Sensitivity Improvement of OCMI Based on the Virtual Enhanced Harmonic Vernier Effect

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High-sensitivity temperature detection plays a crucial role in scientific research and industrial applications. This work aims to enhance the temperature measurement sensitivity of distributed cascaded fiber Fabry-Perot interferometers (FPIs) based on the optical carrier-based microwave interferometry (OCMI) by utilizing the virtual Vernier effect. Considering that the magnification factor of the virtual traditional Vernier effect is constrained by the frequency scanning range, a novel virtual enhanced harmonic Vernier effect is proposed, wherein the two virtual reference FPIs and the sensing FPI form Vernier effects with frequency shifts in opposite directions, thereby achieving an enhanced Vernier effect. By designing the length differences between the two reference FPIs and the sensing FPI, which are twice plus a small detuning, the harmonic Vernier effect is constructed. The virtual enhanced harmonic Vernier effect achieves a sensitivity of 3322.54 kHz/°C and a magnification factor of 130.43, surpassing the sensitivity of the traditional and same-order harmonic Vernier effects. Additionally, the virtual Vernier effect allows for arbitrary adjustments to the length of the reference FPI, demonstrating the applicability of the proposed Vernier effect in distributed sensors composed of cascaded FPIs.