Optical Vernier Effect Enabled Narrow Linewidth All-Fiber SBS-FWM-Based Tunable Optical Frequency Comb Generation

In this paper, we experimentally demonstrate a low-power, broadband, and narrow linewidth SBS-FWM OFC with tunable line spacing of 50 to 600 GHz. The SBS-FWM OFC spans a bandwidth of
$\approx$
100 nm over the C
$+$
L-band region using
$\approx$
15 dBm pump power. The OFC is obtained by cascaded FWM process in highly nonlinear fiber (HNLF), seeded by SBS pumps in an all-fiber ring cavity setup. The linewidth of the SBS-FWM comb lines is measured to be
$\approx$
20 MHz which is largely determined by the SBS pump properties. By using the optical Vernier effect, the linewidth of comb lines is reduced by a factor of more than 2000 to below 20 kHz. We utilize the SBS-FWM OFC to study the performance of binary and 4-PAM transmission at 1 and 10 Gbps in a back-to-back (B2B) setup. The side-modes caused by the FP-cavity, formed by the HNLF modules in the ring cavity in our setup, affects the performance of the B2B transmission of PAM system.