ABSTRACT
Since the introduction of low-loss single mode fiber, there has been a constant effort to exploit the bandwidth of the medium for both digital and analog transmission systems. This has led to transmission systems operating around 1310 nm in the zero dispersion region of standard single mode fiber (SSMF) and later also to transmission in the C-band where the fiber loss reaches a minimum. Multi-channel dense wavelength division multiplexing (DWDM) transmission has increased the bandwidth requirement around 1550 nm extending to the neighboring S-and L-bands for additional channels. While DWDM uses narrow channel spacing of typically 0.4 to 0.8 nm, coarse wavelength division multiplexing (CWDM) follows a different path; here, the channel spacing is increased to 20 nm and the bandwidth covers the entire low-loss region of single mode fiber from 1270 or 1310 nm to 1610 nm [1]. The idea behind wide channel spacing is the use of low-cost uncooled lasers exhibiting a wavelength drift with temperature and relaxed requirements for manufacturing passive components such as multiplexers. One of the simplest and most cost-effective
techniques for modulating a light source is to directly modulate the laser drive current with the desired waveform. This continues to be the most common approach for low-cost optical communication systems while external modulation is used for achieving higher transmission performance in trunk applications where transmission distance noise and distortion requirements are beyond the capabilities of direct modulation. In this approach, a modulator, external to the laser device, is used to impart the intensity modulation on the continuous wave (CW) output of the laser.
