  We describe a superconducting device capable of producing laser light in the
visible range at half the Josephson generation frequency, with the optical
phase of the light locked to the superconducting phase difference. An earlier
proposed device, the so called "half-Josephson laser" [Phys. Rev. Lett. 107,
073901 (2011)], cannot provide long coherence times, because of spontaneous
switchings between the emitter states. To circumvent this we consider N >> 1
emitters driving an optical resonator mode. We derive a general model that
captures essential physics of such devices while not depending on specific
microscopic details. We find the conditions under which the coherence times are
exponentially long, thus surpassing the fundamental limitation on the coherence
times of common lasers. For this we study the noise in the device. In
particular, we are interested in the rate of large fluctuations of the light
field in the limit where the typical fluctuations are small. The large
fluctuations are responsible for switching of the laser between stable states
of radiation and therefore determine the coherence time.
