A method for enhancing the temperature measurement range of fiber specklegram sensors based on the integrated speckle intensity probability density function

Fiber specklegram sensors based on correlation demodulation suffer from rapid loss of correlation when the measured specklegram deviates too far from the reference image, causing their dynamic range to be limited by the saturation of the correlation coefficient. This restricts their application in wide-range temperature sensing. To overcome this limitation, we propose a novel demodulation method based on the integrated speckle light intensity probability density function, adopting the probability value of a specific light intensity interval as the characteristic parameter to enhance the dynamic range of fiber specklegram sensors. This paper elucidates the fundamental mechanism by which ambient temperature variations induce changes in the fiber length, diameter, and refractive index, subsequently leading to variations in the transmission modes and ultimately affecting the parameters of the integrated speckle light intensity probability density function. Theoretical derivation establishes the relationship between the probability value of a specific light intensity interval and the temperature change, thereby clarifying the principle of the proposed method for temperature sensing. A single-mode–no-core–multimode fiber structure is used as the sensing unit to perform principle verification and comparative experiments. The experimental results indicate that, within the operational temperature range of the experimental setup, our method expands the usable temperature range from 60–130 to 60–170 °C compared to the standard zero-mean-normalized cross correlation approach, achieving a 57.14% increase in the dynamic range. A slight reduction in linearity is observed and discussed, but the overall performance confirms that the proposed method substantially enhances the temperature measurement capability of fiber specklegram sensors.