JPH05155694A - Optical waveguide material consisting of lithium niobate added with zinc and its production - Google Patents

Abstract

PURPOSE:To provide the optical waveguide material which is advantageous for use in a short wavelength region of <=500nm and has high resistance to optical damages, and the process for production of the material. CONSTITUTION:This optical waveguide material consisting of lithium niobate added with zinc and having an epitaxially grown layer is formed on a single crystal substrate consisting of lithium niobate by using a lithium niobate melt added with >=11mol% zinc oxide. The melt temp. is 940 to 860 deg.C and the growth speed is 1pm per minute. The propagation loss of the optical waveguide which is formed by epitaxial growth and has 10mum thickness is 2dB/cm after annealing for two hours. The optical waveguide by the epitaxial layer allows propagation of both TM mode and TE mode and the optical damages are not admitted even when an argon laser of 488nm is made incident at 1KW/cm<2> intensity.

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 where a photo-induced refractive index change (optical damage) is likely to occur, particularly 500 nm.
The present invention relates to devices using the following wavelength regions.

[0002]

2. Description of the Related Art Conventionally, as a method for forming a lithium niobate optical waveguide, there are an ion diffusion method such as a titanium diffusion method in which metallic titanium is deposited and diffused, and a proton exchange method using benzoic acid or pyrophosphoric acid as a proton source. In the epitaxial method, there is a method of epitaxially growing undoped lithium niobate on 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, and the optical waveguide obtained by this method has a long wavelength region of 800 nm or more in a usable range. is there. In the wavelength region below this range, particularly in the wavelength region below 500 nm, when strong light is incident, the so-called optical damage in which the refractive index of the titanium diffusion waveguide portion is changed remarkably occurs. Although the proton exchange method is resistant to this optical damage, only the extraordinary light refractive index increases in the proton exchange part. Therefore, there is a drawback that the optical waveguide is only for extraordinary light and ordinary light is not guided.

On the other hand, even when undoped lithium niobate is epitaxially grown, optical damage occurs remarkably. Therefore, lithium niobate containing magnesium has been tried. When 5 mol% of magnesium is added,
Compared to non-added lithium niobate, the resistance to light damage is 1
It has been reported to increase more than 00 times (for example,
DABryan, R. Crouson, HETomaschke, Appl.Phys.Let
t. 44 (1984) 847-849). However, compared with undoped lithium niobate, both the ordinary and extraordinary refractive indices of magnesium-added lithium niobate monotonically decrease with the addition amount (for example, Sato, Furukawa, Japan Society of Applied Physics, Department of Crystal Engineering). The 93rd meeting of the society textbook (1990, 31-38)).

Therefore, even if magnesium-added lithium niobate is epitaxially grown on a non-doped lithium niobate substrate, the refractive index of the epitaxial growth layer is smaller than the refractive index of the substrate, so that it does not serve as an optical waveguide. Therefore, as compared with magnesium-added lithium niobate, magnesium-added lithium niobate is epitaxially grown on a lithium tantalate substrate having a smaller ordinary light refractive index and extraordinary light refractive index. However, since lithium tantalate and lithium niobate have large differences in lattice constant and thermal expansion coefficient, it is not easy to form an epitaxial layer having a small strain.

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

[0007]

The optical waveguide material of the present invention comprises zinc oxide on a lithium niobate single crystal substrate.
It is characterized by having an epitaxial growth layer formed by performing epitaxial growth using a lithium niobate melt added at a mol% or more.

That is, the present inventor has conducted various studies on optical waveguide materials resistant to optical damage. As a result, when zinc oxide of 5 mol% or more is added to the lithium niobate melt, the resistance to optical damage is improved, particularly 10 mol. %, It was found that the resistance to photodamage increased 100 times or more, and when 11 mol% was added, both the ordinary and extraordinary refractive indexes were higher than those of undoped lithium niobate. I found that. Then, if lithium niobate containing 11 mol% or more of zinc oxide is epitaxially grown on an undoped 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 can be formed, and the present invention was completed. This will be described in more detail below.

[0009]

In FIG. 1, the lithium composition is 0.94 with respect to niobium.
2 industrially used 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 added concentration of zinc oxide. It is a thing. As shown in FIG. 1, the ordinary refractive index monotonically increases with an increase in the concentration of zinc oxide added. On the other hand, the extraordinary light refractive index is 5 mol%
In the following region, it decreases as the added concentration of zinc oxide increases. However, in the region of 5 mol% or more, on the contrary, the extraordinary light refractive index also increases with the increase of the added concentration of zinc oxide, and at the added concentration of 11 mol%, the value of the extraordinary optical refractive index of undoped lithium niobate is increased. Is equal to Then, it can be seen that the added amount becomes larger when the added concentration is 11 mol% or more.

FIG. 2 shows that when the lithium composition is 0.
942 industrially used lithium niobate (L
iNbO ₃ ) was added with zinc oxide (ZnO) at various concentrations, and a single crystal grown from the melt by the Czochralski method was subjected to photoinduced refractive index change (photodamage) to an argon laser beam with a wavelength of 488 nm. The time dependence is shown with the added concentration of zinc oxide as a parameter. The added concentration of zinc oxide is 0 (no addition), 3, 5,
It changed to 7, 10 mol% and 5 steps. When the addition concentration is 5 mol% or less, the effect of adding zinc oxide is not seen. However, if the added concentration is higher than that, the resistance to optical damage increases, and if the added concentration is 10 mol% or higher, no change in the refractive index is observed over 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, and a melt thereof is prepared. In the single crystal grown by the Czochralski method, when the concentration of zinc oxide added was 11 mol% or more, not only the ordinary and extraordinary refractive indices were higher than those of non-added lithium niobate, but also the optical damage to Resistance is also greatly improved. Therefore, if a high-refractive-index layer is epitaxially grown on a lithium niobate substrate that is industrially used, using a lithium niobate melt in which zinc oxide is added at 11 mol% or more, both ordinary light and extraordinary light can be conducted. It is possible to form an optical waveguide that is undulating and is resistant to optical damage.

[0012]

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

The propagation loss of the 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
It is considered that the change from trivalent to pentavalent due to annealing. In the optical waveguide formed by the epitaxial layer, both TM mode and TE mode propagated. Furthermore, wavelength 488n
No light loss was observed even when an argon laser of m was incident at an intensity of 1 KW / cm ² .

11 mol% of zinc oxide was added to lithium niobate having a lithium composition of 0.942 with respect to niobium, and lithium vanadate (Li) was used as a flux.
50 mol% of VO ₃ (total amount of niobium and zinc oxide is 10
12 mol% of zinc oxide was added to the melt obtained by adding and heating, and to lithium niobate having a lithium composition of 0.942 with respect to niobium, and lithium vanadate (LiVO 4) was added as a flux. ₃ ) 50 mol%
An optical waveguide similar to that described above could be formed by using the melt obtained by adding (when the total of niobium and zinc oxide was 100) and heating.

[0015]

As described above, the optical waveguide material according to the present invention does not diffuse titanium and is resistant to optical damage, and is therefore advantageous for use in the short wavelength region. Further, since lithium niobate can be used as a 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 drawings]

FIG. 1 is a diagram 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 light-induced refractive index of lithium niobate with the added concentration of zinc oxide as a parameter.

Claims (2)

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