In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

We present analytic expressions for the frequency-domain transfer function of semiconductor optical amplifier Mach-Zehnder interferometric (SOA-MZI) switches that employ a single optical control signal and a continuous wave input optical beam. Our analysis relies on first-order perturbation theory approximations applied both to the SOA response as well as to the SOA-MZI characteristics, yielding a frequency response that enables a qualitative insight into the different SOA-MZI operational regimes. The final transfer function expression is utilized for the analysis and evaluation of the multifunctional potential of SOA-MZI switches, concluding with the necessary conditions for supporting a number of completely different SOA-MZI-based nonlinear signal processing applications that have been demonstrated experimentally: wavelength conversion, packet envelope detection (PED), and clock recovery (CR). The theoretically obtained operational conditions are in close agreement with experimental observations, showing that SOA-MZIs can serve as functional circuit elements in applications with different requirements depending on its operational parameters: as low-pass filtering devices with cut-off frequencies in the megahertz regime or in the multi-gigahertz regime, and as resonant modules resembling band-pass filtering structures. The validity of our theoretical SOA-MZI frequency-domain system model is further confirmed by its successful incorporation in a Fabry-Perot assisted SOA-MZI subsystem, demonstrating PED and CR operations through the exploitation of typical systems theory tools.

We present theoretical and experimental performance analysis of 40 Gb/s Non-Return-to-Zero (NRZ) All-Optical Wavelength Conversion (AOWC) using a differentially-biased SOA-MZI. A frequency domain transfer function model for both the standard single-control SOA-MZI-based AOWC and for the differentially-biased SOA-MZI is analytically derived, exploiting first order perturbation theory techniques and showing that only the differentially-biased scheme can yield an almost flat low-pass filtering response enabling wavelength conversion at 40 Gb/s. The theoretically obtained results are also confirmed through experiments that demonstrate successful 40 Gb/s AOWC functionality for NRZ data signals only when a differentially-biased SOA-MZI configuration is employed, whereas an error-floor is obtained when 40 Gb/s NRZ AOWC with the standard single-control SOA-MZI scheme is attempted. The 1.7 dB negative power penalty obtained by the differentially-biased SOA-MZI architecture confirms its enhanced regenerative properties and its potential for extending 40 Gb/s optical transparent network dimensions by means of cascaded 2R AOWC stages.