High-Precision FOCT With AC–DC Separation Function for Ship’s Shaft-Rate Electric Field Current Measurement

A high-precision fiber-optic current transformer (FOCT) for measuring ship’s shaft-rate electric field current (SREFC) is proposed and demonstrated in this article. The optical path of the FOCT comprises a sensing arm and a reference arm. The light polarization states in two arms are orthogonal, with a specific time delay between them. The superposition of light from the two arms can achieve relative intensity noise (RIN) suppression and improve the output signal to noise ratio (SNR) of the optical path. Compared to traditional nonoptical RIN suppression methods, the proposed all-fiber structure method offers greater simplicity and ease of implementation. Furthermore, a novel extraction algorithm is proposed, which integrates the denoising process with the separation of dc and ac mixed current signals. This algorithm leverages the strengths of fast Fourier transform (FFT) and discrete wavelet transform (DWT), employing a modified soft threshold function in wavelet threshold denoising to perform secondary denoising after initial optical path denoising, thus improving the accuracy of current extraction. Experimental results show that the SNR of the optical path increases by 4.26 dB after inhibiting RIN. In comprehensive testing, the absolute error for dc amplitude is less than 0.4 A, that for ac amplitude is less than 0.1 A, and that for frequency is less than 0.2 Hz, fulfilling the accuracy index requirement of 1% full scale (FS). The proposed FOCT is eminently suitable for SREFC compensation fields and can be extended to other current measurements involving a combination of ac and dc.