JP2005043497A - Optical waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide for improving productivity of the optical waveguide and attaining proper propagation mode refractive index and for improving the propagation loss. <P>SOLUTION: The refractive index of a second upper clad layer is set smaller than that of a first upper clad layer, constituting an upper clad layer covering a core of the optical waveguide. Furthermore, by setting the thickness of the second upper clad layer to be 1/3 or smaller of the thickness, from the upper face of the core to the upper face of the second upper clad layer, the optical waveguide of superior productivity and optical propagation characteristics can be provided. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to an optical waveguide mainly composed of a low-loss quartz-based material suitable for the fields of communication, measurement, and information processing.
[0002]
[Prior art]
In recent years, in optical communication systems, optical waveguides using quartz-based materials are attracting attention because they have low loss and high reliability in optical transmission, and complex circuits can be formed on a flat substrate at once. . These optical waveguides generally have a structure in which a light propagation region called a core having a higher refractive index than that of the lower cladding is formed on the lower cladding, and the core is covered with an upper cladding layer having a refractive index lower than that of the core. It is.
[0003]
Such an optical waveguide is formed by forming a lower clad film and then a core film by using chemical vapor deposition (CVD) or flame deposition (FHD), and patterning the core film. It is manufactured by leaving the core portion of the waveguide pattern in a convex shape and further forming an upper cladding layer (see Patent Document 1). Also disclosed is an optical waveguide that has a multilayer structure of two or more upper clad layers, which reduces polarization-dependent loss and excess loss, and has good filling characteristics in a narrow gap portion of the core, and a method for manufacturing the same. (See Patent Document 2).
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-337241
[Patent Document 2]
Japanese Patent Laid-Open No. 2000-155229
[Problems to be solved by the invention]
In general, light propagating through the optical waveguide spreads not only in the core but also in the upper and lower cladding regions. However, when this light spreads further from the upper and lower cladding regions and reaches the outside of the cladding region, the light is scattered or absorbed depending on the properties of the medium disposed there, so that the propagation characteristics of the optical waveguide Will be affected and deteriorate. Therefore, particularly in the upper clad region, an upper clad layer covering the core has to be deposited in an amount of 30 μm or more in order to avoid deterioration of light propagation characteristics due to this influence.
[0007]
On the other hand, when forming the upper clad layer, heat treatment is required after the film formation in order to remove stress accumulated during film formation or to soften the glass and embed it in the gap between the waveguides. . For this reason, when the upper clad layer is thick, there is a problem that this heat treatment time becomes long, and the film formation time itself becomes long, and the productivity is remarkably deteriorated.
[0008]
Although the above-described Patent Document 1 and Patent Document 2 disclose a configuration in which the upper clad layer has a multilayer structure, the refractive index and thickness of each layer of the upper clad layer composed of a plurality of layers are maintained in an appropriate relationship. Therefore, there is no suggestion of improving productivity and light propagation characteristics.
[0009]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical waveguide in which both the productivity and propagation characteristics of the optical waveguide are improved by maintaining an appropriate relationship between the refractive index and thickness of each layer constituting the upper cladding layer. There is to do.
[0010]
[Means for solving the problems]
The spread of light outside the upper cladding layer is determined by the refractive index distribution between the core and the upper cladding layer. However, if the refractive index difference between the core and the upper cladding layer is large, the confinement of light inside the core becomes stronger, and The spread of light is reduced. However, on the other hand, as the refractive index difference increases, the core size needs to be reduced in order to make the optical waveguide a single mode. If the size of the core is reduced, it becomes difficult to align the optical axis between the optical waveguide and the input / output optical fiber connecting the optical waveguide. Therefore, the refractive index difference between the core and the upper cladding layer must be kept within a certain range.
[0011]
According to a conventionally known technique, in order to set the optical waveguide to a single mode, it is necessary to maintain the refractive index difference between the core and the upper cladding layer in a range of about 0.3% to 1.5%. If the difference in refractive index becomes larger than this, the connection loss with the input / output optical fiber increases, making it difficult to put it to practical use. On the other hand, if the difference in refractive index is further reduced, the allowable radius of curvature of the optical waveguide is increased, leading to an increase in the size of the optical waveguide device.
[0012]
Therefore, in order to reduce the spread of light to the outside of the upper cladding layer while maintaining the single mode of the optical waveguide, among the layers constituting the upper cladding layer, the refractive index of the layer covering the core and the core What is necessary is just to make the refractive index of the layer of the site | part away from the core smaller than the refractive index of the layer which covers this core, keeping the refractive index difference with a refractive index in said range.
[0013]
Therefore, as in the first aspect of the invention, the refractive index of the second cladding layer covering the first upper cladding layer is made smaller than the refractive index of the first upper cladding layer covering the core. The spread of light to the outside of the cladding layer can be efficiently suppressed, and the propagation characteristics of the optical waveguide can be kept good. Further, by making the refractive index in such a relationship, the entire upper clad layer can be suppressed to an appropriate thickness, and productivity can be kept good.
[0014]
In addition to the above-described configuration, as in the invention of claim 2, the thickness of the second upper cladding layer is set to one third of the thickness from the upper surface of the core to the upper surface of the second upper cladding layer. By making the following, better productivity can be realized, and the effect of the present invention becomes more remarkable.
[0015]
As in the third aspect of the present invention, when the thickness from the upper surface of the core to the lower surface of the second upper cladding layer is 5 μm or more, the spread of light can be more effectively suppressed.
[0016]
Furthermore, the effect of the present invention is further improved by adding fluorine having a function of lowering the refractive index to the second upper cladding layer.
[0017]
The upper cladding layer may have a structure in which the refractive index decreases stepwise by continuously changing the dopant concentration during film formation. In this case, the upper cladding layer has a configuration in which there is no boundary between the layers.
[0018]
In the technology applied to the optical waveguide currently in practical use, light having wavelengths of 1.3 μm and 1.5 μm is usually used, and the present invention can deal with light of any of these wavelengths.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a cross-sectional view of an optical waveguide which is an example of an embodiment of the present invention. The optical waveguide 10 according to the example of this embodiment includes a substrate 100, a lower clad 101 provided on the substrate, a core 102 having a refractive index larger than that of the lower clad 101, and an upper clad layer 105 covering the core. .
[0020]
Quartz is used for the substrate 100, and a lower clad 101 made of quartz is formed on the upper surface by plasma CVD. Further, a core film is formed on the lower clad, and unnecessary portions of the core film are removed by dry etching to form the core 102.
[0021]
The core 102 is made of a material having a refractive index larger than that of the lower cladding. Specifically, it is possible to use SiO ₂ or doped with GeO _2, the SiO ₂ doped with TiO _2.
[0022]
Next, a first upper clad layer that is the first layer of the upper clad layer is formed so as to cover the core 102. The refractive index of the first upper cladding layer 103 can be controlled by adding B and P as dopants to the main component SiO ₂ . By controlling the refractive index in this way, the first upper clad layer 103 and the lower clad 101 can have substantially the same refractive index, and light can be efficiently confined in the core.
[0023]
At this time, B added to the SiO ₂ film has an effect of lowering the refractive index of the film, while P has an effect of raising the refractive index. And P can reduce the softening temperature or transparent vitrification temperature of glass. That is, by appropriately adjusting the dopant amounts of B and P, the softening temperature of the glass can be lowered at the same time as controlling the refractive index of the upper cladding layer.
[0024]
Next, a second upper cladding layer is formed so as to cover the first upper cladding layer. B and P are added so that the refractive index of the second upper cladding layer 104 is smaller than the refractive index of the first upper cladding layer 103. By adjusting the refractive index in this way, light can be efficiently confined in the core.
[0025]
When the upper clad layer is a multilayer, for example, when the upper clad layer is three layers, another layer may be deposited between the first upper clad layer and the second upper clad layer covering the core. However, the refractive index of this other layer must be smaller than the refractive index of the first upper cladding layer 103 and larger than the refractive index of the second upper cladding layer 104.
[0026]
Thus, the upper clad layer is composed of a plurality of layers containing SiO ₂ as a main component, and B, P, etc. are added to each layer for adjusting the refractive index. At this time, among the plurality of layers forming the upper cladding layer, the refractive index of the first upper cladding layer is configured to be higher than the refractive indexes of the other layers.
[0027]
In the present invention, the thickness from the upper surface of the core to the surface of the upper cladding layer is preferably 15 μm to 25 μm. If it is thinner than 15 μm, the coverage property covering the core and the effect of confining light in the core are also poor. If the upper clad layer is thicker than 25 μm, it takes time for the upper clad layer to be formed or heat-treated, thereby deteriorating the productivity of the optical waveguide. Further, the difference between the thermal expansion coefficient of the core and the thermal expansion coefficient of the first upper clad layer is preferably 0.5 × 10 ⁻⁶ or less, and if it is larger than this, the polarization dependence loss becomes large.
[0028]
In order to verify that the above-described configuration can achieve the object of the present invention, a structure in which the upper cladding layer of the two-dimensional slab waveguide is composed of two layers (FIG. 1) is used, and the spread of light outside the upper cladding layer is examined. It was.
[0029]
In FIG. 1, the thickness from the upper surface of the core to the upper surface of the first upper clad layer is d1, the thickness of the second upper clad layer is d2, and the first upper clad layer is always d1 + d2 = 16 μm. The thickness of the second upper cladding layer was changed, and the influence on the intrinsic waveguide mode was calculated.
[0030]
At this time, the refractive index of the second upper cladding layer out of the upper cladding layer was made 0.002 smaller than the refractive index of the first upper cladding layer.
[0031]
FIG. 2 shows the result of calculating the light amplitude distribution of the eigenwaveguide mode with respect to the TE mode by changing the values of d1 and d2. The calculation was performed as follows.
[0032]
The electric field distribution in the eigenmode was obtained by CFM (correlation function method), and the light amplitude distribution on the upper surface of the second upper cladding layer immediately above the core was expressed as a relative value. At this time, the thickness and width of the core were both 7 μm, and the refractive indexes of the core and the first upper cladding layer were 1.4517 and 1.4460, respectively.
[0033]
From this result, it can be seen that the light amplitude on the surface of the upper cladding layer decreases as the thickness of the second upper cladding layer d2 increases. This indicates that the light spreading beyond the upper cladding layer is reduced and is less susceptible to the material disposed on the upper cladding layer.
[0034]
FIG. 3 shows the calculation result of the change in the propagation mode refractive index when the thickness d1 from the upper surface of the core to the upper surface of the first upper cladding layer and the thickness d2 of the second upper cladding layer are changed. Show. For the sake of simplification, the calculation model is assumed to be a slab waveguide, and the propagation constant of the fundamental mode when the values of d1 and d2 are changed is obtained by a mode eigenvalue equation. At this time, the thickness and width of the core were both 7 μm, and the refractive indexes of the core and the first upper cladding layer were 1.4517 and 1.4460, respectively.
[0035]
L5 and L6 are curves with respect to the difference in refractive index between the core and the upper cladding layer of 0.4% and 0.3%, respectively. Waveguide when a layer having a refractive index smaller than that of the first upper clad layer is provided on the second upper clad layer on the basis of the propagation mode refractive index when the upper clad layer is composed of only one layer (d1 = 16 μm). The effect on the mode is shown.
[0036]
As can be seen from this result, when d1 is smaller than about 5 μm, the influence of the second upper cladding layer approaching the core increases, and the propagation state of light keeps the refractive index difference within a certain range. This is different from the case where the first upper clad layer and the second upper clad layer provided for this purpose are provided. Therefore, the thickness from the upper surface of the core to the lower surface of the second upper cladding layer must be at least 5 μm or more.
[0037]
From the above results, the upper cladding layer is composed of a plurality of layers including a first upper cladding layer having the same refractive index as the lower cladding and a second upper cladding layer having a lower refractive index. In this case, it is understood that the second upper cladding layer must be disposed in a region separated by 5 μm or more from the upper surface of the core. Thereby, the spread of light beyond the upper cladding layer can be suppressed. As a result, the influence of the material disposed on the upper cladding layer on the optical waveguide can be reduced, and the light propagation characteristics can be maintained well.
[0038]
That is, if the lower surface of the second upper cladding layer constituting the upper cladding layer is separated from the upper surface of the core by 5 μm or more, the light propagating through the core is not greatly affected by the presence of the second upper cladding layer. I understand that.
[0039]
The first upper clad layer 103 to the second upper clad layer 104 may be one upper clad layer 105 whose composition changes continuously. That is, in the upper cladding layer 105, a first upper cladding layer composed of SiO ₂ containing B and P and a _second cladding layer covering the first upper cladding layer are continuously formed, A configuration in which the boundary between the layers does not exist may be used. Such a configuration is possible even when another layer is provided between the first upper clad layer and the second upper clad layer. This can be achieved by changing the composition gradually by controlling the gas during film formation and appropriately adjusting the refractive index of each layer of the upper cladding layer.
[0040]
Next, a method for manufacturing an optical waveguide, which is an example of an embodiment of the present invention, will be described.
[0041]
First, as shown in FIG. 6A, a core film 102a is formed on a lower clad 101 provided on a substrate 100. The substrate may be made of quartz glass and the substrate itself may be a lower clad. Alternatively, the lower clad 101 may be formed by thermally oxidizing the substrate surface using a substrate such as silicon or the like, or by depositing SiO ₂ on the silicon substrate by CVD or flame deposition.
[0042]
Depositing of the core layer 102a is a material having a higher refractive index than the lower cladding, specifically, SiO ₂ or doped with GeO _2, the SiO ₂ doped with TiO ₂ CVD (chemical vapor deposition) or the like method Can be performed.
[0043]
Next, as shown in FIG. 6B, the core 102 is patterned by removing unnecessary portions of the core film 102a by dry etching.
[0044]
In order to stabilize the refractive index of the core 102, it is preferable to perform heat treatment after the patterning of the core 102. The heat treatment conditions may be a heat treatment temperature of 700 ° C. to 1,100 ° C. and an O ₂ atmosphere for about 2 to 24 hours.
[0045]
Next, the process for forming the upper cladding layer will be described. The upper cladding layer is preferably formed by plasma CVD. As shown in FIG. 6C, first, a first upper cladding layer 103 made of SiO ₂ containing B and P is formed on a core 102 having a rectangular cross section so as to cover the core.
[0046]
The formation of the first upper clad layer 103 by the plasma CVD method was performed by mixing TEMB (tetraethoxy orthosilicate), TMB (tetramethoxy borate), TMP (tetramethoxy phosphate), and O ₂ as a source gas. What is necessary is just to carry out using mixed gas. By reacting these source gases in plasma, an SiO ₂ film to which B and P are added can be formed.
[0047]
Next, as shown in FIG. 6D, a second upper cladding layer 104 is formed. Similar to the first upper clad, the second upper clad layer 104 is formed by a plasma CVD method using a mixed gas obtained by mixing TMB, TMP, and O ₂ with TEOS as a source gas. By reacting these source gases in plasma, an SiO ₂ film to which B and P are added can be formed. At this time, the composition may be controlled by adjusting the flow ratio of the mixed gas so that the refractive index of the second upper cladding layer is smaller than the refractive index of the first upper cladding layer.
[0048]
The formation of the second upper cladding layer can also be performed continuously with the formation of the first upper cladding layer. In that case, after forming the first upper cladding layer, the second upper cladding layer can be formed only by switching the source gas without opening the chamber of the film forming apparatus to the atmosphere. That is, the first upper cladding layer 103 to the second upper cladding layer 104 can be formed as one upper cladding layer 105 whose composition changes continuously. In this case, during the film forming process, the amount of TMB, TMP, and O ₂ mixed with TEOS is changed over time, and the refractive index of the SiO ₂ film to which B and P are added is related to the upper cladding layer. It is formed while adjusting the amount of dopant so as to satisfy.
[0049]
Further, in order to stabilize the refractive index of the first to second upper cladding layers, it is preferable to perform heat treatment after forming the upper cladding layer. The heat treatment may be performed for about 3 hours in a heat treatment temperature of 500 ° C. to 1100 ° C. in an O ₂ atmosphere. The heat treatment may be performed before the second upper cladding layer is formed, or may be performed after the second upper cladding layer is formed. That is, in order to stabilize the refractive indexes of the first and second upper cladding layers, heat treatment may be performed after each layer is formed, or may be performed after all layers are formed.
[0050]
The optical waveguide of the present invention is completed through the above steps.
[0051]
【Example】
Next, the present invention will be described in more detail based on examples.
A lower clad made of a 10 μm thick quartz film was formed on a 1 mm thick quartz substrate by a plasma CVD method. At this time, the refractive index of the lower cladding was 1.446, the refractive index of the core was 1.4517, and the difference was 0.0057. Similarly, quartz glass to which Ge was added was formed as a core film. At this time, the thickness of the core film was 7 μm. Next, unnecessary portions of the core film were removed by dry etching to form a core having a width of 7 μm.
[0052]
Thereafter, heat treatment was performed at 1100 ° C. in an O ₂ atmosphere for 3 hours in order to relieve internal stress of the film during film formation.
[0053]
Next, a first upper cladding layer was formed by plasma CVD so as to cover the core. The first upper cladding layer was made of BPSG (abbreviation of Boro-Phosphor Silicate Glass) containing 2 mol% B and 7 mol% P so that the refractive index was the same as that of the lower cladding.
[0054]
The first upper cladding layer is formed in about 3.5 hours with TEOS, TMB, TMP, and O ₂ flow rates of 16.0, 5.0, 0.5, and 680 sccm (standard cc / min), respectively. did.
[0055]
After film formation, heat treatment for softening reflow and stress relaxation was performed in an atmosphere of 1100 ° C. and O ₂ for 24 hours. Here, the softening reflow is a process of increasing the temperature to soften the clad and filling a gap between adjacent cores.
[0056]
The film thickness of the first upper clad layer was formed to be 10 μm in the upper part of the core after the heat treatment. By performing the heat treatment, the upper clad layer becomes thinner than the thickness immediately after the film formation. Therefore, in consideration of this, the film was formed thicker by about 2 μm.
[0057]
Thereafter, a second upper clad layer having a thickness of 6.0 μm made of quartz added with fluorine so as to cover the first upper clad layer was formed. After forming the second upper cladding layer, heat treatment was similarly performed at 500 ° C. in an O ₂ atmosphere for 2 hours. Since the second upper clad layer was also formed in consideration of the heat treatment step, the thickness after the heat treatment was 5.0 μm.
[0058]
The fluorine-added quartz film as the second upper cladding layer contains F in SiO ₂ so that the refractive index thereof is smaller than the refractive index of the first upper cladding layer. The second upper cladding layer was formed by plasma CVD in about 2 hours with the mixed gas flow rates of TEOS, CF ₄ , and O ₂ being 7, 10, and 193 sccm, respectively.
[0059]
When the first upper clad layer and the second upper clad layer were formed as one layer whose refractive index continuously changed, it was performed as follows.
[0060]
First, plasma CVD was performed by setting the mixed gas flow rates of TEOS, TMB, TMP, and O ₂ to 16.0, 5.0, 0.5, and 680 sccm, respectively. After film formation without changing the gas flow rate for about 2 hours, only TMP was gradually reduced over 4 hours, and during that time, film formation was continued without changing the flow rates of other gases. After 4 hours, the film formation was completed when the TMP flow rate became zero. At this time, the thickness of the upper clad layer on the upper portion of the core was about 21 μm. In addition, the film forming process described above was performed by adjusting only the flow rate of the mixed gas without opening the chamber of the plasma CVD apparatus.
[0061]
By gradually decreasing the TMP during film formation, the proportion of P, which has the effect of increasing the refractive index, in the upper cladding layer gradually decreases, so that the refractive index gradually decreases.
[0062]
In order to see the effect of light spreading outside the upper cladding layer, the refractive index (refractive index: Nf) of the first upper cladding layer is 1.446, and the refractive index of the second upper cladding layer is 1.444. Thus, the refractive index of each layer was adjusted. At this time, while keeping the thickness from the upper surface of the core to the surface of the second upper cladding layer constant at 15 μm, only the thickness of the second upper cladding layer is changed, and the change of the loss of the linear waveguide is changed. It was measured.
[0063]
The result is shown by L7 in FIG. The change in loss can be known by applying an absorbent resin on the second upper cladding layer and measuring the loss of the linear waveguide before and after the application.
[0064]
Similarly, the refractive index of the first upper cladding layer was 1.446, the refractive index of the second upper cladding layer was 1.441, and the change in the loss of the linear waveguide was examined. The result is shown as L8 in FIG.
[0065]
According to these results, it can be seen that in any case, the loss gradually decreases when the thickness of the second upper cladding layer is in the range of 0 to 5 μm. This indicates that if the refractive index of the second upper cladding layer is smaller than the refractive index of the first upper cladding layer, the spread of light can be suppressed.
[0066]
Further, according to the results shown in FIG. 4, when the thickness of the second upper cladding layer is the same, the refractive index of the second upper cladding layer is 1.441 in the range of 0 to 5 μm. It can be seen that the loss is smaller than that of .444. This indicates that the larger the difference between the refractive index of the first upper cladding layer and the refractive index of the second upper cladding layer, the more efficiently the spread of light outside the upper cladding layer can be suppressed. Yes.
[0067]
However, when the refractive index is 1.441, the loss gradually increases as the ratio of the second upper cladding layer to the entire upper cladding layer increases. This is because the low refractive index region is formed up to the vicinity of the core, and as a result, the refractive index difference between the core and the clad increases, resulting in a higher-order mode and increased loss. it is conceivable that.
[0068]
Therefore, the thickness of the second upper clad layer constituting the upper clad layer must be less than one third of the thickness from the upper surface of the core to the upper surface of the second upper clad layer.
[0069]
From the above, the distance from the upper surface of the core to the lower surface of the second upper cladding layer is 5 μm or more, and the thickness is one third or less of the thickness from the upper surface of the core to the upper surface of the second upper cladding layer. You can do it. By doing so, both the propagation mode refractive index and propagation loss, which are the propagation characteristics of the optical waveguide, can be achieved, and the upper cladding layer does not need to be thicker than necessary, thus realizing good productivity. it can.
[0070]
【The invention's effect】
The upper clad layer is composed of a plurality of layers, and the refractive index of the second upper clad layer constituting this layer is made smaller than the refractive index of the first upper clad layer. An optical waveguide having excellent propagation characteristics can be provided.
[Brief description of the drawings]
FIG. 1 is a sectional view of an optical waveguide showing the principle of the present invention.
FIG. 2 is a diagram showing a light amplitude distribution of an optical waveguide.
FIG. 3 is a graph showing a change in propagation mode refractive index of the optical waveguide of the present invention.
FIG. 4 is a diagram showing the propagation loss of the optical waveguide of the present invention.
FIG. 5 is a cross-sectional view showing an optical waveguide of the present invention.
FIG. 6 is a cross-sectional view showing a method for manufacturing an optical waveguide according to the present invention.
[Explanation of symbols]
10 optical waveguide 100 substrate 101 lower clad 102 core 102a core film 103 first upper clad layer 104 second upper clad layer 105 upper clad layer

Claims (5)

An optical waveguide having a lower clad, a core having a refractive index larger than that of the lower clad, and an upper clad layer covering the core,
The upper clad layer has a plurality of layers including at least a first upper clad layer covering the core and a second upper clad layer formed on the first upper clad layer, and the second upper clad layer An optical waveguide, wherein a refractive index of the cladding layer is smaller than a refractive index of the first upper cladding layer. 2. The light guide according to claim 1, wherein the thickness of the second upper cladding layer is equal to or less than one third of the thickness from the upper surface of the core to the upper surface of the second upper cladding layer. Waveguide. 3. The optical waveguide according to claim 1, wherein a thickness from an upper surface of the core to a lower surface of the second upper cladding layer is 5 μm or more. The optical waveguide according to any one of claims 1 to 3, wherein the second upper cladding layer contains fluorine. An optical waveguide having a lower clad, a core having a refractive index larger than that of the lower clad, and an upper clad layer covering the core,
The upper clad layer is composed of a plurality of layers, and the refractive index of the plurality of layers continuously changes.

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