4.1 Introduction

The light sources used in DWDM are light-emitting diodes (LEDs), semi-conductor lasers, or glass-doped lasers of different types. Some of these sources are wavelength tunable. The emitters can be directly or externally modulated. For high bit rates an external modulation is generally necessary. Bit rates in excess of 40 Gbps can be obtained from a CW source followed by an external modulator [1, 2]. Publications on external modulation are numerous. For instance, typical examples of high-speed LiNbO3 modulators and a typical application are reported in [1, 2], respectively. But we will not further discuss external modulation, which is outside of the scope of this book. WDM channels can be made from well-separated spectral slices selected in the spectrum of LEDs, superluminescent diodes, doped fiber amplifiers, or other coherent sources. All optical processing in the future telecommunication networks will need wavelength converters that can shift an optical signal from one wavelength to another. In this chapter, we review the main light sources and wavelength-converter techniques for DWDM.

4.2 Semiconductor Lasers