JP2003066257A - Optical waveguide type element and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical waveguide which is strong in the case of trapping a light with less waveguide loss in an optical waveguide element. SOLUTION: The optical waveguide element is constituted of an MgO dope LiNbO3 crystal baseboard 1, an burying type proton exchange optical waveguide 2 which is formed in the neighborhood of the front surface of the crystal baseboard 1 and a loaded layer 3 formed on the optical waveguide 2. The loaded layer 3 is formed by a double-layer structure which is constituted of two layers which are provided with a refractive index being larger than that of the optical waveguide 2, which consist of niobium oxide and which have different refractive index. Then the loaded layer 3 has the refractive index being higher according to the layer which is more separated from the optical waveguide 2.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical waveguide type device using a ferroelectric crystal substrate and a method for manufacturing the same. In particular, the present invention relates to an optical waveguide type wavelength conversion element that uses a coherent light source and is used in the fields of optical information processing and optical application measurement control, and a method for manufacturing the same.

[0002]

2. Description of the Related Art An optical waveguide is a light wave control technique that enables compact and integrated optical elements because it can secure a strong optical confinement and a long propagation distance. Optical waveguides have been studied in a wide range of fields such as communication, optical information processing, and measurement, and development and commercialization of optical waveguide type devices have been actively promoted. Among them, by forming an optical waveguide on a ferroelectric crystal substrate having various optical characteristics, it is possible to apply, for example, an optical modulator or a wavelength conversion element using an electro-optical effect, an acousto-optical effect, or a non-linear optical effect. Proposed. Especially when an optical waveguide is applied as a wavelength conversion element, laser light (fundamental wave) emitted from a semiconductor laser
The wavelength of the light is converted by the wavelength conversion element, and light of short wavelength (harmonics)
It is possible to realize a small-sized short-wavelength light source that can obtain the above.

An element using a loaded optical waveguide has been proposed as one of the optical waveguide elements. This is a structure in which, for example, an embedded optical waveguide is formed on a crystal substrate, and a layer called a loading layer (clad layer) is further formed thereon. It is known that a low loss optical waveguide can be realized by using, for example, a dielectric material (eg, SiO ₂ ) having a refractive index smaller than that of the optical waveguide as the loading layer.

Further, as proposed in Japanese Patent Application Laid-Open No. 9-281536, a material having a refractive index larger than that of the optical waveguide (eg, TiO ₂ ) is used as a loading layer.
Etc.) can be used to enhance the optical confinement of the optical waveguide. In the above-mentioned Japanese Patent Laid-Open No. 9-281536, a loaded optical waveguide is adopted as the structure of the wavelength conversion element, whereby the mode control of the guided light can be performed and the overlap between the fundamental wave and the harmonic can be increased. Is being done.

Another example of using a loading type optical waveguide having a loading layer is a method proposed in Japanese Patent Laid-Open No. 5-249519. Here, by using a two-dimensional slab type optical waveguide and a loading layer having substantially the same refractive index as that of the optical waveguide, the conditions under which both the fundamental wave and the harmonic wave are in a single mode are expanded.

As another example using a loading type optical waveguide having a loading layer, a loading layer having a multi-layered structure (Japanese Patent Laid-Open No. 4-276725) is proposed as disclosed in Japanese Patent Laid-Open No. 4-276725. In the official gazette, it is described as "clad layer"). Here, the loading layer is a dielectric film (SiO ₂ , Ta ₂ O ₅ , Al ₂
O ₃ ) and a metal film (Al, Ag, Cu, etc.) has a multi-layer structure, so that it is possible to independently control the waveguide mode shapes of the fundamental wave and the harmonics in the wavelength conversion element, for example. Has been done.

[0007]

In the optical waveguide type device, it is important to reduce the propagation loss and control the guided mode. In particular, when an optical waveguide is used for the wavelength conversion element, the wavelength conversion efficiency changes greatly due to the overlap of the fundamental wave and the harmonics and the optical power density, so it is necessary to further control the refractive index distribution of the optical waveguide. Becomes In a loaded optical waveguide having a loading layer, optical confinement can be controlled by controlling the refractive index and thickness of the loading layer. Therefore, the refractive index distribution control of the optical waveguide described above (effective refractive index control) It can be said that this is a useful structure that enables

However, in the above-mentioned conventional method,
An optical waveguide with low propagation loss and strong optical confinement has not been realized. For example, when a material having a refractive index smaller than that of the optical waveguide is used as a single-layer loading layer, it is possible to realize an optical waveguide with low loss, but it is difficult to strengthen optical confinement. was there. In addition, when a material having a refractive index higher than that of the optical waveguide is used as the single-layer loading layer, the difference in the refractive index between the optical waveguide and the loading layer becomes large, and the scattering loss due to the distortion at the boundary surface is increased. There was a problem that it occurred. Further, in the case of using a loading layer having a multi-layer structure, there is a problem that scattering loss becomes large due to strain at the boundary of layers and a difference in refractive index because the constituent materials of the layers are different.

The present invention has been made in order to solve the above-mentioned problems in the prior art, and an optical waveguide type element capable of strengthening the optical confinement of the optical waveguide and at the same time realizing an optical waveguide of low loss, and the same. It is intended to provide a manufacturing method.

[0010]

In order to achieve the above object, the structure of an optical waveguide type element according to the present invention is a ferroelectric crystal substrate and an optical waveguide formed near the surface of the ferroelectric crystal substrate. And an optical waveguide type element comprising a loading layer formed on the optical waveguide, wherein the loading layer has a refractive index higher than that of the optical waveguide, and the same constituent element It is characterized by having a multi-layer structure composed of a plurality of layers having different optical characteristics.

According to the structure of the optical waveguide type element, an optical waveguide type element capable of enhancing the optical confinement of the optical waveguide and at the same time realizing an optical waveguide with low loss can be obtained.

In the structure of the optical waveguide type device of the present invention, it is preferable that at least adjacent layers have different refractive indexes in the multilayer structure, and at least adjacent layers have different light absorption coefficients.

Further, in the structure of the optical waveguide type element of the present invention, the multilayer structure is a structure composed of layers of m (m is an integer of 2 or more) having different refractive indexes, and is separated from the optical waveguide. The layers preferably have a higher refractive index.

Further, in the structure of the optical waveguide type element of the present invention, it is preferable that the loading layer is made of a dielectric material. According to this preferable example, the following operational effects are obtained. That is, since many dielectric materials are transparent to the guided light in the infrared region to the near-ultraviolet region, the waveguide loss due to the light absorption of the loading layer is small. Moreover, a desired refractive index can be easily obtained by selecting a material. Furthermore, since the dielectric material has high adhesion to the ferroelectric crystal that is the substrate material, the loading layer can be easily formed by sputtering or vapor deposition. Further, in this case, the dielectric material is preferably niobium oxide.

Further, in the structure of the optical waveguide type device of the present invention, it is preferable that the optical waveguide is formed by subjecting the ferroelectric crystal substrate to ion exchange.

Further, the structure of the optical waveguide type element of the present invention further comprises a polarization inversion layer formed near the surface of the ferroelectric crystal substrate and in which the nonlinear polarization is periodically inverted. Is preferred. According to this preferable example, an optical waveguide type wavelength conversion element capable of increasing the wavelength conversion efficiency can be realized.

In the method of manufacturing an optical waveguide device according to the present invention, the first step of forming the optical waveguide in the vicinity of the surface of the ferroelectric crystal substrate and the second step of forming the loading layer on the optical waveguide. And a step of forming a loading layer having a multi-layer structure including a plurality of layers made of the same constituent element and having different optical characteristics. And the formation conditions are different for each layer.

According to this method of manufacturing an optical waveguide device,
It is possible to manufacture an optical waveguide type device capable of enhancing the optical confinement of the optical waveguide and at the same time realizing an optical waveguide with low loss.

In the method for manufacturing an optical waveguide device of the present invention, it is preferable that the optical characteristic is a refractive index.

In the method of manufacturing an optical waveguide device of the present invention, it is preferable that the optical characteristic is a light absorption coefficient.

In the method of manufacturing an optical waveguide type device of the present invention, in the second step, the forming temperature T of at least the first layer formed in contact with the optical waveguide is formed.
It is preferable that the formation temperature T2 of 1 and the second layer formed on the first layer has a relationship of T1 <T2. According to this preferable example, the refractive index of the second layer can be made larger than that of the first layer. Further, in this case, the formation temperature T1 of the first layer is preferably 100 ° C. or lower. According to this preferable example, it is possible to reduce the difference in the refractive index from the optical waveguide and avoid the absorption of light by the film.

In the method of manufacturing an optical waveguide type element of the present invention, in the second step, at least the formation speed V of the first layer formed in contact with the optical waveguide is formed.
It is preferable that the formation speed V2 of 1 and the second layer formed on the first layer have a relationship of V1> V2. According to this preferable example, the refractive index of the second layer can be made larger than that of the first layer. Further, in this case, the formation speed V1 of the first layer is 0.1 nm / sec.
The above is preferable. According to this preferable example, a layer having a low refractive index or a low light absorption coefficient can be stably realized.

Further, in the method of manufacturing an optical waveguide device of the present invention, it is preferable that the material forming the loading layer is niobium oxide.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below with reference to embodiments. In addition, here, the MgO-doped LiNbO ₃ crystal substrate will be described as an example of the ferroelectric crystal substrate, but the ferroelectric crystal substrate of the present invention is MgO.
The substrate is not limited to the doped LiNbO ₃ crystal substrate.

The optical waveguide type device can have various functions by providing a dielectric or metal layer (hereinafter referred to as "loading layer") on the optical waveguide. Above all, the method of forming a loading layer having a refractive index higher than that of the optical waveguide on the optical waveguide is one of the effective means for strengthening the optical confinement of the optical waveguide. At this time, the loading layer acts as a part of the optical waveguide and increases the effective refractive index of the optical waveguide. Therefore, as shown in FIG. 2, it has a role of controlling the electric field distribution of the guided light propagating in the optical waveguide. Fulfill The change in the effective refractive index depends on the refractive index and the thickness of the loading layer, and the refractive index of the loading layer increases, or the effective refractive index also increases when the thickness of the loading layer increases. Here, FIG.
2A is a waveguide mode profile when no loading layer is provided, and FIG. 2B is a waveguide mode when a loading layer having a refractive index larger than that of the optical waveguide is formed on the optical waveguide. Each profile is shown. Figure 2
In FIGS. 2A and 2B, 4 is a ferroelectric crystal substrate, and 5 is an optical waveguide formed on the ferroelectric crystal substrate 4. Further, in FIG. 2B, 6 is a loading layer formed on the optical waveguide 5 and made of a dielectric material. As described above, when the refractive index of the loading layer 6 is increased, the optical confinement strength of the optical waveguide 5 is also increased. Therefore, the loading layer 6 having a refractive index higher than that of the optical waveguide 5 is formed on the optical waveguide 5. This method is effective for an optical waveguide type optical modulator and a wavelength conversion element that utilize a nonlinear optical effect that increases depending on the optical power density in the optical waveguide 5.

Conventionally, as the loading layer 6, a single layer or a multi-layered structure of a plurality of materials has been used. Further, as a material used for the loading layer 6 composed of a single layer, a dielectric material is generally used.
Such as _{SiO 2, TiO 2, Ta 2} O 5, Nb 2 O 5 and the like. This is because it is possible to select a material that absorbs less light that propagates in the optical waveguide 5.

Optical waveguide type devices having a structure in which the loading layer 6 is provided on the optical waveguide 5 are roughly classified into two types. One is a type in which a material having a refractive index smaller than that of the optical waveguide 5 is used as the loading layer 6, and the other is a type in which a material having a refractive index larger than that of the optical waveguide 5 is used as the loading layer 6. It is a type that uses the material. The former is a generally used structure, and is mainly used for protection of the optical waveguide 5 and reduction of propagation loss. On the other hand, if the latter structure is used, the effective refractive index of the optical waveguide 5 can be increased and the optical confinement of the optical waveguide 5 can be strengthened.

In the present invention, the emphasis is mainly on the loading layer 6 having a refractive index higher than that of the optical waveguide 5. Since the optical confinement effect of the optical waveguide 5 increases as the refractive index of the loading layer 6 increases, it is effective to use a material having a higher refractive index as the loading layer 6. However, it has been newly found that the propagation loss of the optical waveguide 5 increases remarkably as the difference in the refractive index between the optical waveguide 5 and the loading layer 6 increases. Therefore, the optical waveguide 5
It has been empirically revealed that it is difficult to enhance the optical confinement and to realize the optical waveguide 5 with low loss at the same time.

On the other hand, an optical waveguide type element having a multilayer structure as the loading layer 6, for example, an optical waveguide type element having a loading layer made of a dielectric material and a metal material or a loading layer made of a dielectric multilayer film has been proposed. However, for example, the optical waveguide type wavelength conversion element having this structure has an advantage that the modes of the fundamental wave and the harmonics can be controlled independently. Therefore, it is possible to achieve a desired optical confinement state by controlling the constituent materials and thickness of the multilayer structure. However, in such a structure, the following (1)
There was a problem like (3).

(1) Since the constituent elements of each layer are different, it is necessary to manufacture each layer by changing the device or target.

(2) Since the film quality (difference in the film structure at the molecular level) is different for each layer, distortion occurs at the interface between layers, and the distortion causes scattering loss.

(3) Manufacturing takes time (poor throughput).

On the other hand, the point of the present invention is to form the loading layer 6 having a multi-layer structure composed of a plurality of layers made of the same constituent element and having different optical characteristics (refractive index and light absorption coefficient). . By changing the forming condition (film forming condition) for each layer, different optical characteristics are realized with the same constituent element, and the loading layer 6 having a multi-layer structure composed of a plurality of layers made of the same constituent element and having different optical characteristics. Can be formed. Further, since the constituent elements of each layer are the same, the above-mentioned problem in the loading layer 6 having a multilayer structure can be solved. That is, since the constituent elements of the respective layers are the same, the same apparatus is used, and the loading layer 6 composed of a plurality of layers having different refractive indexes is realized only by changing the film forming temperature or the film forming rate of each layer. It becomes possible. Also, the loading layer 6
Among them, the refractive index difference between the optical waveguide 5 and the loading layer 6 can be reduced by reducing the refractive index of the layer that is in contact with the optical waveguide 5, so that the loading layer 6 is as low as the single layer. It is possible to enhance the optical confinement of the optical waveguide 5 while realizing the propagation loss.

The optical waveguide device of the present invention will be described in more detail below.

FIG. 1 (a) is a perspective view showing an optical waveguide device according to an embodiment of the present invention, and FIG. 1 (b) is FIG.
FIG. 4A is a cross-sectional view of (a) viewed from the + Z plane, and a diagram showing a refractive index distribution of a loading layer and an optical waveguide.

As shown in FIG. 1, an optical waveguide 2 is formed near the surface of an X-plate MgO-doped LiNbO ₃ crystal substrate 1 which is a ferroelectric crystal substrate, and a loading layer 3 is formed on the optical waveguide 2. Are formed. Here, the optical waveguide 2 is a buried type proton exchange optical waveguide formed by a proton exchange treatment and an annealing treatment, and the loading layer 3 is
It is composed of niobium oxide (Nb ₂ O ₅ ) which is a dielectric material. The loading layer 3 has a two-layer structure composed of two layers made of niobium oxide and having different refractive indexes. In addition, in the refractive index distribution in FIG.
₀ is the refractive index of the MgO-doped LiNbO ₃ crystal substrate 1, n
₁ is the refractive index of the first layer in contact with the optical waveguide 2 in the loading layer 3, and n ₂ is the refractive index of the second layer formed on the first layer. Further, n _f is the MgO-doped LiN of the optical waveguide 2.
The refractive index on the surface of the bO ₃ crystal substrate 1. Since the optical waveguide 2 is formed by the proton exchange treatment and the annealing treatment, the refractive index of the optical waveguide 2 is M
The gO-doped LiNbO ₃ crystal substrate 1 has a graded change from the surface to the inside to take a value of n _f to n ₀ .

In order to realize the device structure of this embodiment, first, the optical waveguide 2 is formed on the MgO-doped LiNbO ₃ crystal substrate 1. Specifically, 0.5 mm thick Mg
A Ta thin film is formed on the surface of the O-doped LiNbO ₃ crystal substrate 1 by vapor deposition, sputtering, etc., and an optical waveguide pattern is formed by a photolithography process and an etching process.
By immersing the ₃ crystal substrate 1 in high temperature benzoic acid, pyrophosphoric acid, or the like, a desired region of the MgO-doped LiNbO ₃ crystal substrate 1 is exchanged with a proton exchange region (MgO-doped LiNbO ₃
A region where the Li ions in the crystal substrate 1 and the proton ions in the acid were exchanged) was formed. At this time, for example, by immersing it in pyrophosphoric acid at about 200 ° C. for 7 minutes, MgO
About 0.2 μm from the surface of the doped LiNbO ₃ crystal substrate 1
It was possible to form a proton exchange region with respect to the depth of. Then, the Ta thin film was removed by immersing the proton-exchanged MgO-doped LiNbO ₃ crystal substrate 1 in a solution in which hydrofluoric acid and nitric acid were mixed at a ratio of 2: 1 for several seconds. Next, the MgO-doped LiNbO ₃ crystal substrate 1 from which the Ta thin film is removed is further annealed at a temperature of, for example, 300 ° C. to 350 ° C. for 150 to 20.
By applying for 0 minute, proton ions (H ⁺ ) were thermally diffused in the MgO-doped LiNbO ₃ crystal substrate 1. This thermally diffused proton exchange region is formed by MgO-doped Li.
The optical waveguide 2 has a refractive index higher than that of the NbO ₃ crystal substrate 1. The width of the optical waveguide at this time is about 5μ.
m, and the depth in the substrate thickness direction was about 2.5 μm. The length of the optical waveguide 2 in the propagation direction was about 10 mm.
Next, the loading layer 3 was formed by sputtering on the surface of the MgO-doped LiNbO ₃ crystal substrate 1 having the optical waveguide 2 obtained as described above.

The method of forming the loading layer 3, which is the feature of the present invention, will be described in more detail below.

In general, when a dielectric film is formed on a substrate by a method such as sputtering or vapor deposition, the characteristics of the film (refractive index, light absorption, etc.) that are formed depend on the film forming conditions (film forming temperature and film forming atmosphere). It is known that the coefficient) changes. Then, the detailed study by the present inventors has revealed that the refractive index of the niobium oxide used in the present embodiment greatly varies depending on the film forming temperature even under the film forming conditions.

FIG. 3 shows the relationship between the film forming temperature of niobium oxide (the sputtering source is Nb ₂ O ₅ ) during sputtering and the refractive index. Sputtering is
For example, it is a method of irradiating a target with Ar ions and depositing a granular material called sputtering on a desired substrate. When the target is an oxide, film formation is performed in an oxygen supply atmosphere to reduce oxygen loss in the target material during sputtering. At this time, depending on the temperature of the substrate (in particular, the temperature of the substrate surface on which the film is deposited), the composition of the film on the deposition surface (in the present embodiment,
The ratio of niobium to oxygen) and the density change, and the refractive index of the formed film changes. The advantage of the niobium oxide film is that the optical characteristics of the film can be largely changed by changing the film forming temperature in a relatively low temperature region. It has been conventionally known for various materials that the optical characteristics of a film change depending on the film forming temperature. This is due to microcrystallization or crystallization of the material of the film, and is caused by the crystallization of the film being promoted in a high temperature state of 400 to 500 ° C. or higher. However, it was difficult to apply such a high temperature process to the loading layer formed on the proton exchange optical waveguide 2. When subjected to a high temperature process, the optical waveguide 2
This is because the thermal diffusion of the protons in the MgO-doped LiNbO ₃ crystal substrate 1 constituting the above becomes remarkable, the optical waveguide 2 disappears, and the mode size greatly expands.
The present inventors have found that in the formation of a niobium oxide film, the optical characteristics of the film change significantly in a relatively low temperature region where the crystallization temperature is not reached. That is, 200
It has been found that it is possible to greatly change the refractive index of the film at a film forming temperature of ℃ or less. Therefore, even in the optical waveguide 2 formed by a low temperature process (200 to 300 ° C.) such as proton exchange, niobium oxide is used as the material of the loading layer 3 and the film forming conditions are changed for each layer to obtain optical characteristics. It was possible to realize the loading layer 3 having a multi-layer structure composed of a plurality of different layers. Focusing on this point, the present inventors further studied the control of the refractive index of the niobium oxide film by controlling the substrate temperature, and in the niobium oxide used in the present embodiment, for example, the film formation temperature was set to 3
By controlling in the range of 0 ℃ to 200 ℃, the wavelength of 6
It was found that the refractive index for 40 nm light can be controlled between 2.27 and 2.39.

On the other hand, the optical absorption coefficient of the niobium oxide film formed by the above method increases as the refractive index increases, and for example, when formed as a single loading layer on the optical waveguide, it propagates in the optical waveguide. The loss (propagation loss) of guided light is 0.5 dB / for guided light having a wavelength of 800 nm.
The value was varied from cm (the film having the refractive index of 2.27) to 1.5 dB / cm or more (the film having the refractive index of 2.39). From this experiment, it can be said that there is a trade-off relationship between strengthening the optical confinement of the optical waveguide by using the high refractive index film as the loading layer and reducing the propagation loss of the guided light propagating in the optical waveguide. Conceivable.

However, it was found that the fluctuation of the propagation loss is larger than the value of the propagation loss calculated from the fluctuation of the optical absorption coefficient of the film. Therefore, the present inventors considered that the above-mentioned propagation loss includes an increase in the optical absorption coefficient of the film and a scattering loss due to the difference in refractive index between the optical waveguide and the loading layer. Then, the loading layer has a multi-layered structure, the portion in contact with the optical waveguide is formed by a low refractive index film, and by further forming a high refractive index film thereon, the propagation loss of the above-mentioned propagation loss is maintained while maintaining a strong optical confinement state. We thought it would be possible to achieve a significant reduction. Also, paying attention to the fact that it is possible to change the refractive index of the film depending on the substrate temperature during sputtering in the above study, and to form multiple films of the same constituent element (same sputter target) and different refractive indices. I tried to form a loading layer by. By forming a multilayer structure composed of the same constituent element, it is possible to avoid the "distortion at the boundary between layers composed of different constituent elements and the scattering loss due to it" which is the problem of the loading layer of the conventional multilayer structure. It is also expected to be possible.

As a study experiment, the same target (Nb ₂
The loading layer 3 (FIG. 1) having a two-layer structure was formed by O ₅ ). First, a low refractive index film was formed on the first layer of the loading layer 3 (a portion in contact with the optical waveguide 2) at a film forming temperature of 30 ° C., for example. Next, the substrate was heated to, for example, 150 ° C., and after the substrate was heated sufficiently uniformly, a second layer was further formed. That is, the film formation temperature T1 of the first layer formed in contact with the optical waveguide 2 and the formation temperature T2 of the second layer formed on the first layer have a relationship of T1 <T2. In the first layer formed in contact with the optical waveguide 2, the film formation temperature is 100 ° C. or lower in order to reduce the difference in refractive index with the optical waveguide 2 and avoid absorption of light by the film. Is desirable.
Further, in the second layer, the film formation temperature is preferably in the range of 120 ° C. to 200 ° C. in order to strengthen the optical confinement of the optical waveguide 2. When the film formation temperature exceeds 200 ° C., for example, when the proton exchange optical waveguide 2 is used as in the present embodiment, the diffusion of protons into the MgO-doped LiNbO ₃ crystal substrate 1 is affected. For this reason,
The shape of the optical waveguide 2 changes during the formation of the loading layer 3, which makes it difficult to manufacture an element having desired characteristics.

Here, since the inventors of the present invention control the optical confinement strength with the high refractive index film of the second layer, it is necessary to optimize the film thickness of the first layer, and The relationship between the film thickness of the first layer and the propagation loss of the optical waveguide 2 was also examined.
Forming the first layer with a film thickness in the range of 5 nm to 30 nm,
When the waveguide characteristics were examined, the following results were obtained. That is, when the first layer is thin, the influence of the second layer is large and the guided light is attracted to the vicinity of the substrate surface.
Even if the light absorption coefficient of the loading layer 3 is small, the absorption loss becomes large, and as a result, the loss of the waveguide becomes large. Further, when the first layer is too thick, the influence of the second layer becomes very small, and the effect of confining light becomes weak. According to this study, the film thickness of the first layer composed of the low refractive index film is 10 n.
It has been found that it is desirable to be in the range of m to 20 nm. In addition, the present inventors can also study the film thickness of the second layer and enhance the optical confinement of the optical waveguide 2 when the film thickness of the second layer is 130 nm to 200 nm. I found out that The film thickness of the second layer is 200n
When m or more, the waveguide mode condition is satisfied in the loading layer 3, resulting in a loss for the guided light guided in the optical waveguide 2.

From the above examination results, it became clear that an optical waveguide type device having the loading layer 3 and having a propagation loss of 0.5 dB / cm or less and enhanced optical confinement can be manufactured.

The present inventors manufactured an optical waveguide type wavelength conversion device having the above-mentioned device structure in order to investigate the optical confinement strength of the optical waveguide. This wavelength conversion element includes a polarization inversion layer in which nonlinear polarization is periodically inverted, which is formed on a MgO-doped LiNbO ₃ crystal substrate which is a ferroelectric crystal substrate, and an optical waveguide which is orthogonal to the polarization inversion layer. It is composed by. This wavelength conversion element can be manufactured, for example, as follows. That is, first, MgO
The Ta film was deposited on the doped LiNbO ₃ crystal substrate, forming a comb-shaped electrode and the stripe electrode MgO-doped LiNbO ₃ crystal substrate by photolithography and etching processes. Then, by applying an electric field using these electrodes, MgO-doped LiN
A periodic domain inversion layer is formed in the bO ₃ crystal. Next, after removing the electrodes, for example, the proton exchange optical waveguide and the loading layer are formed as described above, and the end surface of the optical waveguide is polished by a surface substantially orthogonal to the optical waveguide. Thereby, the optical waveguide type wavelength conversion element is obtained.

The optical confinement strength of the optical waveguide can be evaluated by the wavelength conversion efficiency of the wavelength conversion element. That is, when the optical confinement of the optical waveguide is strong,
The overlap (mode overlap) of the fundamental wave and the harmonic wave in the optical waveguide type wavelength conversion element becomes strong, and the wavelength conversion efficiency increases. For comparison, an element A having a loading layer made of a single layer having a low refractive index and an element B having a loading layer made of two layers of a low refractive index layer and a high refractive index layer formed in the present embodiment.
Was produced. When wavelength conversion efficiency was measured using these elements A and B, the wavelength conversion efficiency of element B was
The result is that the wavelength conversion efficiency is improved by about 20%.

In the present embodiment, as an example of the method of forming the loading layer 3 having a multilayer structure composed of a plurality of layers made of the same constituent element and having different refractive indices, the deposition conditions for the loading layer 3 are used. Although the case of controlling the film forming temperature has been described above, the film forming conditions such as changing the film forming atmosphere of the niobium oxide film (the flow rates of Ar gas and O ₂ gas, the pressure in the chamber of the sputtering apparatus), or The loading layer 3 having the same multi-layer structure can be formed by a method of controlling the film formation rate by changing the power applied to the sputtering apparatus, and an optical waveguide type device having low propagation loss and strong optical confinement is manufactured. be able to.

FIG. 4 shows the relationship between the deposition rate and the refractive index of the niobium oxide film investigated by the present inventors. in this case,
The film formation rate was controlled by changing the power applied to the sputtering device. As shown in FIG. 4, the refractive index of the niobium oxide film is such that the film formation rate is 0.05 to 0.1 nm / sec.
The area is changing rapidly. From the results of this experiment, it is possible to form a layer having a refractive index of 2.30 or less (for light having a wavelength of 640 nm) by setting the film formation rate to 0.1 nm / sec or more. The film formation rate is 0.
By setting it to be 08 nm / sec or less, the refractive index is 2.
It was possible to form a layer of 33 or more (for light having a wavelength of 640 nm). That is, when the film formation speed V1 of the first layer formed in contact with the optical waveguide 2 and the film formation speed V2 of the second layer formed on the first layer have a relationship of V1> V2. The refractive index of the second layer can be made higher than that of the first layer.

Further, in this embodiment, the loading layer 3
As a constituent material of the above, niobium oxide, which is a dielectric material, has been described as an example, but the constituent material of the loading layer 3 usable in the present invention is not limited to niobium oxide. For example, TiO ₂ , Ta ₂ O ₅ , Sn ₂ O ₅ , ZnO, Ga
Even when N, ZrO ₂ , CeO ₂ , ZnSe, In ₂ O ₃ or the like is used, by making the film forming conditions different for each layer, a loading layer having the same effect as that of the present embodiment can be obtained. Can be formed.

Further, in the present embodiment, the loading layer 3
However, it is easy to form a multi-layered loading layer having a larger number of layers by changing the film forming conditions for each layer. It is also possible to control the optical confinement strength by finely controlling the thickness and film forming conditions.

FIG. 5 shows another example of the refractive index distribution of the optical waveguide and the loading layer. 5A shows a case where a layer having a low refractive index in the loading layer 3 is formed by changing its refractive index distribution to graded, and FIG. 5B shows a low refractive index in the loading layer 3. The case where the layer having is formed by changing the refractive index distribution in a stepwise manner is shown. 5 (a) and 5
In the refractive index distribution in (b), n ₀ is MgO-doped L
The refractive index of the iNbO ₃ crystal substrate 1, n ₁ is the refractive index of a portion of the loading layer 3 having a low refractive index in contact with the optical waveguide 2, and n ₂ is the high refractive index formed on the layer having a low refractive index. It is the refractive index of the layer having the refractive index. N _f is the optical waveguide 2
Is the refractive index on the surface of the MgO-doped LiNbO ₃ crystal substrate 1. Here, since the optical waveguide 2 is formed by the proton exchange treatment and the annealing treatment, the refractive index of the optical waveguide 2 is MgO-doped LiNbO ₃ crystal substrate 1
The value changes from n _f to n ₀ from the surface to the inside of the grading. Further, in FIG. 5A, the refractive index of the layer having a low refractive index is n ₁ from the portion in contact with the surface of the optical waveguide 2 toward the layer having a high refractive index.
Is changed to graded to take the value of n ₂ from. The refractive index distribution shown in either FIG. 5A or FIG. 5B can be realized by controlling the film forming temperature as the film forming condition, for example. That is, the refractive index distribution of FIG. 5A can be realized by gradually increasing the film forming temperature of the layer having a low refractive index at the film forming stage. Also,
The refractive index distribution of FIG. 5B can be realized by repeating the steps of first forming a film at an arbitrary film forming temperature, interrupting the film forming, raising the temperature, and then forming the film again.

In the case of forming a multilayer film, if the surface of the first layer is exposed to the atmosphere after forming the first layer and before forming the second layer, an altered layer due to surface oxidation or the like is formed. Then, due to this, there is strain between the layers even after the formation of the second layer, which causes a propagation loss. To prevent this,
It is desirable to switch the film forming conditions in vacuum.

Further, in the present embodiment, the wavelength conversion element has been mainly described as an example of the optical waveguide type element having the loading layer of the present invention in detail, but the optical waveguide type element of the present invention is For example, it can be applied to an optical modulator or an optical switch that uses the electro-optical effect. In these optical modulators and optical switches, for example, LiNbO
_{(3) An} electrode is formed in the vicinity of the optical waveguide formed on the crystal substrate, and an electric field is applied to the region where the optical waveguide is formed, thereby changing the effective refractive index of the optical waveguide through the electro-optic effect. Such a method is taken. In this case, the change in the refractive index becomes large on the substrate surface most affected by the electric field, that is, in the region near the optical waveguide surface. Therefore,
In the optical modulator, the degree of modulation can be increased by strengthening the optical confinement of the optical waveguide. Therefore, by adopting the structure shown in the present embodiment in this optical modulator, an optical modulator having a low propagation loss and a large modulation degree can be manufactured.

Further, the optical waveguide type device of the present invention is caused to function as a grating (periodic refractive index distribution structure) by, for example, patterning the loading layer 3 periodically in the propagation direction of the guided light. Is also possible. The structure in which the grating is formed on the optical waveguide is considered to be applied to an optical integrated circuit or the like. For example, by forming a grating structure on the slab type optical waveguide, it is possible to add a function of condensing light or switching the propagation direction of light by utilizing the optical diffraction effect of the grating.
The diffraction efficiency (strength of diffraction effect) can be increased by increasing the refractive index of the portion forming the grating. Therefore, by patterning the loading layer according to the present invention in a periodic pattern to form a grating, a grating with low loss and high diffraction efficiency can be obtained.

[0056]

As described above, according to the present invention,
By constructing the loading layer with a plurality of layers having a refractive index higher than that of the optical waveguide and having the same constituent elements and different optical characteristics, the optical confinement of the optical waveguide is enhanced and at the same time, it is reduced. It is possible to realize a lossy optical waveguide.

[Brief description of drawings]

FIG. 1A is a perspective view showing an optical waveguide device according to an embodiment of the present invention, FIG. 1B is a cross-sectional view of FIG. 1A viewed from a + Z plane, and a refractive index of a loading layer and an optical waveguide. Diagram showing distribution

FIG. 2A is a diagram showing a mode profile of guided light propagating in an optical waveguide in which a loading layer is not provided in one embodiment of the present invention, and FIG. The figure which shows the mode profile of the guided light which propagates in the optical waveguide in which the loading layer was provided.

FIG. 3 is a diagram showing a relationship between a film forming temperature and a refractive index during sputtering of niobium oxide in one embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a film formation rate and a refractive index during sputtering of niobium oxide in one embodiment of the present invention.

FIG. 5 is a diagram showing another example of the refractive index distributions of the loading layer and the optical waveguide according to the embodiment of the present invention.

[Explanation of symbols]

1 MgO-doped LiNbO ₃ crystal substrate 2, 5 Optical waveguides 3, 6 Loading layer 4 Ferroelectric crystal substrate

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazuhisa Yamamoto             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F-term (reference) 2H047 KA04 KA06 LA02 NA02 NA08                       PA03 PA04 PA12 PA13 PA21                       QA01 QA03 RA00 RA08 TA35                 2K002 AA01 AA04 AB12 BA01 CA03                       DA06 EA04 FA07 FA26 FA27                       GA07 HA20

Claims (16)

[Claims] 1. An optical waveguide type device comprising a ferroelectric crystal substrate, an optical waveguide formed near the surface of the ferroelectric crystal substrate, and a loading layer formed on the optical waveguide. An optical fiber having a multilayer structure in which the loading layer has a refractive index higher than that of the optical waveguide and is composed of a plurality of layers that are made of the same constituent element and have different optical characteristics. Waveguide element. 2. The optical waveguide device according to claim 1, wherein at least adjacent layers in the multilayer structure have different refractive indexes. 3. The optical waveguide device according to claim 1, wherein in the multilayer structure, at least adjacent layers have different light absorption coefficients. 4. The multi-layer structure has m (m
Is an integer of 2 or more), and a layer farther from the optical waveguide has a larger refractive index.
The optical waveguide device according to any one of 1. 5. The optical waveguide device according to claim 1, wherein the loading layer is made of a dielectric material. 6. The optical waveguide device according to claim 5, wherein the dielectric material is niobium oxide. 7. The optical waveguide is formed by subjecting the ferroelectric crystal substrate to ion exchange.
7. The optical waveguide device according to any of 6. 8. The optical waveguide device according to claim 1, further comprising a polarization inversion layer which is formed near the surface of the ferroelectric crystal substrate and in which the nonlinear polarization is periodically inverted. . 9. A method of manufacturing an optical waveguide device comprising a first step of forming an optical waveguide near the surface of a ferroelectric crystal substrate and a second step of forming a loading layer on the optical waveguide. The second step is a step of forming a loading layer having a multi-layer structure composed of a plurality of layers made of the same constituent element and having different optical characteristics, and the formation conditions are different for each layer. And a method of manufacturing an optical waveguide device. 10. The optical property is a refractive index.
A method of manufacturing the optical waveguide device according to item 1. 11. The method of manufacturing an optical waveguide device according to claim 9, wherein the optical characteristic is a light absorption coefficient. 12. The formation temperature T of at least the first layer formed in contact with the optical waveguide in the second step.
10. The method of manufacturing an optical waveguide device according to claim 9, wherein the formation temperature T2 of 1 and the second layer formed on the first layer has a relationship of T1 <T2. 13. The formation temperature T1 of the first layer is 100.
The method for producing an optical waveguide type device according to claim 12, wherein the temperature is not higher than ° C. 14. The formation speed V of at least the first layer formed in contact with the optical waveguide in the second step.
10. The method for manufacturing an optical waveguide device according to claim 9, wherein the formation speeds V2 of 1 and the second layer formed on the first layer have a relationship of V1> V2. 15. The formation rate V1 of the first layer is 0.1.
15. The method for manufacturing an optical waveguide type device according to claim 14, which is not less than nm / sec. 16. The method of manufacturing an optical waveguide device according to claim 9, wherein the material forming the loading layer is niobium oxide.