Carrier envelope offset frequency of a doubly resonant, nondegenerate, mid-infrared GaAs optical parametric oscillator

We measure the carrier envelope offset (CEO) frequency of the mid-infrared frequency comb (wavelength tunable between 3 and 6 μm) from a doubly resonant nondegenerate synchronously pumped optical parametric oscillator (SPOPO) as a function of the CEO frequency of the Tm-fiber pump laser. We show that the CEO frequency of the SPOPO signal wave is a linear function of the CEO frequency of the pump laser, with a slope determined by the signal to pump center-frequency ratio.