Fiber-optic devices and sensors using fiber grating

A fiber optic apparatus using an optical fiber grating, comprising:

• a. an optical fiber grating in which a grating structure is written in an optical fiber having a longitudinal axis, said optical fiber having a core with a refractive index distribution designed to support a first optical propagation mode and a second optical propagation mode for an optical signal propagating through the optical fiber, said core being doped with germanium, and the germanium being exposed by a writing light having a intensity distribution in a cross-sectional pattern and in a period along the longitudinal axis of said optical fiber, such that the refractive index of said core is increased permanently in said cross-sectional pattern and said period along the longitudinal axis of the optical fiber, said grating structure being defined by an incremental amount of said refractive index, said cross-sectional pattern, and said period along said longitudinal axis, said grating structure causing optical energy coupling of the optical signal between said first optical propagation mode and said second optical propagation mode;
• b. a signal light source for introducing an optical signal into said optical fiber grating;
• c. means, located between said light source and said optical fiber grating, for introducing only the first optical propagation mode into said optical fiber grating;
• d. means, located at a distal end of said optical fiber grating from said signal light source, for selecting only said second optical propagation mode, said second optical propagation mode being generated from said first optical propagation mode by optical energy coupling in said optical fiber grating, such that said second optical propagation mode has an intensity which is a function of the amount of said optical energy coupling;
• and
• e. means for perturbing said optical fiber grating by applying an external perturbation to said optical fiber grating, said perturbing means including a mechanical element for applying a strain to said optical fiber grating along said longitudinal axis, such that said grating structure is changed proportional to the applied strain and the amount of optical energy coupled between said first optical propagation mode and second optical propagation mode is controlled by the applied strain.