Since the telecommunication revolution in the early 90s, that saw massive deployment of optical fibre for high bit rate communications, coherent optical sources have made tremendous technological advances. The technological improvement has been multi dimensional; component sizes have been reduced, conversion efficiencies increased, power consumptions decreased and integrability into compact optoelectronic sub-modules improved. Semiconductor lasers, emitting in the 1.1-1.6 μm range, have been the most prominent beneficiaries of these technological advances. This progress is a result of research efforts that consistently came up with innovative solutions and components, to meet the market demand. This in-phase, demand and supply, problem and solution and consumer need and innovation cycle, has ushered us in to the present information technology era, where stable high speed data links make the backbone of almost every aspect of life, from economy to entertainment and from health sector to defence production. By the start of twenty-first century, a new, low cost, low power consumption and miniaturized generation of lasers had started to capture its own market share. These lasers, named Vertical-Cavity Surface-Emitting Lasers (VCSELs) due to the presence of an optical cavity which is normal to the fabrication plane , have established themselves as premier optical sources in short-haul communications such as Gigabit Ethernet, in optical computing architectures and in optical sensors. While shorter wavelength VCSEL (< 1μm) fabrication technology was readily mastered, due to the ease in manipulation of AlGaAs-based materials, long wavelength VCSELs especially VCSELs emitting in the 1.3-1.5 μ range have encountered several technical challenges. There importance as low-cost coherent optical sources for the telecommunication systems is primordial, since they are compatible with the existing infrastructure. VCSEL utilization in low-cost systems imply the application of direct modulation for high bit rate data transmission which engenders the problems of frequency chirping which increases laser linewidth and severely limits the system performance. Furthermore, relatively lower VCSEL intrinsic cut-off frequencies translated in to impossibility of achieving high bit rates. Optical injection-locking is proposed as a solution to these problems. It enhances the intrinsic component bandwidth and reduces frequency chirp considerably. Source: Advances in Optical and Photonic Devices, Book edited by: Ki Young Kim, ISBN 978-953-7619-76-3, pp. 352, January 2010, INTECH, Croatia, downloaded from SCIYO.COM