Cascaded Brillouin fibre lasers coupled to unitravelling carrier photodiodes for narrow linewidth terahertz generation

Narrow linewidth terahertz (THz) generation using a cascaded Brillouin fibre laser structure and a unitravelling carrier photodiode is investigated. Using two distributed feedback lasers separated by > 1 THz with 1 MHz initial linewidth, the use of the cascaded Brillouin laser greatly increases the linewidth reduction factor compared to a single Brillouin laser structure. The linewidth of the realised THz source is found to be <; 100 Hz at 1014.7 GHz without the use of any active stabilisation of the laser cavity or any microwave reference.