Mode-locking of a Linear Fiber Laser Assisted by a Semiconductor Saturable Absorber

Active harmonic mode-locking is particularly attractive for communication systems which require a high bit-rate locked to an external clock. However, most such lasers are implemented in a ring geometry where pulses pass through the modulator in only one direction. A linear laser has several advantages such as the possibility of integration, the simple incorporation of a semiconductor saturable absorber mirror, and the feasibility of environmental stability through Faraday rotation. But in a linear laser, modulation of the pulse traveling in the direction opposite to that of the traveling wave modulation signal can produce loss and affect timing. In this work we demonstrate modelocking of a linear harmonically mode-locked fiber laser with a traveling-wave phase modulator, and show that for frequencies greater than the inverse of twice the transit time through the modulator, the phase modulation on the counter-propagating wave can be insignificant.