Enhancement of Fiber Optical Environmental Sensor Performance Via All-Optical Signal Processing
Creators: name: Krarup, Ole
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2024
Subjects: optics
nonlinear
fiber
signal
processing
photonics

description: The manipulation of signals is crucial for countless technologies that support the modern world. From electrical circuits turning modulated radio-waves into sound to optical amplifiers and transceivers handling laser pulses used for digital internet communication, the need for flexible methods that enhance signal quality to improve performance has grown steadily. When handling optical signals representing telecommunications data or measurements from fiber optical environmental sensors, limiting factors include the fundamental physics governing the wave-like nature of photons and the electronics utilized to generate and detect light. For example, the spectral width of an optical cavity is determined by the way light reflects off its end faces, while detecting the interference of two lasers with a frequency difference greater than around 100~GHz is difficult because the response time of electrical components used for detection is too long for such fast variations to be measured. To overcome such limitations and maximize performance, methods for all-optical signal processing, where power-dependent effects in waveguides, such as the Kerr effect, are exploited, have been developed to reshape optical signals before they are detected electronically. This thesis presents a collection of novel methods for utilizing the Kerr effect to pre-process laser signals from fiber optical environmental sensors. First, the operational principles of a set of fiber optical sensing techniques based on fiber Bragg gratings, chirped pulse optical time domain reflectometry and polarimetry are discussed along with their associated limitations. Results of theoretical analysis show that propagation of two lasers with different frequencies in an optical fiber having a large power-dependent electric susceptibility gives rise to frequency sidebands. The optical power of these sidebands is expressed in terms of the parameters of the input lasers, and the possibilities for exploiting these sidebands to enhance the performance of environmental sensors are demonstrated via four peer-reviewed papers. The first paper details how to potentiate the power of a laser signal from a fiber-based temperature sensor and thus increase its resolution by extracting the frequency sidebands generated when launching it and a second laser into a nonlinear medium. The second paper shows that launching pulses undergoing a frequency sweep and a fixed frequency laser into a nonlinear medium causes higher order sidebands to sweep frequency ranges that are integer multiples of the incident one. In collaboration with a fellow PhD student, this effect is utilized to extend the sensing range of a distributed temperature sensor based on chirped pulse optical time domain reflectometry. The third paper presents a novel model describing the impact of optical polarization on the generation of sidebands. The effects described by the model are practically utilized for enhancing the sensitivity of a polarimetric optical sensor that measures changes in applied strain. In the fourth paper, the model presented in the third paper is exploited to develop a novel polarimetry method, which exploits the relationship between laser polarization and sideband power to determine the state of polarization of a laser under test by all-optical means. Finally, the contributions of this thesis to research on all-optical signal processing are summarized and avenues for further experimental research enabled by this publication are discussed. The extension of an analytical model of sideband generation to account for polarization and the experimental demonstration of its practical applications constitute the most significant contributions of this thesis to the field of all-optical signal processing as the equations describing laser signals in nonlinear media rarely admit analytical solutions.