  The terahertz (THz) frequency range, spanning 0.1 to 10 THz, is a field ripe
for innovation with vast, developing potential in areas like wireless
communication and molecular spectroscopy. Our work introduces a dual-wavelength
laser design that utilizes stimulated Brillouin scattering in an optical fiber
cavity to effectively generate two highly coherent optical Stokes waves with
differential phase noise inherently mitigated. To guarantee robust operation,
the Stokes waves are optically injected into their respective pump lasers,
which also serves to greatly improve the resulting coherence. The frequency
difference between the two wavelengths is converted into THz waves through a
uni-traveling-carrier photodiode. This innovative design facilitates the
generation of THz waves with phase noise levels of less than -100 dBc/Hz,
translating to timing noise below 10~$\mathrm{as} / \sqrt{\mathrm{Hz}}$ at 10
kHz Fourier frequency, over a carrier frequency range from 300 GHz to 3 THz.
This development in phase noise reduction establishes a new benchmark in the
spectral purity of tunable THz sources. Such advances are pivotal for
applications to move beyond oscillator constraints.
