An inherently temperature insensitive fiber Bragg grating force sensor for in-vivo applications

We present a fiber Bragg grating sensor design that provides an inherently athermal response to a transverse applied load. The active element of the sensor is formed from two fibers helically wound around a common axis each containing an FBG element. The helical winding of the fibers is positioned within the transducer so that the FBG elements are coincident and located at the point where the axes of the fibers are in the orthogonal plane to the base of the sensor. An applied load acting on the sensor deflects the fibers sideways so that the upper FBG is compressed and the lower FBG is stretched causing a differential change in the Bragg wavelengths of each element. For small loads, the differential change in wavelength is linearly proportional to the applied force. A change in temperature causes identical change in Bragg wavelength on both FBG elements and therefore does not affect the differential change caused by the applied load. Using this design we have reduced the temperature dependence of our FBG sensors from ∼13 pm per °C to a variation of less than 0.25 pm over a temperature range of 20 - 60 °C, with the residual temperature dependence being largely made up of temperature variations in the solid state spectrometer used to acquire data. These sensors are ideally suited for forming sensing arrays for monitoring in-vivo pressures and forces where fluctuations in temperature are unavoidable, and have been used successfully for monitoring the pressure induced beneath compression bandages.