Measurement of strain coefficient of unequal cross-section FBG sensor based on double optical levers

When a single optical lever is used to test the strain coefficients of Fiber Bragg Gratings(FBGs), slip effects may take place in the clamping part between the testing machine and the sensor. To effectively eliminate the slip effects and improve the test accuracy, a measurement system based on double optical levers was established to measure the strain coefficient of a FBG sensor with an elastic stress element by unequal cross sections. In the measurement, two optical lever mirrors were put respectively on the end faces of two fixed ends beside the sensor's elastic stress element. By measuring the relative displacement of two fixed ends, the equivalent strain coefficient of the sensor was obtained by data processing with the curve fitting method and cumulative average method. A simulation model was established, and the experiment and simulation were performed and compared. The results indicate that the equivalent strain coefficient of the sensor n_εeff is 0.681 7 με/pm obtained by the experiment and the precision is 0.91%. Compared to the coefficient 0.672 0 με/pm from the software simulation, the difference is 1.42%. It is shown that the measurement method satisfies with the requirements for accurately measuring the strain coefficients of complex-structure sensors.