Optical: systems and elements – Optical frequency converter
Patent
1991-12-02
1994-05-24
Gonzalez, Frank
Optical: systems and elements
Optical frequency converter
359332, 385122, 385129, 385130, H03F 700
Patent
active
053154320
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a thin film of lithium niobate (LiNbO.sub.3) single crystal which is suitably applied in various optical materials including thin film wave-guiding second harmonic generating devices.
BACKGROUND OF THE INVENTION
With the recent development of optics-applied technique, shorter wavelength laser beam sources are increasing in demand.
Such shorter wavelength laser beam sources can improve recording density and photosensitivity and can be applied in the field of optical appliances such as optical memory discs and laser printers.
Thus, second harmonic generating (SHG) devices which can convert the wavelength of the incident laser beam into a 1/2 level have been studied.
A bulk of nonlinear optical single crystal has conventionally been used in such SHG devices utilizing high-output gas lasers as the light sources. However, gas laser is now being replaced mainly by semiconductor laser since there are strong demands for much smaller devices such as optical memory disc apparatus and laser printers, and since the semiconductor laser is less expensive and allows direct light modulation while the gas laser requires an external light modulator. Accordingly, thin film wave-guiding SHG devices are now in demand in order to obtain high conversion efficiency using a low light source output of several mW to several tens of mW.
As the nonlinear optical materials for such thin film wave-guiding SHG devices, those having a waveguide layer of a bulk of lithium niobate single crystal on which Ti or the like is diffused so as to modify refractive index, and those having a waveguide made of a thin film of lithium niobate single crystal formed on a lithium tantalate substrate by high-frequency sputtering process are known. However, it is difficult to obtain a thin film of lithium niobate single crystal having excellent crystallinity in either of these methods, and thus they failed to afford high conversion efficiency.
By the way, the Liquid Phase Epitaxial Method is supposed to be an ideal way of forming a thin film of single crystal having excellent crystallinity.
The Liquid Phase Epitaxial Method for forming a thin film of lithium niobate is described, for example, in:
(1) Applied Physics Letters, Vol. 26, No. 1, January 1975, wherein a thin film of lithium niobate is formed as an optical waveguide on a lithium tantalate substrate by liquid phase epitaxial growth using Li.sub.2 O and V.sub.2 O.sub.5 as a flux;
(2) Japanese Patent Publication No. 9720/1976, wherein a thin film of lithium niobate is formed as an optical waveguide on a lithium tantalate substrate by liquid phase epitaxial growth using Li.sub.2 O and V.sub.2 O.sub.5 as a flux; and
(3) Japanese Patent Publication No. 47160/1981, wherein a solid solution thin film of lithium niobate/lithium tantalate single crystal containing magnesium is formed on a substrate by epitaxial growth using Li.sub.2 O and V.sub.2 O.sub.5 as a flux.
However, the known Liquid Phase Epitaxial Method is neither successful in forming a thin film of lithium niobate single crystal having excellent crystallinity on the lithium tantalate substrate nor can form a film of lithium niobate single crystal having a sufficient film thickness required for producing a SHG device, and thus no practical thin film wave-guiding SHG device is so far known.
The film thickness required for producing a thin film wave-guiding SHG device is such that the effective index of an incident laser beam having a fundamental wavelength of .lambda. can be made consistent with that of the second harmonic with a wavelength of .lambda./2 in order to achieve phase-matching between them. Particularly, when a SHG device for semiconductor laser is produced using a thin film of lithium niobate formed on a lithium tantalate substrate, the thin film of lithium niobate single crystal should have a thickness of not less than 5 .mu.m so as to achieve such consistency between these two effective indexes.
On the other hand, in order to obtain a high-output thin film wave-guiding SHG device,
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Gonzalez Frank
Ibiden Co. Ltd.
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