Development of the variable pulsewidth mode-locked femtosecond laser using coherent coupling

Laser processing field demand high energy pulses, thus widening pulsewidth is necessary for femtosecond pulses [1,2]. We have developed a novel method variable pulsewidth laser, which is different from conventional systems with optical BPF (Band-Pass Filter). Our pulsewidth tuning system applies a coherent coupling. When two laser-beams are coupled, output-longitudinal-modes are emitted by corresponding longitudinal modes of respective laser modules. In this study, one longitudinal-mode-space is changed arbitrary by tuning cavity length to control the output-longitudinal-modes. As the result, an envelope of the longitudinal modes, i.e. spectrum bandwidth is controlled to tune the pulsewidth. In experiment, the laser modules of our system generate about 150 fs pulsewidth and around 30 nm spectrum bandwidth respectively. The result of the coupling, the pulsewidth is tuned with cavity length. The cavity length difference is controlled to 1.5 mm between the laser modules, and we have succeeded in widening the pulse to 1.2 ps at maximum.