 to the laser, leveraging on suit- able fabrication processes. Typical wavelengths for pump diode lasers used in telecommunication amplifiers are 800nm 980nm, and a wide band around 1480nm, where almost all the pump lasers for Raman amplification are located. A key parameter for a power laser that has to work as an amplifier pump is the bright- ness. It is defined as the maximum power density that can flow through the output mirror per unit area and per unit solid angle. Whatever the potential output power, the real laser performance is limited by the bright- ness limit at which the output mirror undergoes catastrophic damage that ruins the laser working. In order to improve this threshold, it is important to coat the output facets of the laser with suitable coatings that tune the mirror reflectivity to the optimal value. Just to give an example, an uncoated InGaAs/GaAs laser emitting at can have a brightness limit of 10MW/cm², while a similar uncoated laser realized in InGaAsP/ InGaAs to pump at 1480n does not go over 5MW/cm2. A suitable coating realized with a set of dielectric films can raise these figures up to 20 and 5MW/cm2 respectively, improving greatly the laser performances. In Table 5.4 a set of characteristics of practical pump lasers for different applications is reported. Telecommunication Networks TABLE 5.4 Characteristics of Practical Pump Lasers for Different Applications Application EDFA Pump EDFA Pump Raman Pump Raman Pump Optical power 100-300mW 160mA 350mW 420mW Threshold current 401 mA 150mA Operating drive current 250-600 mA 600mA 1.2 A 1.7 A Centre wavelength 974nm 1480nm 1420-1510nm 1420-1510nm Spectral width 1.5nm Operating temperature -20°C to 75°C 0°C to 70°C -20°C to 70°C -20°C to 70°C Footprint (w/pins, w/o pigtail) 50x37: mm 50 28 mm 50x40mm 50x40mm 5.2.3 Vertical Cavity Surface-Emitting Lasers The vertical cavity surface-emitting laser (VCSEL) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting s