 expansion. Technology for Telecommunications: Integrated Optics and Microelectronics p electrode Contact Grating p-InP n-InP Waveguide layer Active n electrode FIGURE 5.12 Schematic section of a high-performance DFB laser. Power (dBm) 1 = 1552 nm 31.5 dB Wavelength (nm) FIGURE 5.13 Emission spectrum of a typical DWDM DFB. A change in the refractive index alters the wavelength selection of the grating structure and thus the wavelength of the laser output, producing a narrow band wavelength tunable laser. The tuning range is usually on the order of 6nm for an ~50K (90°F) change in tempera- ture, while the linewidth of a DFB laser is on the order of a few megahertz. Altering of the current powering the laser will also tune the device, as a current change causes a tempera- ture change inside the device. If this temperature sensitivity can be exploited to tune the laser on the opportune wave- length, it also implies that, in order to match the stability requirements of high-performance DWDM systems, the laser has to be temperature-stabilized. This is generally done through a Peltier temperature controller and a feedback control loop that assure sufficient temperature stability in the face of environment changes and the device heat production during working. Telecommunication Networks TABLE 5.3 Characteristics of Practical DFB Lasers for Use as Sources in Telecommunication System CW Source in Single-Channel Short- High-Performance DWDM Reach Interfaces Carrier Source Threshold current Operating current 150mA Modulation current 25mA (Estintion ~6dB) Only CW operation Output power 2.5dBm 13 dBm Slope efficiency 0.025mW/mA Direct mod. extinction ratio Wavelength 1550-1560nm ITU-T grid-100 GHz Spectral width 250KHz Side mode suppression 45 dB Relative intensity noise -120 dB/Hz -140 dB/Hz Polarization extinction ratio 25 dB Operating temperature -20°C to +70°C 15°C-50°C There are generally two distinct types of DFB lasers. Traditionally, DFBs are antireflec- tion-coated on one side of the cavity and coated for high reflec