有源光器件和无源光器件区别及基础介绍.ppt

* Fabry-Perot lasers are the first type of semiconductor lasers to enter the communication market. Also known as multi-longitudinal mode lasers, they produce highly polarized, coherent and low divergence beams. Low spectral width and high optical power enable FP lasers to be used in longer transmission distances. Typical telecom links span about 30 km. Unlike LEDs, lasers operate beyond a threshold condition for normal stimulated emission. Below the threshold, the optical power is negligibly small and the spectrum looks similar to that of an LED. Furthermore, high speed modulation works only above the threshold. Semiconductor lasers need ~10 ns to ~10 μs turn-on time when started below the threshold. * Distributed Feedback Lasers are the highest performance lasers available today. They emit radiation in only one longitudinal mode. The laser cavity is designed with gratings along the active layer. This enables, ideally, one wavelength component to propagate along the cavity and stimulate radiation only at that optical frequency. Like the FP laser, DFB laser produces a highly polarized, low divergent and high power beam. Due to the narrow spectral width, it can be used in long haul fiber optic links; narrow linewidth also enables many lasers to be optically multiplexed. DFB lasers form the core of DWDM systems (more on that later). * VCSELs are used in datacomm applications. These lasers provide radiation in the 850 nm region. Distributed Bragg reflectors are used to reflect a very narrow range of wavelengths, allowing the laser cavity to selectively amplify a narrow range. Spectral widths are narrower than FP lasers but wider than DFBs. They also provide single longitudinal mode beams. * Light sources are primarily used for communication systems. In some cases they are used to characterize passive optical devices such as optical filters. A white light source is a very broadband emission source. It provides the widest frequency range, but does not provide v