The invention relates to a device for doubling the frequency of a light wave, which device comprises a nonlinear optical medium for guiding a fundamental light wave while forming a second harmonic wave, said medium comprising a waveguide having a spatially periodic structure.
The invention also relates to a method of manufacturing a device for doubling the frequency of a light wave, which device comprises a waveguide having a spatially periodic structure.
The device according to the invention can be used, for example, in combination with a semiconductor laser light source which generates red light, as a compact blue light source for optical storage of information, for telecommunication and for laser printers. By virtue of the use of shortwave light, the information density can be increased and the possibilities of writing and erasing information are extended as a result of the larger photon energy.
In devices for doubling the frequency of a light wave the problem arises that the nonlinear optical medium has different refractive indices for the fundamental wave and the second harmonic wave, thus bringing about increasing phase differences between both waves when light is guided through the medium. Owing to destructive interference, a large part of the intensity of the generated blue light is lost. A possible solution to this problem is the use of a birefringent material whose ordinary refractive index at one wavelength should be equal to the extraordinary refractive index at another wavelength. Another solution which is known per se is the use of waveguides having spatially periodic structures. In the latter solution, the nonlinear optical properties in alternate regions are selected such that in regions where the fundamental light wave and the second harmonic wave are out of phase, the second harmonic light wave is generated to a reduced degree or not at all, so that no complete extinction takes place. It is even possible to change the nonlinear properties in such regions in a manner such that the generated blue light is of the opposite phase (domain inversion), so that a further intensification of the blue light takes place.
In PCT Patent Application WO 90/04807, a description is given of such a (domain inversion) device and of a method of manufacturing such a device, in which method domains of opposite ferroelectric polarity are manufactured under a mask layer in a nonlinear optical material by means of annealing. Said annealing treatment takes place at a temperature slightly below the Curie temperature which, for example, for lithium niobate is approximately 1200.degree. C. To obtain sufficient waveguide action in the above device, it is necessary to subsequently locally increase the refractive index, for example, by means of diffusion of protons.
A disadvantage of the known method is the high-temperature annealing treatment (causing the formation of colour centres in the material) and the necessity to carry out both process steps in a patterned manner, i.e. via two masks. The use of one single mask for both steps leads to a waveguide which is segmented according to the domain structure. Investigations leading to the invention have shown that in such a segmented waveguide, light scattering takes place at the interfaces between the domains (grating effect). A further disadvantage of the known device and method is that domain inversion is possible only on the c.sup.+ crystal face of lithium niobate. However, in the c.sup.+ crystal face etching takes place to an undesirable degree during the proton diffusion process.