A non-linear optical effect occurs when light is incident on a non-linear optical element such as inorganic materials KH.sub.2 PO.sub.4 and LiNbO.sub.3. Polarization occurs which is proportional to terms of higher order and are at least equal to the square of the light of the electric field of the incident light. This allows generation of a second harmonic. Another inorganic material having a large non-linear optical constant is 2-methyl-4-nitrile-aniline (MNA) which is widely used today.
It is known to form a second harmonic generating element of the light waveguide type. An elongated light waveguide for shutting in propagating light is formed on a substrate and then an overlayer is applied to the waveguide. In order to obtain second harmonic light from the waveguide, the waveguide must have a structure capable of handling the phase velocity of propagation of the wavelength of the second harmonic. The waveguide must be phase-matched with the second harmonic. The easiest method now known to phase match the waveguide with the second harmonic is to use the Cerenkov radiation system.
The Cerenkov radiation system is explained with reference to FIG. 6. Assume that a second harmonic is generated at a point A from light propagating through a light waveguide portion 11 and the second harmonic leaks out to a substrate 12 and an overlayer 13 at an angle .THETA.. When the equiphase wave surface of the second harmonic generated at point A is coincident with the equiphase wave surface of a second harmonic generated at point B in the .THETA. direction, another second harmonic is emitted in the .THETA. direction. To attain Cerenkov radiation, only one condition must be satisfied: EQU n.sub.s (2.omega.)&gt;n.sub.a (.omega.)&gt;n.sub.s (.omega.)
where n.sub.s (.omega.) is the index of refraction of the substrate with respect to a dominant wave, n.sub.G (.omega.) is the index of refraction of the light waveguide, and n.sub.s (2.omega.) is the index of refraction of the substrate with respect to the second harmonic.
The second harmonic generated by a second waveguide-type second harmonic generator does not have a good wave surface characteristic as it is radiated from a narrow light waveguide onto a substrate. This makes it very difficult to focus the second harmonic light to a small spot. Thus it difficult to use the resulting second harmonic light for writing and/or reading operations on an optical storage medium such as an optical disk having very fine pits.