Abstract

The response of two CO₂ lasers coupled by a common saturable absorber is investigated experimentally when both lasers are operating close to threshold. By contrast to two coupled solid state lasers or two coupled semiconductor lasers [1], our two lasers are strongly pulsating limit-cycle oscillators and the coupling through the saturable absorber is highly nonlinear. A successful synchronization is possible only when the laser pulses overlap in time. The pump parameters can be controlled independently. We observe that one laser is always driving the second laser which responds by harmonic or subharmonic oscillations and with a slight delay. The amplitude of the oscillations of the second laser is typically smaller than the amplitude of the oscillations of the first laser (see Figure). The model equations consist of seven rate equations and are based on a four level model of the CO₂ laser with saturable absorber [2]. Taking advantage of the laser parameters, we show that these equations can be reduced to the equations of one laser subject to the time-periodic modulation of the second laser. Because these modulations are pulsating in time, the solution of the reduced problem can be analyzed in the phase plane. We obtain conditions for a successful 1-1 synchronization as well as conditions for subharmonic entrainment.

© 1998 IEEE