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Absolute Optical Path Length Retrieval of Ultra-Short Fabry-Pérot Cavities Using Hankel-SVD Subspace Circularization
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DOI：10.1109/jlt.2026.3698510.png)
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Abstract
En 中文
Ultra-short fiber Fabry-Pérot (FP) cavities, especially thin-film FP sensors, often exhibit fewer than one observable interference period within a practical finite sweep bandwidth, which significantly challenges robust demodulation. This paper presents a Hankel-SVD Basis-Circularization Phase Demodulation (HS-BCPD) method for absolute optical path length (OPL) estimation under equally spaced frequency sampling. By exploiting a dominant rank-2 approximation of the sub-fringe interference signal, HS-BCPD models the finite-window-induced distortion in the extracted two-dimensional subspace as an ellipse and restores linear phase evolution via basis circularization, enabling direct absolute OPL estimation without iterative fitting or initialization. Simulations under a fixed C-band sweep window demonstrate monotonic OPL demodulation over the investigated range, overcoming the restricted monotonic interval of peak/valley tracking. Compared with cosine-based nonlinear least-squares fitting (NLSF), HS-BCPD requires no initial guess and achieves nanometer-level accuracy at high signal-to-noise ratios. Experimental validation is performed using a 5 µm thin-film FP temperature sensor, for which stable demodulation is achieved with a maximum temperature deviation below 0.15 °C over 50-300 °C. The results indicate that the proposed HS-BCPD method provides an effective and robust demodulation solution for ultra-short FP interferometers under practical measurement constraints.
Keywords:
Ultra-short Fabry-Pérot interferometer
Optical path length
Hankel matrix
singular value decomposition
basis circularization
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