The phase noise of wavelength tunable semiconductor lasers is becoming increasingly more important in fast optical switching networks employing advanced modulation formats. The objective of this thesis is to develop techniques to investigate the phase noise of different types of tunable lasers, and investigate the performance of these devices when used in dynamic coherent networks. Specifically sampled-grating distributed Bragg reflector (SG-DBR) lasers and a three-section tunable slotted Fabry-P´erot (SFP) laser are studied in this thesis. The optical linewidth of the devices is measured using a standard delayed selfheterodyne method and a new measurement technique using an optical quadrature front end. The effect of laser phase noise in system environments is examined by employing the lasers in a static 1.25 Gbit/s differential binary phase shift keying (DPSK) transmission system and a 10.7 Gbaud differential quadrature phase shift keying (DQPSK) packet switching system. The work presented in this thesis also introduces a new method for calibrating SG-DBR lasers to achieve the best possible phase noise. The method is based on using the voltage signal of the gain section of the SG-DBR laser, and can achieve a linewidth reduction of up to 80% compared to techniques that only calibrate for the side-mode suppression ratio (SMSR) and the output power of SG-DBR lasers. The results in this thesis show the importance of characterization of laser phase noise when tunable lasers are used in dynamic coherent systems.