Narrow Linewidth, Arbitrarily Tuning DFB Laser Using 3 x 3 Optical Coupler-Based Fully Digital Feedback

Abstract

Active feedback has been employed in the context of DFB lasers to generate chirp signals with narrow linewidth. While recent works often adopt heterodyne structures, homodyne structures, despite their simplicity, struggle to stabilize lasers at the in-phase point of delay line frequency discriminators for long term. This study introduces an active feedback method employing a 3 x 3 optical coupler and digital signal processing to break the phase limitation of homodyne delay line frequency discriminators, thus facilitating the generation of arbitrary frequencies. The proposed structure enables precise control of laser frequency with high linearity. This approach also achieves a linewidth reduction in DFB laser from 2.4 MHz to 5.8 kHz. The jitter is reduced from 56 ps to 30 fs (integrated within 10 Hz - 1 MHz). Additionally, the frequency tuning range of laser exceeds 6 GHz and its time response achieves a level on the order of a few hundred nanoseconds.