  We construct a phase-conjugate resonator which passively produces stable
pulses that alternate between the probe and the conjugate colors. The requisite
phase-conjugate mirror inside the resonator is constructed using non-degenerate
four-wave mixing (4WM) in rubidium vapor. The glancing-angle phase-conjugate
mirror is a 100\% output coupler, and therefore this resonator is unusual in
that no light circulates the cavity more than once. Without the gain of the
phase-conjugate mirror, the cavity boundary conditions, and thus resonant
modes, are not defined and therefore can be tuned by the pump. The output of
the optical parametric oscillator that is formed above threshold can passively
mode-lock. The phase-conjugate mirror removes thermal or acoustic instabilities
that are on a MHz or slower timescale. This work provides a new method for
stable pulsing using phase-conjugate optics, and suggests a platform for
producing mode-locked pulses with squeezed light, as the 4WM process has
already demonstrated quantum correlations.
