Non hydrogenated a-SiCN films have been prepared by Radio Frequency (RF)reactive sputtering of a SiC target in an Ar + N-2 plasma. The N content has been varied from 0 to 66 at.% of the sample composition. The structural and chemical orders have been investigated by X ray diffraction and X ray Absorption Near Edge Spectroscopy. The optical absorption properties of the films have been studied by transmittance measurements and by ellipsometry. A particular attention has been paid to the relation between the local order and the complex dielectric function. It has been shown that the atomic Short Range Order (SRO) rules the optical absorption while the index of refraction is dominated by the Medium Range Order (MRO). The as deposited samples have an heterogeneous microstructure containing zones where "mixed" tetrahedra Si (C1-x N-x)(4) have been formed contiguously with other zones inside of which the basic SiC4 and SiN4 tetrahedra of the carbide and nitride are already present. The optical absorption edges of the SiCN deposits, remain close to the one of a-SiC, up to about 50% of nitrogen. The absence of hydrogen in a sample containing less than 1% of N allowed the study of the nitrogen induced relaxation of the network during heating treatements at 1200degreesC. The crystallization temperature of a-SiC is increased of about 200degrees and N atoms are located in substitution sites with a four fold coordinence. The optical gap widens after the annealing at 1200degreesC, from 1.6 to 2.4 ev which last value is in the same range that the gaps of non annealed a-SiC:H materials containing about 10% of hydrogen.