Virtual Vernier modulation for overcoming peak-tracking limits in dense-fringe fiber interferometry

Fiber-based sensing systems that rely on wavelength interrogation for real-time operation can encounter peak-tracking limitations under small free spectral range (FSR) conditions. In such cases, conventional wavelength interrogators using local peak-tracking algorithms can underestimate the amplitude of dynamic signals when neighbouring spectral fringes are closely spaced. This underestimation arises from algorithmic constraints that restrict the tracked peak excursion to avoid ambiguity between adjacent fringes. In this study, we experimentally investigate this limitation and introduce a virtual-Vernier approach based on computational spectral processing. The experimental validation is carried out using a tunable Fabry–Perot (FP) cavity microphone, which provides adjustable FSR and enables controlled sparse- and dense-fringe conditions. Under the tested dense-fringe condition, conventional local peak tracking records an apparent peak-to-peak wavelength modulation of approximately 10 pm for a 60 Hz acoustic excitation. A virtual reference interferometer is generated numerically and combined with the measured sensing spectrum to produce a slowly varying Vernier envelope with an FSR of approximately 28 nm. Envelope dip tracking yields a clear peak-to-peak modulation of approximately 4.25 nm. The corresponding Vernier magnification factor was approximately 25.1, yielding an equivalent single-fringe tracking amplitude of approximately 170 pm. This value is approximately 17 times larger than the amplitude obtained using conventional local peak tracking under the same dense-fringe condition. The virtual-Vernier method therefore provides a trackable wavelength-domain coordinate for dense-fringe fiber interferometers without modifying the physical interferometer.