Strain measuring apparatus/method having a sensor and a reference

Abstract

A condition sensing apparatus/method includes a sensor-fiber-grating that is subjected to a physical condition or measurand (strain/temperature/pressure) that is to be sensed. A broad-band light beam is directed into the core of the sensor grating, and the constructive-interference light beam that is reflected from this sensor grating is directed into a reference-fiber-grating. The reference grating is subjected to a servo-controlled elongation force or strain, and in this manner the length of the reference grating is varied until the reference grating operates to reflect a maximum intensity of its received light beam, and to transmit a minimum intensity of its received light beam. This maximum/minimum condition of the reference grating is sensed by detecting the minimum intensity of a light beam that is transmitted through the reference grating. The physical strain of the reference grating at which this condition occurs is used as a measure of the measurand to which the sensor grating is subjected. Sensing the mechanical resonant frequency of the reference grating provides an output that indicates the magnitude of the measurand currently at the sensor grating.