Improved Composite Optical Current Sensing With Polarization Offset and Self-Calibration Demodulation

In optical current sensing, existing polarimetric optical structures have relatively high measurement error and low signal-to-noise ratio (SNR). To resolve these challenges, this article proposes an improved composite polarimetric structure with polarization offset (PO) and self-calibration demodulation. The proposed structure contains two single-polarization optical paths (SOPs), one of which incorporates PO to induce unidirectional changes in the polarization angle. In the SOP with PO, a waveform that matches the shape of the current waveform can be precisely obtained by demodulating the sensing output. Besides, in the SOP without PO, only the effective value of the current can be precisely obtained due to bidirectional changes in the polarization angle. Therefore, the current waveform can be calculated by self-calibration between the obtained results. The Jones matrix derivation shows that the output of the improved structure has fewer noise terms and does not require approximation such as existing structures. The test results reveal that the small-current error, large-current error, and SNR of the proposed structure have been improved to 0.8%, 0.2%, and 37.42 dB, respectively, compared to 1.00%, 0.5%, and 30.45 dB of the state-of-the-art structures, and high-frequency fault currents are used to demonstrate the wide bandwidth of the proposed structure.