Simultaneous Existence of Ultrahigh and Ultralow Spectral Sensitivity for Directional Couplers

Abstract

We present the experimental evidence for the existence of two very unique wavelengths, termed critical and cross-over wavelength in the transmission spectrum of the 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 ( cross-over ) wavelength corresponding to the same external perturbation parameter. We also present a theoretical explanation of the observed experimental behavior, showing 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 and for designing extremely sensitive fiber optic sensors.