The present work deals with the study of combustion instability of a non premixed flame stabilized in a backward facing step combustor. The bifurcation resulted in a shift in the dominant frequency from acoustic to a hydrodynamic mode accompanied by a rise in the amplitude. The dominant frequency scales with the Strouhal number substantiating the role of hydrodynamic mode in combustion instability. Simultaneous time resolved PIV and CH∗ chemiluminescence was performed for condition of low and high pressure amplitudes. The dynamics of the flow and the flame is distinguished across the onset of instability using Proper orthogonal decomposition (POD). POD showed the hydrodynamic frequency to be dominant in velocity as against the dominant acoustic mode in pressure time trace but in the high amplitude condition, the dominant frequency in POD matched with that of pressure. The acoustic modes in velocity is shown in the higher mode indices for both of the amplitude condition. Further, the existence of intermediate non-linear modes are also retrieved.