Feedforward Laser Linewidth Narrowing Scheme Using Acousto-Optic Frequency Shifter and Direct Digital Synthesizer

A digital feedforward-type linewidth narrowing scheme for single tune lasers is presented in this paper. An acousto-optic frequency shifter driven by RF signal generated by a direct digital synthesizer is used to externally manipulate the phase of the laser. The RF signal from the direct digital synthesizer carries real-time controlled phase variation, which is opposite to the laser phase noise, suppressing the linewidth of the laser. With the help of a compact electro-optics system and digital control algorithm, the linewidth of a commercial DFB laser can be narrowed by nearly three orders of magnitude from 2.2 MHz to 3.1 kHz. In addition, the scheme supports extensions such as the linewidth narrowing for frequency swept lasers, which is also demonstrated and discussed. Owing to the performance, cost, and flexibility, this scheme has the potential to be applied in optical fiber sensors, LIDARs, laser spectroscopy systems, and other related applications.