, “Adaptive optics for fast optical communication: Spatial solutions for spectral issues
,” 2013. Publisher's VersionAbstract
In recent decades, the use of fast optical signals has become increasingly dominant, both in scientific research and in engineering applications. High speed photonics serves as the core of modern worldwide communication networks, as well as in many optical signal processing applications. Such applications rely on the ability to control, filter and manipulate large bandwidth signals. Traditionally, such control can be realized using fast electronics. However, continuous growth in data rates makes this option impractical, since the signals become too fast to control even for cutting edge electric circuit technology. The alternative is to use an all-optical system, where signal control is done in the frequency (spectral) domain. Such a system must be capable of manipulating large bandwidth signals with high spectral resolution. Such optical systems are essential in optical communication networks, for performing signal conditioning, impairment mitigation and WDM channel power equalization.
In this work I explore a family of optical sub-systems combining guided-wave and free-space optics for spectrally resolving optical signals at unprecedented resolution, and actively manipulating the spectral components with spatial light modulator (SLM) technology. The ability to combine the employed cutting edge technologies, including a high resolution planar lightwave circuit (PLC) arrayed waveguide grating (AWG), together with the state-of-the-art phase SLM, which was adapted from the light projection industry, enables the design and demonstration of high resolution photonic spectral processors (PSP). This system is capable of applying arbitrary spectral phase and amplitude at high spectral resolution to an optical signal and of controlling its properties in the time domain. A PSP can be configured for addressing the entire conventional optical communication band, at a price of poor resolution due to the finite space-bandwidth trade-off. Alternatively, the PSP can be designed as a colorless adaptive device, operating with a free spectral range (FSR) matching the channel plan, e.g. with a 100-GHz FSR, for in-band high-resolution wavelength division multiplexing (WDM) filtering applications. By using two-dimensional free-space optics achieved by crossing the PLC AWG with a bulk grating, a new broadband processor was introduced. This PSP is capable of controlling independent WDM channels on the 100 GHz grid at the high resolution of the colourless solution, thereby shattering the space-bandwidth limitation.
Based on these concepts, a family of novel systems and implementations were developed and investigated. In this thesis I introduce six papers which demonstrate the design and implementation of three PSP systems, based on hybrid waveguide/free space optics arrangements. The papers are divided into two groups: in the first group, three papers present the evolution of the spectral processing device, from the simplest version of colorless PSP up to two dimensional PSP arrangement with full spectral, control along the c-band. The second group contains three papers describing several implementations of these technologies, including amplitude filtering applications (Nyquist-WDM generation), phase filtering applications (tunable chromatic dispersion compensation and group delay stairs generation) and a demonstration of a new fiber laser which was built using the PSP platform. These high spectral resolution devices and systems can serve as an important element in controlling dispersion, enhancing signal quality and optimally filtering a distorted signal, and their development is essential for the progress in the optical fiber communication world.