In an optical system for an oblique incidence interferometer, first and second prisms are used for luminous flux dividing and for luminous flux combining, respectively. Data modulated on first and second carrier frequencies is received via respective first and second lines of the powerline system. Based on a given duty cycle, the fail-safe circuitry selects, actuates, deactuates, andor maintains in the actuated or unactuated state one or both of the first and second solenoid-controlled valves.