Coherent narrow-linewidth optical frequency synthesis across the optical telecommunication band

Broadband, coherent narrow-linewidth optical frequency synthesis is of crucial importance in dual-comb interferometric measurement. Here we present a detailed description of the construction and performance characterization of a hertz-level linewidth coherent optical frequency synthesizer across the optical telecommunication band. A narrow-linewidth cavity-stabilized laser at 1565.00 nm is built and coherently transferred through a fiber link with an additional fractional frequency instability of 2.0×10^-16 at 1 s averaging time. Broadband, coherent optical frequency synthesis is then achieved by steering one mode of a laser frequency comb with the transferred optical frequency oscillation. By beating with a 1542.14 nm ultra-stable cavity-stabilized laser, the evaluated fractional frequency stability and absolute linewidth of the nearest synthesized optical oscillation are 3.5×10^-15 at 1 s averaging time and 1.8 Hz, respectively. According to the ultra-low-noise feature of the utilized laser frequency comb of 4.7×10^-17 at 1 s averaging time, the synthesized optical frequency oscillations could maintain the high coherence across the comb's output bandwidth.