Optical: systems and elements – Optical frequency converter
Reexamination Certificate
2001-03-02
2003-09-23
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Reexamination Certificate
active
06624923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optically functional device to control light by utilizing polarization-inverted structures in a LiTaO
3
single crystal substrate, which is useful in various fields of e.g. optical information processing, optical processing technology, optical communication technology and optical measurement using a laser beam, a single crystal substrate for such an optically functional device, and a method of using such a single crystal substrate for the optically functional device.
2. Discussion of Background
A lithium tantalate (LiTaO
3
) single crystal (hereinafter sometimes referred to simply as “LT”) known as a typical ferroelectric single crystal, is used mainly as a substrate for a surface acoustic wave device. With respect to this crystal, a single crystal having a large diameter and a high compositional homogeneity can be supplied at a relatively low cost. Further, this crystal is transparent within a wide wavelength range of from ultraviolet to infrared, and it is possible to invert the ferroelectric polarization even at room temperature by applying a high electric field at a level of a few tens kV/mm. Accordingly, it has attracted attention in recent years also as a substrate for various optically functional devices such as non-linear optical devices or electro-optical devices, using the polarization-inverted structures.
Particularly, in recent years, it is expected to have second harmonic wave generating (SHG) devices of waveguide type developed to convert a semiconductor laser having a near infrared wavelength to a blue light having a half wavelength by a non-linear effect. Among them, a frequency conversion device is most thoroughly studied which employs an element of a structure having the polarization of an inorganic ferroelectric single crystal such as LT or a lithium niobate (LiNbO
3
) single crystal (hereinafter sometimes referred to simply as “LN”) periodically inverted, as a light source for high density recording/read out of optical disks. Such a frequency conversion device is based on a quasi phase matching (QPM) system, which is a system to carry out phase matching by compensating the difference between propagation constants of the fundamental wave and the higher harmonic wave with the periodic structures.
This system has many excellent characteristics such that high conversion efficiency can be obtained, collimation or diffraction limit condensation of the output light is easy, and useful materials or wavelengths are not particularly limited. As the periodic structures for QPM, structures having the sign of the SHG coefficient (the d
33
coefficient) periodically inverted, are most effective to obtain high efficiency, and with a ferroelectric crystal, positive or negative of the d coefficient corresponds to the polarity of the ferroelectric polarization, whereby the technology to have the ferroelectric polarization domains periodically inverted, becomes important.
As a device employing this system, a green SHG device by means of an internal resonator using a Nd:YVO
4
single crystal as the fundamental wave, has been reported, which employs a QPM element having periodically inverted structures formed by applying an electric field of about 21 kV/mm to a LT single crystal, as disclosed in a literature (Y. Kitaoka et al., Optics Letters, 21, p1974, 1996). Particularly, the LT single crystal has a large non-linear optical constant (d
33
being 26 pm/V) comparable to the LN single crystal and is strong against optical damage as compared with the LN single crystal, and its fundamental absorption range extends to 280 nm, whereby it is expected to be useful as a frequency conversion material for short wavelengths.
Further, with respect to an optical device utilizing an electro-optical effect, for example, according to a literature (M. Yamada et al., Appl. Phys. Lett., 69, p3659, 1996), attention has been drawn to a cylindrical lens, a beam scanner, a switch, and an optical device to form polarization-inverted structures of a lens or prism shape in a LN single crystal as a ferroelectric crystal by applying a high electric field to the crystal and to polarize a laser beam passed therethrough by utilizing the electro-optical effect, as new optical devices. The LT single crystal which is transparent to a shorter wavelength than the LN single crystal, is considered to be promising as an excellent substrate material for an optical device using ultraviolet to visible light.
With respect to a frequency conversion device or an electro-optical device utilizing the polarization-inverted structures of a ferroelectric LT single crystal, so far reported, in each case, a commercially available LT single crystal having a congruent composition with no additive, has been employed as the substrate crystal, because the LT single crystal so far available has been limited to a crystal having a congruent composition grown by a Czochralski method which is capable of growing a crystal having a large diameter inexpensively, from the industrial viewpoint. With respect to the LT crystal, it is well known from the correlation diagram of temperature/compositional ratios (phase diagram) that the stoichiometric composition and the congruent composition do not agree with each other.
Only the congruent composition is a composition whereby the composition of the melt and the composition of the crystal will agree, and a crystal having a uniform composition can be grown over the entire crystal. Accordingly, the composition of the LT single crystal which is presently produced and used for various applications, is a congruent composition having a molar fraction of Li
2
O/(Ta
2
O
5
+Li
2
O) of about 0.483 (the molar ratio of Li/Ta being about 0.93). Accordingly, in the conventional LT single crystal having a congruent composition, the Ta component is excessive, whereby Ta ions as much as a few percent are substituted for Li ions (anti-site defects), and Li ion sites also have a few percent of vacancy defects. The influence of these defects may not be serious for an application to a surface acoustic wave device, but can not be neglected for an application to an optical device. Accordingly, it has been desired to develop a crystal having a composition close to the stoichiometric composition and having non-stoichiometric defects reduced, as a substrate to be used for optically functional devices.
As is apparent from the phase diagram, for example, in the case of a LT single crystal, a crystal having a composition close to the stoichiometric composition can be precipitated from a melt having a composition having a Li concentration higher than the stoichiometric composition. However, when it is attempted to grow a crystal having the stoichiometric composition by the Czochralski method which has been used as a means to produce a LT crystal having a large diameter in a large amount on an industrial scale, the excessive Li component will remain in a crucible along with the precipitation of the crystal, and the compositional ratio of Li/Ta in the melt will gradually change, whereby the compositional ratio in the melt will reach the eutectic point soon after the initiation of growing. Accordingly, the solidification ratio of the crystal is restricted to a level as low as about 10%, and the quality of the precipitated crystal has been so poor that it can not be used for an optically functional device.
The present inventors have previously invented a lithium tantalate single crystal close to the stoichiometric composition having a molar fraction of Li
2
O/(Ta
2
O
5
+Li
2
O) of from 0.495 to 0.50 (the molar ratio of Li/Ta being from about 0.98 to 1.00) and having the non-stoichiometric defect concentration of the congruent composition substantially reduced, as a novel substance different from the conventional commercially available LT crystal having a congruent composition, and have filed a Patent Application (JP-A-11-35393). Further, they have reported about this novel crystal in a literature as follows. Namely, as a means to de
Furukawa Yasunori
Kitamura Kenji
Nakamura Masaru
Takekawa Shunji
Lee John D.
National Institute for Research in Inorganic Materials
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