Characteristics of Critical-Wavelength-Existed Fiber-Optic Mach–Zehnder Interferometers and Their Sensing Applications

Abstract

In this paper, we review the characteristics of critical wavelength (CWL)-existed fiber-optic Mach–Zehnder interferometers (MZIs), including special few-mode fibers and microfibers, and their sensing applications in physical, chemical, and marine fields. Owing to the existence of CWL in the transmission spectra, the in-line MZIs show some specific characteristics. The closer the peak/dip wavelength to the CWL, the larger the wavelength shift or the related sensitivity when the interferometer is under testing. Meanwhile, CWL shifts monotonically with the variations in measurands, such as temperature (in the air or seawater), axial strain, water pressure, surrounding refractive index, etc., when they are applied to the sensing fibers. These characteristics of the CWL-existed in-line MZIs make them appealing solutions for fabricating various interferometric sensors, with the advantages of large measurement range, high sensitivity, multiparameter sensing, etc. Theoretical and experimental studies on the properties of the CWL-existed in-line MZIs are reviewed and discussed in this paper.