ology for Telecommunications: Integrated Optics and Microelectronics Modulation response (dB) 9.2 mA 0.5 mA 4.3 mA Frequency (GHz) FIGURE 5.17 Spectral response to amplitude direct modulation for different values of the bias current of a long-wavelength VCSEL emitting at 1550nm (After Hofmann, W. et al., Uncooled high speed (>11 GHz) 1.55 um VCSELs for CWDM access networks, Proceedings of European Conference on Optical Communications-ECOC 2006, s.n., Cannes, France, 2006.) where W R is the natural resonance frequency of the VCSEL is the dumping factor of the relaxation oscillations Wp is a characteristic parameter whose expression can be derived from more fundamen- tal characteristics of the VCSEL and that represents the intrinsic low-pass response of the structure in the absence of laser effect due to parasitic and thermal effects The frequency response deriving from Equation 5.13 is plotted in Figure 5.17 [19] for an experimental VCSEL emitting at 1550 nm up to 2mW. The resulting modulation bandwidth is never lower than 10.8GHz if the bias current is greater than 4.3mA, qualifying this experimental VCSEL as a very interesting source for high-capacity access networks like G-PON and 10G-PON and for coarse-wavelength division multiplexing (CWDM) systems. 5.2.4 Tunable Lasers Almost all semiconductor lasers are tunable by controlling the laser working tempera- ture. Thermal tuning is generally limited to few nanometers, but for devices specifically designed for tuning where it can reach 5-6 nm. Thermal tuning is generally used to maintain the laser on the correct wavelength through contrasting changes during an operation due to aging and environment. Another way of tuning a standard semiconductor laser is to change the current injec- tion. As a matter of fact, continuous current injection changes the carrier's density inside the laser cavity and causes a slight change of the emission wavelength. Although this second method of tuning is even less effective than thermal tuning it indicates that there are two fundamen