JP2958408B2 - Zinc-doped lithium niobate optical waveguide material and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device using a lithium niobate crystal, in a short wavelength region in which a photoinduced refractive index change (optical damage) is liable to occur, particularly 500 nm.
The present invention relates to a device using the following wavelength region.

[0002]

2. Description of the Related Art Conventionally, as a method for forming a lithium niobate optical waveguide, there are a titanium diffusion method in which metal titanium is deposited and diffused, and a proton exchange method using benzoic acid or pyrophosphoric acid as a proton source in the ion diffusion method. In addition, in the epitaxial method, there is a method of epitaxially growing lithium niobate without addition of lithium niobate to which magnesium is added, or a method of epitaxially growing lithium niobate on a lithium tantalate substrate.

[0003]

Among the above, the method of forming an optical waveguide by the titanium diffusion method is most often used. However, the optical waveguide obtained by this method has a long wavelength region of 800 nm or more. is there. In a wavelength region equal to or less than this, particularly a wavelength region equal to or less than 500 nm, when strong light enters, the refractive index of the titanium diffusion waveguide portion changes, that is, so-called optical damage occurs remarkably. The proton exchange method is resistant to this optical damage, but only the extraordinary light refractive index increases in the proton exchange portion. Therefore, there is a defect that the optical waveguide is only for the extraordinary light and does not guide the ordinary light.

On the other hand, when undoped lithium niobate is epitaxially grown, optical damage is remarkable. Therefore, lithium niobate to which magnesium is added has been attempted. When 5 mol% of magnesium is added,
1 less damage to light damage compared to lithium niobate without additives
Has been reported to increase by over a factor of 00 (eg,
DABryan, R. Crouson, HETomaschke, Appl.Phys.Let
t. 44 (1984) 847-849). However, lithium niobate to which magnesium is added monotonically decreases in both ordinary light refractive index and extraordinary light refractive index as compared with non-added lithium niobate (for example, Sato, Furukawa, Crystal Engineering Division of the Japan Society of Applied Physics). 93rd Workshop Textbook (1990, 31-38)).

[0005] Therefore, even if magnesium-doped lithium niobate is epitaxially grown on a non-doped lithium niobate substrate, it does not become an optical waveguide because the refractive index of the epitaxially grown layer is smaller than that of the substrate. Therefore, magnesium-added lithium niobate is epitaxially grown on a lithium tantalate substrate having a smaller ordinary light refractive index and extraordinary light refractive index than magnesium-added lithium niobate. However, since lithium tantalate and lithium niobate have greatly different lattice constants and thermal expansion coefficients, it is not easy to form an epitaxial layer with small distortion.

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide an optical waveguide material which is formed by an epitaxial layer and which forms an optical waveguide that guides both ordinary light and extraordinary light and is resistant to optical damage, and a method of manufacturing the same.

[0007]

According to the optical waveguide material of the present invention, zinc oxide is deposited on a lithium niobate single crystal substrate in an amount of 11%.
It is characterized by having an epitaxial growth layer formed by epitaxial growth using a lithium niobate melt to which mol% or more is added.

That is, the present inventor has studied various optical waveguide materials which are resistant to optical damage. As a result, when zinc oxide is added in an amount of 5 mol% or more to a lithium niobate melt, the resistance to optical damage is improved, and in particular, 10 mol% is added. %, The resistance to photodamage is increased by 100 times or more. When 11 mol% is added, both ordinary refractive index and extraordinary refractive index are larger than that of lithium niobate without addition. I discovered that it would be. If lithium niobate to which zinc oxide is added in an amount of 11 mol% or more is epitaxially grown on a non-added lithium niobate substrate, both ordinary light and extraordinary light are guided to the epitaxial growth layer,
Moreover, it was confirmed that an optical waveguide resistant to optical damage could be formed, and the present invention was completed. Hereinafter, this will be described in more detail.

[0009]

FIG. 1 shows that the lithium composition is 0.94 with respect to niobium.
2 lithium niobate (LiN
bO ₃ ), zinc oxide (ZnO) was added at various concentrations, and the refractive index at a wavelength of 632.8 nm of a single crystal grown from the melt by the Czochralski method was shown with respect to the zinc oxide addition concentration. Things. As shown in FIG. 1, the ordinary light refractive index monotonically increases with an increase in the added concentration of zinc oxide. On the other hand, the extraordinary light refractive index is 5 mol%
In the following regions, it decreases with an increase in the concentration of zinc oxide. However, in the region of 5 mol% or more, the extraordinary light refractive index also increases with an increase in the concentration of zinc oxide, and at an addition concentration of 11 mol%, the value of the extraordinary light refractive index of lithium niobate without addition increases. Becomes equal to Thus, it can be seen that when the concentration is 11 mol% or more, the addition becomes larger.

FIG. 2 shows that the lithium composition is 0.1 to niobium.
942 of industrially used lithium niobate (L
iNbO ₃ ) was added with zinc oxide (ZnO) at various concentrations, and the single crystal grown from the melt by the Czochralski method showed a change in photoinduced refractive index (optical damage) with respect to argon laser light having a wavelength of 488 nm. The time dependency is shown by using the zinc oxide addition concentration as a parameter. The addition concentration of zinc oxide is 0 (no addition), 3, 5,
It was changed to 7,10 mol% in 5 steps. When the addition concentration is 5 mol% or less, the effect of adding zinc oxide is not observed. However, at an added concentration higher than this, the resistance to optical damage increases, and at an added concentration of 10 mol% or more, no change in the refractive index is observed even with the passage of time.

As described above, zinc oxide (ZnO) is added at various concentrations to industrially used lithium niobate (LiNbO ₃ ) having a lithium composition of 0.942 with respect to niobium. When the concentration of zinc oxide added is 11 mol% or more, the single crystal grown by the Czochralski method has not only a large ordinary light refractive index and an extraordinary light refractive index as compared with lithium niobate without addition, but also a light damage. Resistance is also greatly improved. Therefore, if a high refractive index layer is epitaxially grown on an industrially used lithium niobate substrate using a lithium niobate melt containing zinc oxide in an amount of 11 mol% or more, both ordinary light and extraordinary light can be guided. An optical waveguide that undulates and is resistant to optical damage can be formed.

[0012]

EXAMPLE 13% of zinc oxide was added to lithium niobate having a lithium composition of 0.942 with respect to niobium,
Furthermore, lithium vanadate (LiVO
₃ ) was added at 50 mol% (when the total amount of niobium and zinc oxide was 100) and heated to prepare a melt. This melt was used for epitaxial growth on a lithium niobate substrate having a lithium composition of 0.942 with respect to niobium and having the -Z axis as a main surface. At this time, the temperature of the melt is 940 ° C.
From 860 ° C., and the growth rate was 1 μm / min.

The propagation loss of an optical waveguide having a thickness of 10 μm formed by epitaxial growth was 30 dB / cm or more, but when this was annealed in ozone (O ₃ ) for 2 hours, the propagation loss was reduced to 2 dB / cm. This is because the valence of vanadium contained in the epitaxial layer is slightly
It is considered that the annealing changed from trivalent to pentavalent. Further, the optical waveguide of the epitaxial layer propagated in both the TM mode and the TE mode. Further, the wavelength 488n
No light loss was observed when an argon laser of m m was incident at an intensity of 1 KW / cm ² .

11% by mole of zinc oxide is added to lithium niobate having a lithium composition of 0.942 with respect to niobium, and lithium vanadate (Li) is added as a flux.
VO ₃ ) (50 mol% (total of niobium and zinc oxide is 10
0%), and 12 mol% of zinc oxide was added to a melt obtained by heating after addition and lithium niobate having a lithium composition of 0.942 with respect to niobium, and lithium vanadate (LiVO2) was added as a flux. ₃ ) 50 mol%
An optical waveguide similar to that described above could be formed using the melt obtained by adding and heating (when the total amount of niobium and zinc oxide was 100).

[0015]

As described above, the optical waveguide material according to the present invention does not diffuse titanium and is resistant to optical damage, so that it is advantageous for use in a short wavelength region. In addition, since lithium niobate can be used as the substrate for epitaxial growth, an optical waveguide with less lattice distortion is formed, and the obtained optical waveguide can guide both ordinary light and extraordinary light.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the concentration of zinc oxide added to lithium niobate and the refractive index.

FIG. 2 is a diagram showing the time dependence of the amount of change in the photo-induced refractive index of lithium niobate using the addition concentration of zinc oxide as a parameter.

Claims (2)

(57) [Claims] 1. A zinc-doped niobium having an epitaxial growth layer formed on a lithium niobate single crystal substrate by epitaxial growth using a lithium niobate melt containing zinc oxide in an amount of 11 mol% or more. Lithium oxide optical waveguide material. 2. A method of manufacturing an optical waveguide material, comprising: using lithium niobate as a substrate and growing an epitaxial growth layer from a lithium niobate melt obtained by adding zinc oxide to lithium niobate by 11 mol% or more by an epitaxial method. .