DFB fibre laser sensor for simultaneous strain and temperature measurements in concrete structures

a distributed feedback (DFB) fibre laser sensor for simultaneously measuring strain and temperature has been developed. The DFB fibre laser consists of a single fibre Bragg grating written in a low birefringent rare-earth doped fibre. By measuring the rf beat frequency between the two orthogonal polarised lasing modes and the absolute wavelength of one mode, both strain and temperature can be determined simultaneously to an accuracy of +/-3 mu epsilon and +/-0.04 degrees C. Multiplexing capabilities make this sensor ideal for monitoring several locations within a civil engineering structure. Three gauge protection systems were developed to prevent damage to the fibre during embedment and insulate it from the high alkaline environment of the concrete. This sensor is easy to install, provides excellent strain transfer from the concrete to the optical fibre and is thin enough not to degrade the concrete structure.