Fiber optical time delay resonant oscillating strain gauge

Abstract

Longitudinal tensile and/or compressive strain in optical fibers is detered by an entirely optical technique. A test optical fiber optically coupled to optical injection and extraction couplers form an optically recirculating loop. A semiconductor laser diode feeds a series of narrow light pulses into the loop via the injection coupler and an oscilloscope or signal peak detector give visual indications of optical correlation when an avalanche photodiode provides responsive signals coming from the extraction coupler. Straining the optical test fiber will change the loop's length and, hence, the time delay between reoccurring pulses so that the loop frequency of the narrow optical pulses must be correspondingly changed to provide maximum signal correlation. Changing the pulse repetition rate of the laser diode until a maximum correlated signal is observed at the scope or detector provides a new resonant loop frequency that is proportional to strain. Stresses in the optical fiber due to temperature, pressure, installation, etc. can thereby be noted. Because the system employs an optical technique for measuring strain, errors which might otherwise be due to electronic time delay variations and drift are eliminated.