A Narrow-Linewidth On-Chip Toroid Raman Laser

In this paper, we report a narrow-linewidth on-chip toroid Raman laser. Under free-running condition, we have obtained the minimum fundamental linewidth of 3 Hz and the maximum unidirectional output power of 223 μW. Lasing under the same condition in continuous-wave mode over 90 min is achieved at an average power level of 21 μW and a standard deviation of 0.17 μW. We further derived the frequency noise spectrum and identified an enhancement of frequency noise due to Kerr non linearity. In addition, we have observed the shifting of relaxation oscillation frequency as a consequence of weak mode splitting.