JPH06281829A - Single mode optical waveguide - Google Patents

Abstract

PURPOSE:To decrease light damages in a visible light region, to assure desired emitting power, to enhance optical damage resistance and to lessen light absorption by incorporating boron into an optical waveguide layer consisting of a lithium niobate single crystal. CONSTITUTION:The boron (B) is incorporated into the optical waveguide layer of the LiNbO3 single crystal. The thickness of the optical waveguide layer of the single mode optical waveguide is defined as T(mum) and the wavelength of incident guided light as lambda(mum). The thickness and wavelength mentioned above are then so set as to satisfy conditions 1.9<(T+0.7)/lambda<5.7. Further, the waveguide width of the optical waveguide layer is defined as W(mum), the depth of etching as DELTAT(mum) and the wavelength of the incident guided light as lambda(mum). The width, depth and wavelength mentioned above are then so set as to satisfy conditions W<=(4lambda-0.5)X(lambda<2>/DELTAT+2.0). The respective relations are preferably 0.29<(T+0.04)/lambda<1.19 (where T>0), W<=(0.04lambda<3>+0.1lambda<2>)/DELTAT+2.5lambda in the case of propagation of a TE mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single mode optical waveguide suitable as a component of optical devices such as optical modulators and switches incorporated in optical communication systems, optical information processing systems, and optical sensor systems.

[0002]

2. Description of the Related Art In recent years, due to technological advances in optical fibers and monochromatic lasers, high-speed optical transmission on the order of Gb / sec is becoming possible. In order to construct an optical fiber network in the high-speed optical communication field, optical transmission is required. Development of optical devices such as switches and optical modulators is essential. The required characteristics of the optical waveguide as a component of such an optical device include a large nonlinear optical constant, a large electro-optical constant, an excellent thermo-optical effect, an acousto-optical effect, and the like. It is required to have various characteristics such as a single mode, little light damage, reduction of emission power, and little light absorption.

In order to meet such required characteristics, conventionally, "optical integrated circuit" (Asakura Shoten 1988, Japan Society of Applied Physics,
As described in P158-), many of the channel-type optical waveguides made of lithium niobate (hereinafter referred to as “LiNbO ₃ ”) single crystal material are used as optical waveguides applied to this type of optical device. Being developed. This L
Since iNbO ₃ has a considerably large electro-optic constant as an inorganic optical crystal, iNbO ₃ is often used in optical devices utilizing the electro-optic effect.

In order to manufacture such an optical waveguide, a method such as a Ti diffusion method, a proton exchange method, or a liquid phase epitaxial method (LPE method) has been proposed. Among them, the LiNbO ₃ optical waveguide manufactured by the Ti diffusion method is particularly prone to optical damage in the visible light region and cannot be guided. Further, when used as an electro-optical device, DC drift occurs and there is a problem in reliability. The LiNbO ₃ optical waveguide manufactured by the proton exchange method is strong against optical damage, but has a problem that the nonlinear optical constant and the electro-optical constant are lowered. Whatever is manufactured by the Ti diffusion method or the proton exchange method, if it is annealed at 800 ° C. or higher, the optical waveguide mode is changed or disappeared, or the profile is changed due to the diffusion of Ti or proton into the substrate. There is a problem in terms of etc.

From the above, Li by the LPE method
NbO ₃ single crystal optical waveguide is excellent in nonlinear optical effect, electro-optical effect, acousto-optical effect, etc., and especially has better crystallinity and less optical loss than those by Ti diffusion method or proton exchange method. I came to collect. And this LP
As an example for obtaining a LiNbO ₃ single crystal thin film wafer by the E method, for example, Japanese Patent Application Laid-Open No. 4-120 by the present applicant is proposed.
As disclosed in Japanese Patent Laid-Open No. 95-95, a Li ₂ O—V ₂ O ₅ —Nb ₂ O ₅ flux-based melt is used for liquid phase epitaxial growth, and lithium tantalate (LiTaO ₃ ) is added to the melt.
A manufacturing method is known in which a single crystal substrate is dipped and a lithium niobate single crystal thin film for an optical waveguide is formed on this lithium tantalate single crystal substrate by liquid phase epitaxial growth.

[0006]

However, LiO
_{2 -V 2 O 5 -Nb 2 O} 5 flux system (hereinafter, "LV flux system" hereinafter) those prepared using melt, V ₂ O ₅ (V ³⁺ in LiNbO ₃ single crystal from the melt ion)
However, there is a problem in that the V ³⁺ ions increase the optical absorption, especially the propagation loss due to the optical absorption in the visible light region, and thus impair the optical characteristics.

Therefore, focusing on such a problem, this is not intended especially for a single mode optical waveguide, but as an attempt to improve general optical waveguide characteristics,
For example, Hitachi Metals, Ltd.
Appl.Ph presented by 3rd Japan Society of Applied Physics Academic Lecture (17a-ZK-9) and by Mr. Yamada of Sony Corp.
ys. Lett., Vol.61, No.24, pp.2848 (1992), Li ₂ O-B ₂ O ₃ -Nb ₂ O ₅
It has been proposed to form a LiNbO ₃ single crystal thin film optical waveguide by the LPE method using a flux system (hereinafter referred to as “LB flux system”) melt. In each of these examples, V ions are not contained in the LiNbO ₃ single crystal thin film, thereby suppressing optical absorption and reducing optical waveguide loss.

However, these are all LiNb.
A LiNbO ₃ single crystal thin film is formed on an O ₃ single crystal substrate. The substrate used here is a so-called homo-substrate (LiNbO ₃₎ whose crystal growth surface of the LiNbO ₃ single crystal thin film is the same material as this LiNbO ₃ single crystal. ₃ single crystal with different elements added), and L on this homo substrate
The iNbO ₃ thin film is formed. Therefore, L
The refractive index of the thin film (n ₁ )> the refractive index of the substrate (n ₂ ) is maintained by lowering the refractive index by incorporating MgO in the iNbO ₃ single crystal substrate, and therefore MgO doped in the substrate is , Because it lowers the ordinary refractive index of the substrate, TE
Although the mode propagates, there are problems that the TM mode does not propagate because it does not change the extraordinary light refractive index, and the light damage resistance cannot be improved because MgO cannot be doped in the thin film. As is clear from these, the propagation loss in the visible light region is low, the light damage resistance is excellent, and TE and TM
It was not possible to propagate the guided light in any of the modes without loss.

On the other hand, in this type of optical device, as described in "Optical Integrated Circuit" by Nishihara et al., P36 (Ohm Co., 1985), interference between modes in an optical waveguide is caused when the mode is not a single mode. The inability to stably propagate the 0th-order mode has been a particularly serious problem because unnecessary mode conversion occurs due to slight disturbance. Therefore, in order to obtain a practical optical device, it must be configured as a single mode channel type optical waveguide capable of propagating only the fundamental mode (TM mode or TE mode).

The present invention has been made to solve the above-mentioned problems, and is particularly excellent in non-linear optical effect, electro-optical effect, thermo-optical effect, acousto-optical effect, and particularly visible light. It is an object of the present invention to provide a single-mode optical waveguide in which light damage is small in a region, a desired emission power can be secured, light damage resistance is excellent, and light absorption is small.

[0011]

To achieve the above object, a single mode optical waveguide according to the present invention comprises a LiNbO layer formed on a hetero substrate made of a hetero material having a basic composition different from that of a lithium niobate (LiNbO ₃ ) single crystal. _{3 A} single crystal optical waveguide layer formed in a lattice-matched state,
The gist is that the optical waveguide layer of iNbO ₃ single crystal contains boron (B).

The hetero substrate is a single crystal substrate having a hexagonal structure, such as sapphire (Al ₂ O ₃ ) and quartz (Si).
O ₂ ), ZnO, MgO, Gd ₃ Ga ₅ O _12, etc. can also be used, but in particular, the crystal system of the lithium tantalate substrate is similar to the crystal system of lithium niobate and is advantageous in that epitaxial growth is easy. Is. FIG. 1 shows an electron micrograph of an example of a lithium niobate single crystal thin film formed by a LPE method on a lithium tantalate single crystal substrate. The lithium niobate single crystal thin film is formed on the (0001) crystal plane of the hetero substrate.
The (0001) plane of this substrate refers to a plane perpendicular to the c-axis of the hexagonal crystal structure as shown in FIG. 2, but the (0001) plane of the hexagonal substrate is the crystal growth plane of the lithium niobate single crystal thin film. Since it is composed only of the a-axis, it can be easily lattice-matched with the lithium niobate single crystal thin film simply by changing the lattice constant of the a-axis.

Here, the content of boron (B) contained in the LiNbO ₃ single crystal optical waveguide layer is preferably 5 to 1000 ppm. The content of this boron (B) is 1000ppm
If it exceeds 5 ppm, the electro-optical characteristics of the LiNbO ₃ single crystal will deteriorate, and if it is less than 5 ppm, optical damage will occur in the visible light range. Boron (B) is preferably contained in the LiNbO ₃ single crystal in the state of trivalent cation (B ³⁺ ) since the subsequent bonded state is most stable. By containing boron (B) in place of vanadium (V) conventionally used as an additional element in the LiNbO ₃ single crystal optical waveguide layer formed on the substrate,
This boron (B) itself not only has the function of increasing the light damage resistance, but also because this boron (B) is dissolved or substituted in the crystal lattice, it is an impurity element that causes optical damage or visible light absorption, such as LiNbO. _{3 It} is considered that the inclusion in the single crystal can be avoided, and therefore, optical damage or visible light absorption does not occur.

The a-axis lattice constant of the crystal structure of the LiNbO ₃ single crystal optical waveguide layer is preferably within the range of 99.81-100.07% of the a-axis lattice constant of the hetero substrate, particularly 99.92-100.03. The range of% is preferred. For example, when the lattice constant of the hetero substrate is 5.153A, L
The lattice constant of the iNbO ₃ single crystal is in the range of 5.150 to 5.155A. Here, "A" means "angstrom" and is simply represented by "A" hereinafter. If outside the scope of this lattice constant, the substrate and the LiNbO ₃ difficult to match the lattice constant of the single crystal, by such a large number of defects generated during LiNbO ₃ single crystal, superior optical properties that can be used as an optical waveguide Also, the LiNbO ₃ single crystal cannot be formed sufficiently thick, and the waveguide is deteriorated with time. The a-axis lattice constant of the hetero substrate is 5.128.
More preferably, it is ~ 5.173A.

The means for lattice-matching the LiNbO ₃ single crystal on the hetero substrate is not particularly limited, but Li
Lattice matching can be advantageously performed by making the NbO ₃ single crystal thin film contain a different element to increase the lattice constant. L formed on the hetero substrate by such lattice matching
iNbO ₃ single crystal does not have many defects,
A LiNbO ₃ single crystal having a large film thickness can be obtained. From such an idea, in the LiNbO ₃ single crystal optical waveguide layer, as a different element for lattice matching,
Na in the range of 0.02 to 10.0 mol% and / or 0.8 to 10.8
It is preferable to contain Mg in the range of mol%. When the content of Na is less than 0.02 mol%, the lattice constant does not become large enough to be lattice-matched with the hetero substrate, and also 10.0 mol%
On the other hand, if it exceeds, the lattice constant becomes too large, and in any case, the lattice matching between the hetero substrate and the LiNbO ₃ single crystal cannot be obtained. If the Mg content is less than 0.8 mol%, the effect of preventing optical damage is insufficient, and if it exceeds 10.8 mol%, magnesium niobate-based crystals are precipitated.

Here, Mg is added to the LiNbO ₃ single crystal.
When Mg is included, the refractive index of the LiNbO ₃ single crystal changes due to Mg, so it is preferable that the thickness of the optical waveguide layer and the wavelength of the guided light be formed so as to satisfy the specific relational expression. Assuming that the thickness of the optical waveguide layer of the single mode optical waveguide according to the present invention is T (μm) and the wavelength of the incident guided light is λ (μm), the relationship between T and λ is when the guided light is the TM mode. The thickness of the optical waveguide layer is preferably set so as to satisfy the condition of 1.9 <(T + 0.7) / λ <5.7. Thereby, the single guided light of the TM mode is efficiently propagated. There is no unnecessary propagation of guided modes, and unstable phenomena such as mode conversion due to disturbance do not occur.

Further, if the waveguide width of the optical waveguide layer of the single mode optical waveguide is W (μm), the etching depth is ΔT (μm), and the wavelength of the incident guided light is λ (μm), then W and ΔT. When the guided light is in the TM mode, the waveguide width and the etching depth of the optical waveguide layer are set so that the condition of W ≦ (4λ−0.5) × (λ ² /ΔT+2.0) is satisfied. If so, the optical waveguide characteristics are further improved. In the case of propagating the TE mode, the above relationships are as follows: 0.29 <(T + 0.04) / λ <1.19 (T>
0) It is preferable that W ≦ (0.04λ ³ + 0.1λ ² ) /ΔT+2.5λ.

[0018]

EXAMPLES Examples of the single mode waveguide according to the present invention will be specifically described below. First, the single mode optical waveguide according to the present invention starts by forming a lithium niobate single crystal thin film on a hetero substrate made of a hetero material having a basic composition different from that of the lithium niobate single crystal. Then, an optical waveguide layer is formed on the lithium niobate single crystal thin film by subjecting the wafer having the lithium niobate single crystal thin film formed on the hetero substrate to predetermined patterning, etching, and the like.

Explaining the first step of forming a lithium niobate single crystal thin film, for example, if a lithium tantalate single crystal is used as a hetero substrate, Na ₂ O is used.
Lithium tantalate single crystal substrate is dipped in a melt composed of Li, O ₂ , B ₂ O ₃ , Nb ₂ O ₅ , MgO, etc., and a lithium niobate single crystal thin film is crystal-grown on the substrate surface by the LPE method. It is something. Li ₂ O, B
The composition range of ₂ O ₃ and Nb ₂ O ₅ is Li ₂ O- shown in FIG.
In the trigonometric diagram of the three-component system of B ₂ O ₃ -Nb ₂ O ₅ , A (8
8.90, 2.22, 8.88), B (55.00, 43.00, 2.00), C
(46.50, 51.50, 2.00), D (11.11, 80.00, 8.89),
E (37.50, 5.00, 57.50) is in the composition region surrounded by 5 composition points, F (47.64, 46.12, 6.24), G (27.0
1, 64.69, 8.30), H (36.71, 37.97, 25.32), I (4
The range surrounded by four composition points (4.05, 32.97, 22.98) is particularly preferable. In the melt composition, Na ₂ O is mixed in a molar ratio of Na ₂ O / Li ₂ O of 0.1 / 99.9 to 50.00 / 50.0, preferably in a molar ratio of 1.0 / 99.0 to 10.0 / 90.0. In addition, the MgO / MgO ratio in the lithium niobate single crystal thin film is 0.1 / molar ratio.
It is mixed in the range of 99.9 to 25.0 / 75.0, preferably 5.0 / 95.0 to 9.0 / 91.0.

The above-mentioned melt composition is heated and melted at 800 to 1300 ° C. in an air atmosphere or an oxidizing atmosphere. The temperature for growing the single crystal thin film is preferably 800 to 1250 ° C. The melt is left stirring for 6 to 96 hours. When the stirring time is short, crystal nuclei remain in the melt, and crystals grow centering on the crystal nuclei, so that irregularities may occur on the surface of the lithium niobate single crystal thin film and the crystallinity may decrease. Next, the heated melt is cooled at a rate of 0.5 to 300 ° C./hour to be in a supercooled state, and then the substrate is immersed in the melt while rotating, and in this rotating state, lithium niobate monolith is placed on the substrate. The crystal thin film is grown by crystal growth.

In this case, the surface of the hetero substrate on which the lithium niobate single crystal thin film is formed is chemically polished or chemically etched,
It is desirable to set the surface roughness to JIS B0601, R _MAX = 10 to 1000A. When the value of R _MAX is larger than 1000 A, the crystallinity of the formed lithium niobate single crystal thin film deteriorates. Furthermore, it is advisable to chamfer the edges. If the edge is not chamfered, fine scratches may be formed on the edge portion and a crack may occur due to thermal shock. The chamfer may be either the R surface or the C surface. The substrate thickness is 0.5 ~
Use 2.0 mm. If it is thinner than 0.5 mm, cracks are likely to occur, and if it is thicker than 2.0 mm, the pyroelectric effect (electric discharge effect due to heating) poses a problem.Because it is charged by heating or polishing, polishing debris will adhere and scratches will occur. easy.

At the time of growth, by rotating the lithium tantalate substrate, crystals having a uniform film thickness can be obtained and stable optical characteristics can be obtained. The rotation is performed in a horizontal state, and the rotation speed is generally selected to be 5 to 150 rpm. By appropriately selecting the contact time between the substrate and the melt and the temperature of the melt, the thickness of the lithium niobate single crystal thin film deposited on the substrate can be controlled. Generally, the growth rate of a lithium niobate single crystal thin film is 0.01 to 1.0.
μm / min is desirable. If the growth rate is faster than this, undulation may occur in the lithium niobate single crystal thin film, and if the growth rate is slower than this, it takes time to grow the thin film.

After a lapse of a predetermined time, the substrate is taken out of the melt, the excess melt is removed, and then the substrate is cooled. The melt may be removed by rotating the wafer on which the lithium niobate single crystal thin film is formed at a high speed of 100 to 10,000 rpm. Cooling after removing the melt is 0.5 to 3
It is recommended to perform the cooling at a rate of 00 ° C / hour and to cool exponentially from 400 ° C. At the temperature of the Curie point of the lithium tantalate substrate (generally 650 ° C.), it is desirable to keep the temperature for a certain period of time or cool at a rate of 0.1 to 5 ° C./min. As a result, it is possible to prevent the occurrence of cracks due to the phase transition of crystals at the Curie point.

Next, a process of forming an optical waveguide layer on the lithium niobate single crystal thin film thus formed will be described. Finally, as shown in FIG. 4, (1) linear type, (2) ) Curved type, (3) Y branch type, (4) X crossing type,
And (5) various types of channel type optical waveguides such as tapered type can be obtained. An example of the manufacturing process is shown in FIG. 5, which will be described step by step. A wafer 3 formed by forming a lithium niobate single crystal thin film 2 on a single crystal substrate 1 has the above-described single crystal thin film 2 surface. The optical waveguide forming regions of various forms are patterned by photolithography, and then a titanium (Ti) thin film 4 is formed on the entire surface. Subsequently, the photosensitive resist 7 and the Ti thin film 4 thereon are removed by a lift-off method to form a Ti mask. And then argon (A
r) The area outside the optical waveguide forming region on the surface of the single crystal thin film 2 is removed by plasma etching, and the Ti thin film 4 is peeled off to form the optical waveguide layer 5, and the above-mentioned channel type optical waveguide 6 is obtained.

First, various experimental results at the wafer (slab waveguide) stage will be described. For the product of the present invention,
The present samples 1 to 6 were prepared, and as the comparative product, comparative samples 1 to 3 were prepared. Table 1 summarizes the composition of these melts, the type of substrate used for thin film formation, the film thickness of the thin film formed on this substrate, and various analysis results and various experimental data of these sample specimens. Showed. All of the samples 1 to 6 had a melt composition of Na ₂ CO ₃ , Li ₂ CO ₃ , B ₂
It was prepared by using one composed of O ₃ , Nb ₂ O ₅ and MgO. The compounding amount of each composition is as shown in Table 1. MgO is LiNbO that can be precipitated from its melt composition.
It is shown as the amount added to the theoretical amount of ₃ . It goes without saying that the melt composition does not contain V ₂ O ₅ .

The substrate is 1 in each of the present samples 1-6.
A hexagonal single crystal of lithium tantalate with a thickness of mm is used. Then, a test piece is obtained by optically polishing the (0001) plane of this lithium tantalate single crystal substrate. This lithium tantalate single crystal substrate was brought into contact with the melt composed of the above-mentioned melt composition, and the (0001)
A single crystal thin film of lithium niobate is formed on the surface by crystal growth. In any of Samples 1 to 6, the melt composition is put in a platinum crucible in advance, and an air atmosphere is used in an epitaxial growth and growth apparatus. It is heated and melted to 1100 ℃ under.

Next, the melt was heated to 930-9 at a rate of 60 ° C./hour.
After cooling to 50 ° C. to be in a supercooled state, the above-mentioned lithium tantalate single crystal substrate was immersed in this melt for a predetermined time while rotating at a rotation speed of 20 rpm, and the lithium tantalate single crystal substrate was placed on the (0001) plane. Crystal growth of a lithium niobate single crystal thin film is performed. In all of Samples 1 to 6, the immersion time was 5 minutes. After forming a lithium niobate single crystal thin film on a lithium tantalate single crystal substrate by the LPE method, pulling it up from the melt, the rotation speed is 1000r.
The melt is shaken off from the surface of the lithium niobate single crystal thin film by rotating at 10 pm for 10 minutes, and then slowly cooled to a low temperature. Thus, in any case of the present samples 1 to 6, 5
A μm thick lithium niobate single crystal thin film was obtained.
The components of Na, Mg, and B in the lithium niobate single crystal thin film were analyzed by ICP emission analysis. The results are also shown in Table 1. It can be seen that all of Samples 1 to 6 contain appropriate amounts of Na, Mg, and B, respectively.

Further, the lattice constant in the a-axis direction of the crystal structure of the lithium niobate single crystal thin film was measured by the usual powder X-ray analysis. The results are also shown in Table 1. Si is used as an internal standard in the measurement, and the lattice constant is Cu-2θ.
It is calculated by the method of least squares from the angles of the 15 diffraction peaks of lithium niobate detected in the range of 45 to 90 ° and their surface indices. For the measurement, lithium niobate single crystal grains, so-called restmelt, deposited in a platinum crucible were ground and used as a sample. According to the measurement results, the samples 1 to 6 show values of 5.151A to 5.156A. It can be seen that this is a value that is approximately approximated to the lattice constant 5.154 A in the a-axis direction of the lithium tantalate single crystal substrate crystal structure. This means that the lattice matching is well done.

On the other hand, comparative samples 1 and 2 shown as comparative products
Uses a lithium niobate single crystal substrate, that is, a homo substrate. Then, as a melt for immersing this substrate, Comparative Sample 1 uses the LB flux system, while Comparative Sample 2 uses the LV flux system. In all the samples, due to the fact that the lithium niobate single crystal substrate was doped with MgO, care was taken not to mix Na and Mg in the thin film. Therefore, the melt contains Na ₂ CO ₃ and MgO.
Is not compounded. The method for forming a lithium niobate single crystal thin film on the lithium niobate single crystal substrate by crystal growth by the LPE method is substantially the same as that of the above-described products of the present invention (Samples 1 to 6), and therefore a detailed description will be given here. Omit. However, the final thickness of the lithium niobate single crystal thin film is 5 μm.
I am trying. Further, as can be seen from the component analysis results (Table 1), Na and Mg are not mixed in the thin film. Furthermore, the lattice constant in the a-axis direction of the single crystal thin film by X-ray analysis was 5.150.
It was about A.

Comparative sample 3 is a product of the present invention (samples 1 to 6)
Similarly to the above, a lithium niobate single crystal thin film is formed on a lithium tantalate substrate (hetero substrate).
Li ₂ CO ₃ —V ₂ O ₅ —Nb ₂ having different melt compositions
O ₅ type is used. Then, Na ₂ CO ₃ is added to this for lattice matching. The method of forming a single crystal thin film by the LPE method is almost the same as that of the case of the product of the present invention, and the thickness of the thin film is also 5 μm. I
As a result of CP emission analysis, Na and V coexist, and due to the coexistence of Na, the lattice constant in the a-axis direction of the thin film is 5.154A, which is close to the lattice constant of the single crystal substrate, as can be seen from the X-ray analysis result. Showed the value. This means that the lattice matching is well done.

Although not shown in Table 1, if the content of B ₂ O _{3 in} the composition of the melt is too large, other than lithium niobate such as lithium tetraborate (Li ₂ B ₄ O ₇ ). And the points A, B, C, D and the points A, B, C and D in the three-component triangular diagram of Li ₂ CO ₃ —B ₂ O ₃ —Nb ₂ O ₅ shown in FIG. 3 as the melt composition. It has been found that the single crystal thin film is not formed when it is out of the composition region surrounded by the five composition points of E. Further, it was also found that when the content of Na ₂ CO ₃ in the melt composition was too large, not the lithium niobate single phase but sodium niobate (NaNbO ₃ ) as the second phase was precipitated.

Next, the optical waveguide characteristics of TE mode and TM mode of the lithium niobate single crystal thin film were measured. The results are as shown in Table 1. That is, in the products of the present invention (the present samples 1 to 6), good results were obtained in the optical waveguide characteristics together with the TE mode and the TM mode, whereas in the conventional products (comparative products), the comparative samples 1 and 2 were Both are TE
Since the mode can be guided but the TM mode cannot be guided, the optical waveguide characteristic in the TM mode is inferior. In the case of the product of the present invention, the lithium niobate single crystal thin film was formed on a hetero substrate different from this thin film material, while in the case of Comparative Samples 1 and 2, the lithium niobate single crystal thin film was the same as this thin film material. It is believed that this is due to the fact that it was formed on a homo substrate. This means that Comparative Sample 3 has a lithium niobate single crystal thin film formed on a hetero substrate, and this sample has TM
It is possible to agree because it was possible to guide the modes of light.

FIG. 6 shows the relationship between the substitution amount of Na in the melt and the lattice constant in the a-axis direction of the lithium niobate single crystal thin film. The horizontal axis shows the amount of Na substitution in the melt, and the vertical axis shows the lattice constant in the a-axis direction of the single crystal thin film. The lattice constant in the a-axis direction of the lithium tantalate single crystal substrate is approximately 5.154A.
It shows a weak value. As can be seen from FIG. 6, the lattice constant of the thin film increases as the Na content in the thin film increases both in the case of using the LV flux type melt and in the case of using the LB flux type melt. It can also be seen that the lattice constant is larger when MgO is diffused (doped) in the thin film than when undoped.

As is clear from the measurement data in this figure, the use of the LB flux type melt has a higher tendency of increasing the lattice constant with respect to the Na content and a higher sensitivity to Na than the LV flux type melt. I can say. Moreover, a lattice constant close to (or substantially the same as) the lattice constant of the lithium tantalate single crystal substrate can be obtained quickly at a low Na content level. Therefore, by using the LB flux-based melt, impurities such as Na in the thin film can be reduced, and a dense film with no disorder in the crystal structure of the thin film can be obtained, which is expected to improve optical constants. Is.

Next, the result of measurement of the optical propagation loss of the TM mode guided light is also shown in Table 1, but in the visible light range of the wavelength used of 0.633 μm, all of the samples 1 to 6 were 1 dB / cm or less. Yes, good results with little optical propagation loss were obtained. On the other hand, the comparative sample 3 of the conventional product using the LV flux-based melt was significantly damaged by light at the used wavelength of 0.633 μm, and the optical propagation loss could not be measured. In Samples 1 to 6, it is considered that B contained in an appropriate amount in the LiNbO ₃ single crystal waveguide layer contributes to the low optical propagation loss. The optical transmission loss was not measured for Comparative Samples 1 and 2 in which the TM mode light could not be guided.

Next, the samples 1 to 6 and the comparative sample 3 were subjected to a light damage resistance evaluation test. The test results are also shown in Table 1. This optical damage resistance evaluation test was conducted by guiding TM mode guided light having an incident light wavelength of 0.633 μm by the prism coupling method and measuring the change over time in the output power. If optical damage occurs, scattering occurs due to a change in the refractive index and the output power becomes small. If optical damage does not occur, the output power does not change. The results of this test are shown in FIG. The horizontal axis represents time (measurement constant), and the vertical axis represents changes in output power. In the case of the comparative sample 3 using the LV flux system, the output power sharply decreases after 1 minute from the light emission. It is considered that this is because the refractive index changes due to the optical damage of the thin film, which causes the scattering of light and the decrease of the output power. On the other hand, in each of Samples 1 to 6 using the LB flux system, there was almost no decrease in output power, and the result was excellent in light damage resistance.

Further, for the present samples 1 to 6 and the comparative sample 3, a silica buffer layer was formed on the LiNbO ₃ single crystal layer by chemical vapor deposition, and then an aluminum electrode was formed by vacuum vapor deposition to obtain an electro-optical constant. Of the bulk LiNbO ₃ single crystal, which was 30 ± 3 (pm / V) in the TM mode and 9 ± 1 (pm / V) in the TE mode. From the above, it can be understood that the comparative samples 1 and 2 cannot guide the TM mode, while the present samples 1 to 6 and the comparative sample 3 can guide both the TE mode and the TM mode. Further, although the comparative sample 3 has serious problems in the lattice matching property and the TM mode optical waveguiding property itself, the optical propagation loss and the optical damage are severe, but the present samples 1 to 6 do not have such problems. I understand.

[0038]

[Table 1]

Next, ridge-type channel waveguides were prepared from the LiNbO ₃ single crystal thin films of the present samples 1 to 6 which could obtain such good characteristics, and the characteristics in the TM mode were tested. The test sample of the ridge-type channel waveguide was prepared by patterning the substrate on which the LiNbO ₃ single crystal optical waveguide layer was formed by the above-mentioned method, forming a Ti mask, and performing Ar plasma etching. That is, a linear channel waveguide as shown in FIG. 4 (1) was produced from the LiNbO ₃ single crystal optical waveguides of Samples 1 to 6 described above. Waveguide shape is 10 μm wide, etching depth 1
μm. The ridge type waveguide thus produced was guided by a TM mode laser beam having a wavelength of 1.55 μm for testing. The film thickness (T: 5 μm), the waveguide width (W), the etching depth (ΔT), and the wavelength (λ) of the guided light are given by the above desirable relational expression 3.5 <(T + 0.7) / λ <5. .7 W ≦ (4λ−0.5) × (λ ² /ΔT+2.0) is satisfied.

A thickness of 1 was obtained by the same manufacturing method as the samples 1 to 6.
A μm LiNbO ₃ single crystal thin film was prepared, and a linear channel waveguide having a width of 1.5 μm and an etching depth of 0.3 μm was prepared by the same method as described above. In this ridge type waveguide,
A test was conducted by guiding a TM mode laser beam having a wavelength of 0.48 μm. In this case as well, the desired relational expression is satisfied similarly to. LiN having a thickness of 2 μm is manufactured by the same manufacturing method as the present samples 1 to 6.
A bO ₃ single crystal thin film was prepared, and the width was 10 μm in the same manner as in.
m, the etching depth was 1 μm, and a linear channel waveguide was prepared. This ridge type waveguide has a T of 1.55 μm wavelength.
The test was conducted by guiding M-mode laser light. In this case, unlike in, the desired relational expression is not satisfied.

That is, the ridge type waveguides (12 types in total) are the samples according to the present invention, and the ridge type waveguides (6 types) are the samples for comparison. Table 2 collectively shows the waveguide shape, the waveguide wavelength, etc. of each of these ridge type waveguides. With respect to such a total of 18 types of ridge type waveguides, single mode property of TM mode light, low propagation loss property,
The light damage resistance and the anneal resistance were tested. Table 2 also shows the results of these tests. First, regarding the single mode property, a TM mode laser light having a predetermined wavelength was guided in each ridge type waveguide in the direction indicated by the arrow in FIG. As a result, a single waveguide mode was observed in all of the ridge type waveguides (12 types in total), and good single mode property was shown. However, in the ridge type waveguides (6 types), the guided light was It was not observed. This is considered to be because the waveguide shape does not satisfy the above-described desirable relational expression in the ridge type waveguide, and thus no waveguide mode exists.

Next, the propagation loss was measured for 12 types of ridge-type waveguides showing good single-mode characteristics. All ridge-type waveguides were 1 dB / cm, similar to the value of the slab waveguide. The following good values were shown. In order to evaluate the optical damage resistance of these twelve types of ridge type waveguides, the change over time in the output light output was measured and no change was observed for at least 24 hours. That is, each of these ridge type waveguides has a single mode property similar to that of a slab waveguide in propagating TM mode light.
It can be seen that it has low propagation loss and light damage resistance.

Further, a silica buffer layer was formed directly above the waveguide by chemical vapor deposition, and then an aluminum electrode was formed by vacuum vapor deposition, and the electro-optical constant was measured. As a result, it was found to be almost the same as a bulk LiNbO ₃ single crystal. The same value was shown. Furthermore, to investigate the heat resistance of these waveguides, 1000 W
An accelerated deterioration test, that is, an annealing test was carried out by keeping the temperature at 12 ° C. for 12 hours, and then TM-mode laser light of a predetermined wavelength was guided again. As a result, the waveguide mode was single and the propagation loss was the same as that of the slab waveguide. Moreover, when the change with time of the emitted light was measured, at least 24
The time did not change. Furthermore, after forming a silica buffer layer by chemical vapor deposition directly above the waveguide after the annealing test,
When an aluminum electrode was formed by vacuum evaporation and the electro-optical constant was measured, it showed about 30 pm / V, which was almost the same as that of the bulk LiNbO ₃ single crystal. That is, all of these ridge type waveguides have the same single mode property, low propagation loss property, optical damage resistance, and electro-optic constant after the annealing test as before the annealing test. That is, it can be seen that it has excellent annealing resistance.

[0044]

[Table 2]

Next, the TE mode characteristics of the ridge type channel waveguides prepared from the LiNbO ₃ single crystal thin films of the present samples 1 to 6 were tested. The test sample of the ridge type channel waveguide was manufactured by the same method as described above. That is, with the same manufacturing method as the above-mentioned samples 1 to 6, the thickness is 1.5 μm.
A single crystal thin film of LiNbO ₃ was prepared and used as a linear channel waveguide. The shape of the waveguide was 2 μm in width and 0.5 μm in etching depth. The ridge type waveguide thus manufactured was guided by a TE mode laser beam having a wavelength of 1.55 μm for testing. The film thickness (T), the waveguide width (W), the etching depth (ΔT), and the wavelength (λ) of the guided light are the above-mentioned desirable relational expressions 0.29 <(T + 0.04) / λ <1.19. W ≦ (0.04λ ³ + 0.1λ ² ) /ΔT+2.5λ is satisfied.

The thickness was measured by the same manufacturing method as in Samples 1 to 6.
A 0.4 μm LiNbO ₃ single crystal thin film was prepared, and a linear channel waveguide having a width of 0.8 μm and an etching depth of 0.1 μm was prepared by the same method as described above. A TE mode laser beam having a wavelength of 0.48 μm was guided through this ridge type waveguide for testing. In this case as well, the desired relational expression is satisfied similarly to. Lithium having a thickness of 0.4 μm was manufactured by the same manufacturing method as in Samples 1 to 6.
NbO ₃ single crystal thin film was prepared, and the width was
A linear channel waveguide having a thickness of 2.0 μm and an etching depth of 0.2 μm was manufactured. A wavelength of 1.55
A test was conducted by guiding a TE mode laser beam of μm. In this case, unlike in, the desired relational expression is not satisfied.

That is, the ridge-type waveguides (12 types in total) are the samples according to the present invention, and the ridge-type waveguides (6 types) are the samples for comparison. Table 3 collectively shows the waveguide shape, the waveguide wavelength, etc. of each of these ridge type waveguides. About 18 kinds of such ridge-type waveguides in total, single mode property of TE mode light, low propagation loss property,
The light damage resistance and the annealing resistance were tested by the same method as in the TM mode light. Table 3 also shows the results of these tests. As a result, all of the ridge type waveguides (12 types in total) showed good single-mode characteristics, while no guided light was observed in the ridge type waveguides (6 types). This is considered to be because the waveguide shape does not satisfy the above-described desirable relational expression in the ridge type waveguide, and thus no waveguide mode exists.

The 12 types of ridge-type waveguides exhibiting good single-mode characteristics were good in both propagation loss and optical damage resistance as in the case of TM-mode light. That is, it is understood that all of these ridge-type waveguides have the same single mode property, low propagation loss property, and optical damage resistance as the slab waveguide in guiding the TE mode light as in the case of the TM mode light. Furthermore, when the electro-optical constant was measured, it was about the same as the bulk LiNbO ₃ single crystal.
It showed 9 pm/V. Furthermore, after the annealing test, the same single-mode property, low propagation loss property, optical damage resistance, and electro-optic constant as before the annealing test were shown. That is, it can be seen that it has excellent annealing resistance.

[0049]

[Table 3]

As described above, the single-mode optical waveguide according to the present invention has the same single-mode property, low propagation loss property, and optical damage resistance as a slab waveguide when formed into a ridge type waveguide, and , Has an electro-optical constant comparable to that of a bulk LiNbO ₃ single crystal. Furthermore, such excellent characteristics are maintained even after the annealing test, and the annealing resistance is also excellent.

[0051]

As described above, the single mode optical waveguide according to the present invention is obtained by forming a LiNbO ₃ single crystal thin film containing B on a LiTaO ₃ single crystal hetero substrate and processing it into a shape satisfying a predetermined relational expression. is there. Therefore, the single mode optical waveguide of the present invention can propagate both TE and TM mode guided light because the difference between the ordinary and extraordinary refractive indexes of the LiNbO ₃ single crystal thin film and the hetero substrate is sufficiently large. It has excellent single mode property, low optical propagation loss, and excellent light damage resistance in the visible light range.
Good anneal resistance and bulk LiNbO ₃
Exhibits electro-optical effects comparable to single crystals. Therefore, a waveguide type optical device suitable for various optical applications can be provided, and the effect of industrial contribution is extremely large. Particularly, in the case where the waveguide is channeled into a predetermined ridge type, light can be confined in a very small area. Therefore, by coupling the light source and the waveguide with an optical fiber or a lens to form an electrode, etc. It is easy to manufacture.

[Brief description of drawings]

FIG. 1 is an electron micrograph showing a cross section of a lithium niobate single crystal thin film formed on a lithium tantalate single crystal substrate according to the present invention. The waveform on the photograph is the result of EPMA analysis of boron.

FIG. 2 is a schematic view showing a (0001) plane of a lithium tantalate substrate which is a growth surface of a lithium niobate single crystal.

FIG. 3 shows Li ₂ O—B ₂ O ₃ − in the melt according to the present invention.
FIG. 3 is a triangular diagram of a ternary system of Nb ₂ O ₅ . Each composition point is (Li
₂ mol%, B ₂ O ₃ mol%, Nb ₂ O ₅ mol%). Li ₂ O / B ₂ O ₃ / Nb ₂ O ₅ A (88.90, 2.22, 8.88) B (55.00, 43.00, 2.00) C (46.50, 51.50, 2.00) D (11.11, 80.00, 8.89) E (37.50, 5.00) , 57.50) F (47.64, 46.12, 6.24) G (27.01, 64.69, 8.30) H (36.71, 37.97, 25.32) I (44.05, 32.97, 22.98)

FIG. 4 is a diagram illustrating a single mode optical waveguide according to the present invention. (1) linear type, (2) curved type, (3) Y branch type, (4) X cross type, and (5) taper type are shown respectively.

FIG. 5 is a diagram illustrating a manufacturing process of a single mode optical waveguide according to the present invention.

FIG. 6 shows the relationship between the substitution amount of Na ₂ CO ₃ in the melt and the lattice constant of the a-axis of the lithium niobate single crystal thin film in the comparison between the LB flux melt and the LV flux melt. It is a figure.

FIG. 7 is a diagram showing the results of light damage resistance test of a lithium niobate single crystal thin film in comparison between an LB flux-based melt and an LV flux-based melt.

[Explanation of symbols]

1 LiTaO ₃ single crystal substrate 2 LiNbO ₃ single crystal thin film 3 single crystal thin film wafer 4 titanium thin film 5 optical waveguide layer 6 channel type optical waveguide

Claims (7)

[Claims] 1. An optical waveguide layer of a lithium niobate single crystal is formed in a lattice matching state on a hetero substrate made of a hetero material having a different basic composition from the lithium niobate single crystal. A single-mode optical waveguide comprising a lithium oxide single crystal optical waveguide layer containing boron. 2. The single mode optical waveguide according to claim 1, wherein the content of the boron is 5 to 1000 ppm. 3. The a-axis lattice constant of the crystal structure of the lithium niobate single crystal optical waveguide layer is within the range of 99.81 to 100.07% of the a-axis lattice constant of the hetero substrate. The single mode optical waveguide according to claim 1 or 2. 4. The a-axis lattice constant of the hetero substrate is 5.
The single mode optical waveguide according to claim 3, wherein the single mode optical waveguide is 128 to 5.173A. 5. The lithium niobate single crystal optical waveguide layer contains sodium in the range of 0.02 to 10.0 mol% and / or magnesium in the range of 0.8 to 10.8 mol%. The single mode optical waveguide according to any one of claims 1 to 4. 6. When the thickness of the optical waveguide layer is T (μm) and the wavelength of the incident guided light is λ (μm), the relationship between T and λ is 1.9 when the guided light is TM mode. <(T + 0.7) / λ <5.7, when the guided light is in TE mode, the optical waveguide layer satisfies the condition of 0.29 <(T + 0.04) / λ <1.19. 6. The single mode optical waveguide according to claim 5, wherein the thickness of the single mode optical waveguide is set. 7. The waveguide width of the optical waveguide layer is W (μ
m), the etching depth is ΔT (μm), and the wavelength of the incident guided light is λ (μm), the relationship between W and ΔT is W ≦ (4λ−0.5) when the guided light is the TM mode. When the guided light is in the TE mode, the condition of x (λ ² /ΔT+2.0) is satisfied. W ≦ (0.04λ ³ + 0.1λ ² ) /ΔT+2.5λ 7. The single mode optical waveguide according to claim 5, wherein the waveguide width and the etching depth are set.