect to one or more dopants affecting gas diffusion characteristics most;
• E. repeating steps A-D until the gas diffusion characteristics measured in step C are the same for two cores;
• and
• F. using the two cores to make a twin-core fiber and characterize thermal, strain and gas diffusion effects.