New method of measuring ultra-narrow laser line-width

A new method of measuring Ultra-narrow laser line-width using the second Stokes wave of Brillouin fiber ring laser as reference laser is proposed. The characteristic of line-width narrowing of the first and second Stokes wave in Brillouin fiber ring laser is analyzed, which shows line-width of the second Stokes wave could fall in the sub-Hertz domain. The characteristic of the method using a reference laser with zero line-width is theoretically analyzed, which shows the spectrum line-shape of beat is identical with that of the laser under tested. Further analysis shows that this method is suitable for the measurement of laser line-width below 100 kHz, with the advantages of simple set-up, high accuracy and a wide measurable waveband.