A Two-Path Fiber-Optic AE Demodulation Algorithm Based on Numerically Stable Direct Least Squares Fitting of Ellipses

The 3 × 3 coupler demodulation algorithm commonly employs ellipse fitting algorithms (EFAs) to eliminate coupler asymmetry. In the detection scenario of fiber-optic acoustic emission (AE) sensor arrays, the amount of data increases significantly due to the high sampling rate and multi-channel characteristics. Traditional EFAs require substantial computational resources and involve complex calculations, which severely hinder demodulation efficiency. To address these issues, we propose a two-path demodulation algorithm based on Numerically Stable Direct Least Squares Fitting of Ellipses (NSDLS). The algorithm only two input signals and divides the fitting process into linear and quadratic components, reducing computational complexity. The triggered ellipse fitting judgment method is introduced to mitigate background noise impact on fitting results and enhance demodulation efficiency. Experimental results demonstrate that the proposed two-path demodulation algorithm based on NSDLS reduces time consumption by approximately 40 times and the CPU utilization by 40% compared with traditional EFAs demodulation algorithm, improves SINAD by 18.93 dB compared with the improved Naval Postgraduate School (NPS) method. The algorithm achieves a dynamic range of 126.04 dB @ 1 kHz, 0.075% THD, and 99.999% response linearity. This method maintains high demodulation accuracy while significantly reducing computational resource demands, lowering hardware costs, and improving overall demodulation efficiency.