Abstract: Method and apparatus for determining the wave number of optical spectrum radiation, particularly for use in controlling lasers. Two parts of a beam provided by a source to be monitored, particularly a laser, are directed to two double-wave interferometers. The differences of the optical path lengths .delta. of the interferometers are assigned a difference of .lambda./4 which corresponds to a phase difference of .pi./2. The two optical output signals emerging from the interferometers are converted into sinusoidal electrical waves in quadrature as a function of the wave number .sigma. = 1/.lambda. of the laser and the period 1/.delta. of the two sinusoidal electrical waves. The electrical waves are processed in suitable circuitry to obtain a modulo value 1/.delta. of the wave number. These steps are repeated for different values of .delta. so as to progressively eliminate uncertainty in the value of the wave number.