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 mu W. Lasing under the same condition in continuous-wave mode over 90 min is achieved at an average power level of 21 mu W and a standard deviation of 0.17 mu W. 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.