Graphene-Based Optical Fiber Sensing and All-Optical Switching

基於石墨烯的光纖傳感和全光開關

Student thesis: Doctoral Thesis

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Award date2 May 2019

Abstract

As the first two-dimensional material successfully isolated in 2004, graphene, which is a one-atom-thick carbon crystal, has been extensively studied in this decade. Graphene exhibits many excellent optical properties and has been incorporated into optical fiber/waveguide devices and sensors for various applications. In particular, incorporating graphene onto optical fibers is appealing because of its compatibility with present fiber systems.

There are two main challenges in the development of graphene-based fiber devices. The first challenge is the selection of an appropriate optical model of graphene in analyzing graphene-based fiber structures. Three optical models of graphene, namely the isotropic, anisotropic, and interface models, are available. In Chapter 2, the modal properties of graphene-attached microfibers and D-shaped fibers are analyzed with the three optical models of graphene. The numerical results show that the interface model and the anisotropic model generate accurate results, while the isotropic model can significantly overestimate the graphene-induced modal losses unless the graphene thickness assumed is sufficiently small. The study clarifies the accuracies in various analyses of fiber structures based on different models and confirms that the interface model is the preferred model for a reliable analysis of graphene-based fiber structures.

The second challenge is the achievement of sufficiently strong light-graphene interaction with a fiber structure. Graphene-attached microfibers, D-shaped fibers, and special fibers with cores located near the surface have been proposed to improve light-graphene interaction by enhancing the evanescent field. However, microfibers and D-shaped fibers are fragile, and special fibers are expensive to make and difficult to align. In Chapter 3, a new mechanism to solve this problem is proposed and demonstrated, which is based on applying the cladding modes of a fiber excited by a long-period fiber grating. The dependences of the graphene-induced losses of the cladding modes on the mode order, the fiber dimension, the number of graphene layers, and the surrounding refractive index are studied. Experimental results on the measurement of the variations of the graphene-induced losses of the cladding modes with the surrounding refractive index agree qualitatively with the theoretical results. This mechanism can be explored for the realization of intensity-based refractive-index sensors.

According to the mechanism proposed in Chapter 3, a robust graphene-coated in-fiber Mach-Zehnder interferometer formed with a pair of 3-dB long-period fiber gratings is proposed and demonstrated for ammonia gas sensing in Chapter 4. The sensor operates on the principle of changing the phase of the cladding mode of a fiber through changing the conductivity of the graphene coating by adsorbed ammonia molecules, which gives rise to a shift in the interference spectrum with an amount that depends on the gas concentration. The experimental sensor shows a high sensitivity of ~3 pm/ppm for an ammonia gas concentration from ~10 ppm to ~180 ppm. The proposed in-fiber Mach-Zehnder interferometer can serve as a generic platform for the development of fiber sensors and devices that incorporate two-dimensional materials.

The excellent thermal property of graphene also allows the development of graphene-based fiber devices. In Chapter 5, an all-optical switch based on a graphene-coated fiber Mach-Zehnder interferometer is proposed, where the phase of the signal light in one arm of the interferometer is changed by the heat generated from the absorption of external pump light by the graphene coating. The external pumping scheme allows efficient pump absorption with multiple layers of graphene coated on an ordinary fiber or a slightly tapered fiber without introducing significant additional signal loss. Without using any wavelength multiplexer/demultiplexer, the switch can be pumped at any convenient wavelength or even with broadband light. The experimental device based on a standard 125-μm-diameter single-mode fiber with a 5-mm-long graphene coating can be switched with a pump power of 5.3 mW at an extinction ratio of 19 dB with no additional signal loss. The switching power is insensitive to the length of the graphene coating and can be reduced to 4.8 mW with the fiber tapered to 40 μm. The measured switching powers agree well with the theoretical values obtained by treating the graphene coating as an ideal uniform sheet of heat source. The response time of the switch decreases with the fiber diameter and inversely with the length of the graphene coating. The rise and the fall times of the switch based on a 40-μm tapered fiber with a 20-mm-long graphene coating are 30 ms and 50 ms, respectively.

The studies reported in the thesis provide theoretical guidance and implementation approaches for incorporating graphene onto optical fibers, and make contribution to the development of applications of graphene-based fiber sensing and all-optical switching.