Experimental Study of the Timing Jitter of a Passively Mode-Locked External-Cavity Semiconductor Laser Subject to Repetition Rate Transitions and Optical Feedback

We experimentally investigate the timing jitter (TJ) of a passively mode-locked external-cavity diode laser. Variation of the gain current and the absorber reverse bias voltage allows transitions from fundamental mode-locking up to seventh harmonic mode-locking. Hereby, a reduction of the TJ as a function of the harmonic mode-locking order is found. Furthermore, the application of optical feedback results in an additional reduction of TJ for almost the whole investigated operation range. In particular, the reduction increases with harmonic mode-locking order. The highest observed reduction of TJ amounts to a factor of 10 as compared with the free-running case which corresponds to a repetition-rate linewidth reduction by a factor of 100.