Optical receivers based on multistage laser preamplifiers

An examination is made of the performance of optical preamplifier receivers using cascaded semiconductor laser amplifiers (SLAs), emphasis being placed on the influence of signal frequency fluctuations (chirp). The analysis makes use of a lightwave simulation package designed to accommodate laser chirp, fiber dispersion, laser preamplifiers, and receiver characteristics. The signal-to-noise characterization of cascaded structures is assessed. To address the system performance in the presence of laser chirp, simulation results are presented for a 1.5-μm optical system assuming 2-Gb/s RZ transmission through a fiber with 20-ps/(nm-km) dispersion. Also discussed is a polarization-insensitive receiver which is based on cascaded laser preamplifiers. This approach requires only a single photodetector and hence it is potentially simpler than a polarization-diversity receiver. The difference in sensitivity for the two orthogonal polarizations is estimated to be less than 0.7 dB for the cascaded preamplifier receivers