  We report on the development, testing, and performance analysis of a bow-tie
resonant cavity for terahertz (THz) radiation, injected with a continuous-wave
2.55 THz quantum cascade laser. The bow-tie cavity employs a wire-grid
polarizer as input/output coupler and a pair of copper spherical mirrors coated
with an unprotected 500 nm thick gold layer. The improvements with respect to
previous setups have led to a measured finesse value F=123, and a quality
factor Q = 5.1x10^5. The resonator performances and the relevant parameters are
theoretically predicted and discussed, and a comparison among simulated and
experimental spectra is given.
