Optical waveguides – Planar optical waveguide
Patent
1991-06-24
1992-12-08
Healy, Brian
Optical waveguides
Planar optical waveguide
385 14, 385122, 385131, 437 20, 437 21, 437 26, 437 51, 437134, 437153, 359326, 359332, G02B 610, H01L 21265, H03F 700
Patent
active
051704609
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to waveguides (for quasi-phase-matched frequency conversion) in optically non-linear materials such as LiNbO.sub.3, LiTaO.sub.3 and KTiOPO.sub.4 and a method for fabrication of such waveguides.
BACKGROUND OF THE INVENTION
It is well known that, in optically non-linear materials, light or other kinds of electromagnetic radiation of certain wavelength can be converted to light or other kinds of electromagnetic radiation of another wavelength. This conversion can be in the form of frequency doubling, sum frequency generation, difference frequency generation, parametric amplification, parametric oscillation, etc., utilizing the so-called second order optical non-linearity. For the conversion to be efficient, so-called phasematching has to be achieved. A problem is, however, that in all homogenous, optically non-linear materials with a uniform structure, the phase-matching condition can only be fulfilled within a rather limited wavelength interval, if it can be fulfilled at all. Another problem associated with conversion is that for conventional non-linear optical materials and methods, the intensity in the light from the pump source has to be high for the efficiency of the frequency conversion to be acceptable.
One way of achieving efficient frequency conversion is to use so-called quasi-phase-matching, thereby making the crystal orientation vary periodically in the optically non-linear material so that the non-linearity changes its sign with the same period. The method implies that when the interacting waves have come 180 degrees out of phase and no longer interact constructively, the crystal structure is altered so that the waves continue to interact constructively.
Crystal structure variation has been realized in the form of plate stacks and in the form of crystals grown in such a way that the crystal orientation alters periodically during the growth. Both methods, however, are complicated and expensive.
Further, it is known that by coupling the light/radiation into a waveguiding layer or channel, one can achieve a high radiation intensity over a long interaction length, thereby reducing the demands on the intenstity of the pump source. This means that low power lasers can be used for frequency conversion. Examples of materials in which waveguides with low optical losses can be fabricated are LiNbO.sub.3, LiTaO.sub.3 and KTiOPO.sub.4.
SUMMARY OF THE INVENTION
The purpose of the present invention is to realize waveguides in a simple and inexpensive way, with such a periodically altered crystal structure so that quasi-phase-matching can be obtained, in order to achieve efficient frequency conversion of desired wavelengths from lasers with low output powers, such as semiconductor lasers.
The invention provides solutions to the above described problems as summarized below. Here it is assumed that a waveguide for electromagnetic waves is provided in an optically non-linear crystal substrate, e.g., LiNbO.sub.3, LiTaO.sub.3 or KTiOPO.sub.4. According to the invention, the waveguide passes a periodically domain-inverted structure, with peridically inverted crystal orientation, arranged in the surface of the crystal substrate. The regions with inverted crystal orientation (and thereby opposite sign of the non-linearity) are realized in the form of ferroelectric domains of opposite ferroelectric polarity with respect to the intermediate regions. The invention is characterized in that this periodically domain-inverted structure is obtained by means of a heat treatment of the substrate that previously has been furnished with a periodic perturbation in the surface layer, achieved by use of a periodic mask structure.
The periodically domain-inverted structure according to the invention is preferably produced by a method characterized in that a periodic mask structure consisting of metal or dielectric is applied to the surface of the crystal substrate by known techniques, and that a heat treatment is performed up to a temperature slightly below the Curie temperat
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Arvidsson Gunnar
Laurell Fredrik
Webjorn Jonas
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