FIELD OF THE INVENTION
The present invention relates to a method of heat-treating an oxide optical crystal which is to be employed in optical devices, optical waveguides, optical recording media or optical second harmonic generators (hereinafter abbreviated to "SHGs") for the crystal's electro-optic effect, acousto-optic effect, magneto-optic effect or nonlinear optical effect, to improve the crystal's light absorption characteristics, and the heat treatment apparatus for carrying out the method.
The oxide optical crystal is applied widely, for its electro-optic effect, acousto-optic effect, magneto-optic effect or nonlinear optical effect, to various optical devices and optical waveguides. Particularly, the application of the nonlinear optical effect of the optical oxide crystal to SHG devices for reducing the wavelength of laser light emitted by a semiconductor laser improves the recording density and resolution of optical recorders using light of a short wavelength such as laser light.
Restrictions are placed on the use of ordinary oxide optical crystals, such as lithium niobate (LiNbO.sub.3) (hereinafter referred to as "LN") having a nonlinear optical effect and expected to be a potential material for application to SHGs, because of the high light absorption characteristics of the host crystal of the optical crystalline material for visible radiation, for example, green light of a wavelength of 0.5145 .mu.m.
It has been reported that a dark optical crystal as grown by the Czochralski process becomes transparent when subjected to a high-temperature annealing in an oxygen atmosphere. However, the heat treatment of such an optical crystal in an oxygen atmosphere is unable to satisfactorily improve the light absorption characteristics of the optical crystal.
On the other hand, the rapid diffusion and development of optical communication systems and optical disk systems has increased the necessity of inexpensive optical devices having a high light transmissivity, such as optical isolators, optical circulators and optical switches.
Bulk type optical devices have made a remarkable development through the employment of a magnetic garnet, particularly a Bi-substituted magnetic garnet, owning to the high magneto-optic figure of merit of the magnetic garnet.
Bi-substituted magnetic garnets have intrinsic light absorption characteristics as shown in FIG. 12. Bi-substituted garnets have a low light absorption for electromagnetic radiation of wavelengths in a wavelength band longer than 1 .mu.m and in a 0.8 .mu.m band. Therefore, Bi-substituted garnets are applicable to optical devices, for example, optical isolators dealing with semiconductor laser light of a wavelength in the 0.8 .mu.m band.
It is desirable, in view of mass production, to produce a Bi-substituted garnet, namely, a magnetic garnet produced by substituting part of the component rare earth metals of a magnetic garnet by Bi, by the liquid-phase epitaxial growth process (hereinafter, referred to as "LPE process"). However, Fe.sup.2+ or Fe.sup.4+ is liable to be included in the Bi-substituted magnetic garnet grown by the LPE process due to defects (Oxygen vacancies) and impurities. As shown in FIG. 13, Fe.sup.2+ (solid line) and Fe.sup.4+ (broken line) have light absorption characteristics in a wide wavelength band, which reduces the forward transmissivity of optical isolators. To improve the light absorption characteristics of the Bi-substituted magnetic garnet, methods have been proposed in Japanese Patent Laid-open (Kokai) Nos. 61-150303, 61-265809, 61-265810 and 62-79608. These previously proposed methods dope the Bi-substituted magnetic garnet with bivalent ions, such as Mg.sup.2+ or Ca.sup.2+. A method proposed in Japanese Patent Laid-open (Kokai) No. 62-95812 subjects a Bi-substituted magnetic garnet to heat treatment in an atmosphere (an oxygen atmosphere) in addition to doping the Bi-substituted magnetic garnet with bivalent ions. This method is fairly satisfactory.
Despite the improvement in the light absorption characteristics of Bi-substituted magnetic garnets, the recent optical devices require higher light transmissivity, and studies of devices of optical waveguide type have been made mainly in the field of optical communication. An optical signal processing devices employing a Bi-substituted magnetic garnet as a thin film optical waveguide is described in an article by H. Tamada et al, Journal Of Applied Physics, Vol. 64, page 558, (1988) (hereinafter referred to as "cited reference 1"). Since the waveguide length of such a thin film optical waveguide is not less than one cm, and for example, is as long as 5 cm or longer, it is essential that propagation loss in guiding light is of the order of one dB/cm or less, which corresponds to a very small light absorption of about 0.2 cm.sup.-1 or below. A material with a 70% light transmissivity in the 0.8 .mu.m wavelength band and 100% light transmissivity in the wavelength range of 1.3 to 1.5 .mu.m, which are theoretical limit values with the material, is found in a recent report. However, it is difficult to obtain such a material having a limit light transmissivity by a practical LPE process with a satisfactory reproducibility.
Furthermore, it is important with a garnet optical waveguide utilizing magnetostatic waves to reduce propagation loss of magnetostatic wave as well as guided-optical-wave propagation loss.