tal ways to tune a semiconductor laser: changing the index through a change in carrier concentration or changing the laser cavity length. In all the cases, if tuning is achieved by index change, A2/2 ~ Ar/r; if tuning is achieved by changing the cavity length, AN/2 ~ AL/L. Telecommunication Networks TABLE 5.5 Characteristics of Practical Tunable Lasers for Use as Transmitters in Telecommunication Systems External Cavity with Technology Liquid Crystals Mirror Laser Array Multisection DBR Output power 13 dBm 15 dBm 13 dBm Output power adjust Tuning range 35nm (C band) 40nm (C band) 35nm (C band) Tuning mode Discrete (ITU-T grid) Discrete (ITU-T grid) Discrete (ITU-T grid) Channel number Channel spacing 50 GHz 50 GHz 50 GHz 50 dB 45 dB 40 dB Linewidth 250 KHz 10MHz -155dB/Hz -145 dB/Hz -140dB/Hz Laser gain current 250 mA 125 mA 100mA Reflector currents Phase current 7.5mA SOA Current 90-160mA Case operating -5°C to 70°C 0°C to 70°C -5°C to 75°C temperature Network applications involving DWDM systems require a much wider tuning with respect to the potentiality of thermal or current injection tuning in order to use tunable laser effectively. Generally tuning over an extended C band is required, that is, tuning over 40nm, while retaining the characteristics of fixed-wavelength lasers. Today technology offers essentially three types of widely tunable lasers: multisection lasers, external cavity lasers, and laser arrays [1]. In Table 5.5 the main parameters of prac- tical tunable lasers for use as transmitter in telecommunication systems are summarized. 5.2.4.1 Multisection Widely Tunable Lasers The simpler type of multisection widely tunable laser is a multisection DFB. Since a stan- dard DFB can be tuned thermally only about 5 nm or less, wider tunability is achieved by creating a laser with multiple cavities. Nonuniform excitation along the cavity in DFB laser enables a change in the lasing condition. Multi-electrode DFB lasers have been realized to achieve nonuniform excitation where the electrode is divided into two