We report on an investigation of optical properties of multiferroic CoCr$_{2}$O$_{4}$ at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr$_{2}$O$_{4}$ is dominated by a mag
non mode, which shows a steep magnetic-field dependence. We ascribe this mode to an exchange resonance between two magnetic sublattices with different $g$-factors. In the framework of a simple two-sublattice model (the sublattices are formed by Co$^{2+}$ and Cr$^{3+}$ ions), we find the inter-sublattice coupling constant, $lambda = - (18 pm 1)$ K, and trace the magnetization for each sublattice as a function of field. We show that the Curie temperature of the Cr$^{3+}$ sublattice, $Theta_{2}$ = $(49 pm 2)$ K, coincides with the temperature range, where anomalies of the dielectric and magnetic properties of CoCr$_{2}$O$_{4}$ have been reported in literature.
