We investigated the thermal induced optical nonlinearity of a CdSSe-doped glass JB-7 made by Changchun Institute of Optics and Fine Mechanics.A slice of this glass coated on its both surfaces with high reflective dielectric films formed a Fabry-Perot resonator.The reflected light signal from the F-P resonator was coupled into a hybrid ring cavity

the round trip time of which was much longer than that in the F-P resonator and even longer than the thermal relaxation time of the glass.The self-oscillations occuring for certain input parameters were regular and the appearence of different modes as a function of the light intensity falling onto the resonator was observed.Due to the broad hysteresis and the high inclination of both branches of the bistability large numbers of increasing and decreasing steps in the output intensity were observed.A simple model of adiabatical simulation by an iterated map shows that the mode structure follows the Farey-tree construction.