Design and Application of Anti-Symmetric Grating for Optical Communication Systems

by Castro, Jose M.

In this dissertation, theoretical and experimental investigations leading to the development of a novel type of Bragg grating, denominated Anti-Symmetric Waveguide Bragg Grating (AWBG), are presented. This type of Bragg grating can be applied in diverse areas of optical communication and optical processing by providing compact, integratable devices which do not need circulators to separate incoming from outgoing signals. The principle of the AWBG is derived from Coupled-Mode Theory, and for the experimental demonstration, designs were fabricated using silica-on-silicon technology. The advantages of the AWBG are compared with the previously studied Tilted Bragg grating. The Anti-Symmetric grating exclusively produces a reflection with mode conversion in a two-mode waveguide. This improves the performance by minimizing noise and crosstalk produced by reflection without mode conversion. In addition, the operational bandwidth and versatility are enhanced while keeping the compactness and simplicity of the devices. In this work, the AWBG concept is experimentally demonstrated using several devices: an optical add/drop multiplexer based on AWBG, sampled Bragg gratings, a novel type of interleaved sampled Bragg gratings, and spectral amplitude encoders/decoders. In addition, theoretical work on several applications of the AWBG in spectral phase encoding/decoding and optical cryptography are presented. 20 1. INTRODUCTION 1.1 Introduction to Anti-Symmetric Waveguide Bragg Gratings In recent years, important advances in optical communication and processing have been reported. Key technologies, such as novel schemes to slow light propagation [Vlasov et al. (IBM), 2005; Lenz et al., 2001] and the first laser semiconductors and modulators [Rong et al. (INTEL), 2005] in silicon are being developed. The implementation of these technologies, which represent advances toward all optical signal processing and buffering, may also result in a significant cost reduction due to welldeveloped silicon processing. Considerable research has taken place in emerging technologies to improve security in optical networks. These technologies include quantum cryptography (or communication) developed by Wiesner in the 1970s [Bennett & Brassard 1982], chaotic communication proposed by Louis Pecora in the 1980s [Mullins, 2006], and other schemes for optical encryption that will be presented in this dissertation. Bragg gratings are key elements for optical processing and communications. They are produced by a periodic perturbation in the waveguide geometry or material properties. Depending on the application, this perturbation can be advantageous or detrimental. In copper communication cables, for example, periodic oscillations during fabrication can produce periodic variation in the characteristic impedance. The period of the perturbation in the cable is in the order of several decimeters and can match the size 21 of utilized wavelengths. Therefore, undesired reflections are produced, reducing the operational bandwidth of the cable [Castro, 2004 (l)]. Unlike communication systems utilizing copper cables, optical communication and microwave systems take advantage of this physical phenomenon to successfully produce important components such as Fiber Bragg Gratings (FBG), integrated waveguide Bragg gratings, photonic bandgap crystals and corrugated waveguides among others. FBGs were feasible after the discovery of the photosensitivity in optical fibers (Hill et al., 1978). The periodic modulation of their refractive index in single mode fibers has been the basis to produce spectral filters, WDM, Optical Add Drop Multiplexers (OADM), and dispersion compensators [Kashyap, 1999]. More recently, they have been used in signal processing applications, such as encoder/decoders for optical code division multiplexing [Teh et al, 2001]. Other applications include sensors [Srinivasan & McFarland, 2001], fiber lasers, and pulse reshaping [Petropoulos et al., 2005] . Devices based on single mode FBGs usually require circulators to separate incoming signals from processed (reflected) signals as shown in Figure 1-1. This requirement increases the size, complexity, and cost of the device. ?? ??