Simultaneous existence of ultra-high and ultra-low spectral sensitivity for Directional Couplers

TL;DR

This paper provides experimental evidence and theoretical explanation for the coexistence of ultra-high and ultra-low spectral sensitivities at specific wavelengths in fiber-optic directional couplers, which is crucial for accurate sensing.

Contribution

It identifies and explains the existence of critical and cross-over wavelengths with conjugate properties in fiber-optic directional couplers, supported by experimental data and theoretical analysis.

Findings

Identification of critical and cross-over wavelengths with conjugate properties

Spectral shift behavior flips around these wavelengths

Theoretical model explaining the observed phenomena

Abstract

We present the experimental evidence for the existence of two exciting wavelengths, termed critical and cross-over wavelength in the transmission spectrum of fiber-optic directional coupler, whose properties are conjugate to each other. The spectral shift associated with the transmission maxima/minima suddenly flip around these wavelengths and the spectrum shows largest (nil) spectral shift for the transmission maxima/minima closest to the critical ( at the cross-over) wavelength, corresponding to the same perturbation parameter. A theoretical explanation of the observed experimental behavior has also been presented, highlighting that the underlying mechanisms for the existence of these wavelengths are entirely different. The knowledge of the precise spectral location of these wavelengths is necessary to avoid false alarms.