Multiorder Cavity Length Estimation Algorithm of Low-Fineness Sapphire Fabry–Perot Interferometers for Strain Measurement Under Ultrahigh Temperature

sapphire Fabry-Perot (FP) interferometers are useful sensors for realizing high-precision strain measurement under ultrahigh temperature. The low fineness of sapphire FP cavities, due to the relatively low endface reflectivity, limits the accuracy of the cavity length demodulation and hence the strain measurement. Here, we propose a multiorder optimal cavity length estimation algorithm to increase the demodulation accuracy of sapphire FP cavities especially under high temperature. The algorithm is designed to evaluate the optimal cavity length by exploiting all valleys in the reflection spectrum and to determine possible cavity lengths of each interference order and then obtaining the optimal cavity length with the minimum deviation between the valleys. Using the algorithm, the repeatability of the cavity length demodulation can reach 1 nm, corresponding to a strain measurement repeatability of 0.1 µε . The zero drift and the repeatability precision of the cavity demodulation are measured as 0.1% (FS) and higher than 0.5% (FS), respectively. As an application, the method is exploited on the thermal wind tunnel testing experiment. Results show that the strain measurement error is less than 5% (FS) at 1000 ^◦C, fulfilling the requirements for strain monitoring of wind tunnel balances.