All-Optical Signal Processing Using the Kerr Effect for Fiber-Based Sensors

Abstract

All-optical signal processing has grown over the last decade due to the demand for high-speed and high-bandwidth data processing. The main objective of all-optical signal processing is to avoid signal conversions from the optical domain to electrical domain and then back to optical, which introduces noise and bottlenecks data transmission speeds. These conversions can be avoided by manipulating light using an optical medium, e.g. an optical fiber, and taking advantage of the nonlinear response of the medium's dipoles to an external electric field. Nonlinear effects arising from the third-order nonlinearities, such as the Kerr effect, allow for an intense light beam to modify the refractive index of a medium through which it propagates. As a consequence, the phase of the light beam changes as it propagates and new frequencies are generated; this phenomenon is referred to as self-phase modulation (SPM). Light's ability to modify not only its own properties but also the properties of other co-propagating beams has been widely applied in telecommunications to create integrated all-optical data regenerators. While optical fibers are mainly utilized to transmit data at extreme speeds, they can also act as sensors when considering the reflected signal as opposed to the transmitted signal. Surprisingly, most of the fiber sensing field relies on electrically-driven components for manipulating light and does not take advantage of all-optical signal processing capabilities.

In this thesis, we demonstrate the use of the nonlinear Kerr effect to improve aspects of both fiber point and distributed sensing. These sensing scenarios respectively refer to the use of a fiber as a single sensing element, and to the detection of external perturbations continuously along the entire length of the fiber. The sensing improvement are obtained by first inducing a sinusoidal modulation on the light before it experiences self-phase modulation in a nonlinear medium, leading to the generation of optical sidebands. By judiciously adjusting the peak power of the light and extracting a specific sideband, multiple all-optical signal processing functions are achieved. First, high extinction ratio pulses can be generated by extracting a higher-order sideband, which allows for extending the sensing distance of distributed fiber-based sensors. The extinction ratio refers to the ratio between the pulse peak and pedestal powers. To quantify the generated extinction ratios, we develop a measurement technique based on a single-photon counter and measure a pulse exhibiting a 120 dB extinction ratio, which was originally created by an electro-optic modulator with a 20-dB extinction ratio. Second, all-optical peak power stabilization can be achieved by extracting the first-order SPM-generated sideband. We utilize this technique to stabilize the peak power of an optical pulse sent to a distributed fiber sensor. We demonstrate that this stabilization technique allows for the detection of applied vibrations that would otherwise remain buried in the background noise. Third, we demonstrate an all-optical scheme, based on sinusoidally-modulated light experiencing SPM, that enables the magnification of fluctuations in the peak power intensity of a pulsed signal. The light's peak power at the entrance of the nonlinear medium is adjusted to reach a power regime yielding a magnification factor of 2m+1, when extracting the mth-order SPM-generated sideband. Finally, we propose a new sensing scheme composed of two all-optical signal processing steps to allow for the detection of environmental perturbations previously too small to be detected by a given intensity-based fiber sensor.