100-Hz-Linewidth, Tunable Terahertz-Wave Generation Using Two Mutually Injected Semiconductor Lasers

This study experimentally investigates the generation of terahertz (THz) waves with high spectral purity and wide frequency tunability using two mutually injected semiconductor lasers with highly asymmetric injection strength. A simple system architecture is proposed, where only one optical injection route consisting of an optical fiber and an optical attenuator between the two lases is required. No optical circulator nor optical isolator is needed. The strong injection from the first laser induces a self-sustained, single-period intensity oscillation of the second laser, namely period-one nonlinear dynamics, for the generation of THz waves. The THz frequency is demonstrated tunable from 100 GHz up to at least 360 GHz by adjusting the frequency difference between the two lasers beyond multiple integrals of the laser free spectral range. Such tunability is highly likely to work over the entire THz band if an adequately large frequency difference between the two lasers is experimentally accessible. The weak injection from the second laser induces regeneration of the period-one dynamics in the first laser and thus equivalently forms an external cavity to establish high spectral purity for THz waves. A linewidth of less than 160 Hz and phase noise of -80 and -95 dBc/Hz at the 10- and 100-kHz offset frequencies, respectively, are demonstrated achievable. Such high spectral purity is highly likely to further improve if an adequately long optical injection route is experimentally accessible.