A tunable optical filter is formed in the structure of an etalon. A thin electro-optic ceramic substrate is fixed between two end substrates. Each end substrate has an inner parallel surface toward said electro-optic ceramic substrate covered by an electrode layer and a reflecting layer. An adhesive which attaches the electro-optic ceramic substrate to each first and second end substrates has a consistency so as to avoid stress on the electro-optic ceramic substrate. A voltage imposed on the electro-optic ceramic substrate by the electrode layers on the inner parallel surfaces of the first and second end substrates effectively controls an optical distance between the reflective coating layers on the inner parallel surfaces of the first and second end substrates of the etalon structure. The electro-optic ceramic substrate is preferably PMN-PT ((1-x)Pb(Mg⅓Nb⅔)O3-x—PbTiO3) and no more than 160 μm thick.