Tuning range extension by active mode-locking of external cavity laser including a linearly chirped fiber Bragg grating

We demonstrate electrical wavelength tuning by mode locking of an external cavity laser (ECL) with linearly chirped fiber Bragg grating (LCFBG). The configuration consists of a laser chip providing the gain coupled to an LCFBG with a large chip rate of 10 or 55 nm/cm providing the counter-reaction for laser oscillation. The laser chirp is electrically modulated by a sinusoidal signal in such a way that the ECL is mode locked. By changing the modulation frequency, a wavelength tuning range of 27 nm is achieved with the 10 nm/cm LCFBG, and a partial tuning range over 41 nm is demonstrated with the 55-nm/cm LCFBG. The output pulse stream at a specific mode-locking frequency and a corresponding wavelength is obtained for both positively and negatively chirped grating. A time bandwidth product reduction is measured in the case of negatively chirp grating when compared with positively chirp grating. A simple general law between the laser parameters is given (locking frequency, tuning range, and FBG chirp value). The parameters for a 40-nm tunable source modulated at 10 GHz are given. This simple tuning mechanism is very well adapted for a structure that requires accurate wavelength monitoring.