. . . "Next, as shown in FIG. 9B, grooves 4 for forming a ridge 3 were formed by dry etching with a reactive gas ions, for example, halogen ions such as chlorine ions, bromine ions, hydrocarbon ions such as those derived from methane, ethane, etc., ion milling with inert gas ions such as Ar ions, or wet etching with an etchant such as chlorine, sulfuric acid, bromine or the like." . .