JP2006030734A - Optical waveguide and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical waveguide capable of reducing a coupling loss between the optical waveguide and an optical fiber and minimizing a variation of coupling rate of a directional coupler in the surface of substrate, and to provide a manufacturing method of the optical waveguide. <P>SOLUTION: The optical waveguide is manufactured by forming an underclad 12 on a substrate 11, forming a core layer on the underclad 12, forming an upper part core 13a of rectangular cross-section and a lower part core 13b made of flat plate part of prescribed thickness D by making an etching depth of the core layer less than the thickness of the core 13 when forming the core 13 from the core layer by means of photolithography and etching and forming an overclad 14 which embeds the core 13 by sintering soot deposited by flame hydrolysis method. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical waveguide used in a planar lightwave circuit that performs optical communication and optical signal processing, and a method for manufacturing the same.

  In recent years, planar lightwave circuits using optical waveguides are actively used in the fields of optical communication, optical signal processing, and the like. In planar lightwave circuits, elemental circuits such as directional couplers composed of optical waveguides, Mach-Zehnder interferometers or AWGs (Arrayed Waveguide Grating) are used alone or in combination to demultiplex, combine, or optical path Functions such as switching are realized.

JP 2001-56415 A JP 2003-35833 A

  However, a planar lightwave circuit using an optical waveguide has the following two problems as characteristics.

One is a problem of loss (connection loss) generated at the connection between the planar lightwave circuit and the optical fiber.
The cause of the connection loss is that the mode field of light is different between the optical fiber and the optical waveguide. For example, when connecting a silica-based optical waveguide and a silica optical fiber, the core of the optical waveguide is generally a few μm square or a rectangle close to this, whereas an optical fiber (here, a single mode optical fiber is used). The core dimension (for example) is about 10 μmφ. Corresponding to this difference in shape, the mode field diameter of light also differs, and due to this mismatch, light leaks at the connection portion between the optical fiber and the optical waveguide, resulting in connection loss.

  In order to reduce the connection loss, the mode field in the optical waveguide may be enlarged and brought closer to the mode field of the optical fiber. From this point of view, the connection loss is reduced by increasing the thickness of the core only at the connection point with the optical fiber, and bringing the size of the mode field in the optical waveguide closer to that in the optical fiber (hereinafter referred to as “the connection loss”). (Referred to as a vertical and horizontal taper method) (see Patent Document 1). However, this vertical / horizontal taper method increases the thickness of the core at the connection with the optical fiber, so it takes time to deposit and etch the core, and the process is complicated because a shadow mask is used during core deposition or etching. There's a problem.

  As an improved method of the vertical and horizontal taper methods, there has been proposed a method of shortening the etching time by etching the core of the connection part only halfway in the thickness direction (see Patent Document 2). In this improved method, it is stated that even if the core is etched only halfway, if the etching depth is set appropriately, a reduction effect almost equal to that obtained when completely etched can be obtained. However, even in this improved method, the process using the shadow mask cannot be omitted, so the complexity of the process remains essentially unchanged.

  Another problem is a variation in the coupling rate of the directional coupler in the substrate (wafer) plane. This problem is caused by the fact that the core collapses during the production of the optical waveguide, as will be described below.

  The basic structure of the optical waveguide will be described with reference to FIG. 7A. As the structure of the optical waveguide formed on the substrate 21, the core 23 has a rectangular shape and the periphery of the core 23 is completely formed. In many cases, a so-called channel-type structure surrounded by the clad (underclad 22 and overclad 24) is used. This is because this structure is simple both in terms of designing a planar lightwave circuit and in producing an optical waveguide.

  Normally, in manufacturing an optical waveguide having a channel structure, when etching the core 23, the etching depth is slightly larger than the thickness of the core 23 so that the etching cross section is vertical and the core 23 does not remain in the etched portion. To do. For example, when the thickness of the core 23 is 6 μm, the etching depth is set to about 7 μm (that is, the overetching amount is 1 μm). At this time, a convex underclad 22 a is left below the core 23. When embedding the core 23 with the overclad 24, a method of depositing the overclad 24 by a flame deposition method and sintering it is often used. At this time, if the underclad 22 and 22a are not deformed, the core 23 does not collapse as shown in FIG.

  However, in reality, when the over clad 24 is sintered, the under clads 22 and 22a are deformed, and the core 23 may collapse. More specifically, the overclad 24 deposited by the flame deposition method is in the form of soot (fine powder), and a transparent glass film is formed by sintering this at a high temperature. In the heating process at the time of sintering, the soot of the overclad 24 softens once, fills the surface irregularities formed by etching, and becomes a transparent glass film through a subsequent cooling process to room temperature. When the over clad 24 is sintered, it is necessary to raise the temperature to 1000 ° C. or higher. Due to this high temperature, the under clads 22 and 22a may also be deformed.

  For example, the directional coupler has a structure in which two cores 23 are arranged close to each other at a distance of about 1 to 3 μm, but the underclad 22 (particularly the underclad 22b) is deformed during sintering. Then, the core 23 disposed on the deformed underclad 22b falls down, and in this state, the overclad 24 flows between the two adjacent cores 23, so that the core 23 falls down. An optical waveguide is formed (see FIG. 7B). When the core 23 collapses, the distance (gap) between the cores 23 of the directional coupler changes. In addition, since the degree of collapse of the core 23 is not necessarily uniform within the wafer surface, the change in the gap is not uniform within the wafer surface, and the coupling rate of the directional coupler varies within the wafer surface. It becomes. Among planar lightwave circuits, filter circuits such as interleavers and optical switches determine the circuit characteristics based on the light coupling rate in the directional coupler, and variations in coupling rates increase the yield of these planar lightwave circuits. Making it difficult.

  Although a chemical vapor deposition method or the like is also used as a method for depositing the overclad 24, it is difficult to bury the processed core 23 without any gaps. The embedded surface reflects the unevenness of the core 23. There are problems such as non-flatness, and the flame deposition method is more practical in practice. Therefore, it is desired to solve the above problems even if a flame deposition method is used as the deposition method of the overclad 24.

The present invention has been made in view of the above problems, and an object thereof is to provide an optical waveguide that realizes the following two points and a method for manufacturing the same, in order to solve the above problems.
1) The connection loss between the optical fiber and the optical waveguide is reduced.
2) To reduce the in-plane variation in the coupling rate of the directional coupler.

An optical waveguide according to claim 1 of the present invention for solving the above-described problems is
A lower cladding layer formed on the substrate;
A core formed of a lower core formed of a flat plate portion of a predetermined thickness and an upper core of a rectangular cross section projecting from the lower core, formed on the lower cladding layer;
And an upper clad layer formed around the core.
In other words, the core composed of the lower core and the upper core is integrally formed so as to have an inverted T shape, and the lower core itself of the flat plate portion has the remaining thickness of the core.

The optical waveguide according to claim 2 of the present invention for solving the above-described problems is
A lower cladding layer formed on the substrate;
A core composed of a laminate of two or more layers formed on the lower clad layer, wherein the layer in contact with the lower clad layer has a flat plate portion having a predetermined thickness, and has a rectangular cross section protruding from the flat plate portion When,
And an upper clad layer formed around the core.
That is, the laminated core is formed in an inverted T shape, and the flat plate portion of the lowermost layer (the layer in contact with the lower cladding layer) has a remaining thickness of the core.

An optical waveguide according to claim 3 of the present invention for solving the above-mentioned problems is
In the optical waveguide,
When the core is formed of a single layer, the thickness of the flat plate portion is set to 17% or less of the thickness of the core.
In other words, by making the flat plate portion under the core of the single-layer core structure 17% or less of the thickness of the core, the mode field is not excessively spread to the flat plate portion so that the connection loss does not increase. can do.

An optical waveguide according to claim 4 of the present invention for solving the above-described problems is
In the optical waveguide,
The thickness of the flat plate portion is 30% or less of the thickness of the core.
In other words, by making the flat plate portion under the core of the laminated core structure 30% or less of the thickness of the core, the mode field is not excessively spread to the flat plate portion so that the connection loss does not increase. be able to.

An optical waveguide according to claim 5 of the present invention for solving the above-described problems is
In the optical waveguide,
The thickness of the flat plate portion is at least 5% of the thickness of the core.
That is, the collapse of the core can be suppressed by providing a flat plate portion having a thickness of 5% or more of the core thickness at the lower portion of the core.

An optical waveguide according to claim 6 of the present invention for solving the above-described problems is
In the optical waveguide,
The core is made of a material whose deformation is induced (hereinafter referred to as a deformation temperature) higher than a sintering temperature of the upper clad layer.
That is, since the core having a high deformation temperature is hardly deformed when the upper clad layer is sintered, deformation of the core itself can be suppressed. In consideration of the collapse of the core, it is sufficient that at least the flat plate portion of the core has a high deformation temperature.

An optical waveguide according to claim 7 of the present invention for solving the above-described problems is
In the optical waveguide,
The flat plate portion and its vicinity are made of a material having a smaller refractive index than other portions of the core.
In particular, when it is desired to increase the flat plate portion (remaining thickness of the core), a configuration in which the refractive index of the flat plate portion is smaller than that of other portions is preferable. For example, in the case of a laminated core structure, the refractive index of the flat plate portion and its vicinity is reduced by changing the concentration of the additive material (dopant) of the layer to be the flat plate portion.

An optical waveguide according to an eighth aspect of the present invention for solving the above-described problems is
In the optical waveguide,
The flat plate portion is made of a material having a higher refractive index than the lower clad layer and the upper clad layer.

An optical waveguide according to a ninth aspect of the present invention for solving the above-described problems is
In the optical waveguide,
The flat plate portion is disposed on the entire surface of the substrate.
By disposing the flat plate portion, which is the remaining thickness of the core, on the entire surface of the substrate, the process for manufacturing the optical waveguide can be simplified.

An optical waveguide according to claim 10 of the present invention that solves the above problems is
In the optical waveguide,
The lower clad layer, the upper clad layer, and the core are made of a quartz-based material.

An optical waveguide according to an eleventh aspect of the present invention that solves the above problems is as follows.
In the optical waveguide,
When the relative refractive index difference between the core having the highest refractive index and the lower cladding layer and the upper cladding layer is 0.75% and the core thickness is 6 μm, the thickness of the flat plate portion Is 0.3 μm or more and 1.0 μm or less in the case of a single-layer core, or 1.8 μm or less in the case of a multi-layer core.

The method for manufacturing an optical waveguide according to claim 12 of the present invention for solving the above-described problems is as follows.
Forming a lower cladding layer on the substrate;
Forming a core layer composed of one or more layers on the lower cladding layer; and
Forming a core from the core layer by photolithography and etching;
A step of forming an upper cladding layer for embedding the core by sintering soot deposited by a flame deposition method,
When the core layer is processed by etching, the etching depth of the core layer is made smaller than the thickness of the core layer, and a flat plate portion is left under the core.

The method for manufacturing an optical waveguide according to claim 13 of the present invention for solving the above-described problems is as follows.
In the method of manufacturing the optical waveguide,
When the core is composed of a single layer, the thickness of the flat plate portion is set to 17% or less of the thickness of the core.

A method for manufacturing an optical waveguide according to claim 14 of the present invention for solving the above-described problems is as follows.
In the method of manufacturing the optical waveguide,
When the core is composed of a plurality of layers, the thickness of the flat plate portion is 30% or less of the thickness of the core.

The method for manufacturing an optical waveguide according to claim 15 of the present invention for solving the above-described problems is as follows.
In the method of manufacturing the optical waveguide,
The thickness of the flat plate portion is at least 5% of the thickness of the core.

The method for manufacturing an optical waveguide according to claim 16 of the present invention for solving the above-described problems is as follows.
In the method of manufacturing the optical waveguide,
The temperature at which the deformation of the core is induced is higher than the sintering temperature of the upper clad layer.

The method for manufacturing an optical waveguide according to claim 17 of the present invention for solving the above-described problems is as follows.
In the method of manufacturing the optical waveguide,
The flat plate portion and its vicinity are made of a material having a smaller refractive index than other portions of the core.

An optical waveguide manufacturing method according to an eighteenth aspect of the present invention that solves the above problems is as follows.
In the method of manufacturing the optical waveguide,
The flat plate portion is made of a material having a higher refractive index than the lower clad layer and the upper clad layer.

The method for manufacturing an optical waveguide according to claim 19 of the present invention for solving the above-described problem is as follows.
In the method of manufacturing the optical waveguide,
The lower clad layer, the upper clad layer, and the core are made of a quartz-based material.

The method for manufacturing an optical waveguide according to claim 20 of the present invention for solving the above-described problem is as follows.
In the method of manufacturing the optical waveguide,
When the relative refractive index difference between the core having the highest refractive index and the lower cladding layer and the upper cladding layer is 0.75% and the core thickness is 6 μm, the thickness of the flat plate portion Is 0.3 μm or more and 1.0 μm or less in the case of a single-layer core, or 1.8 μm or less in the case of a multi-layer core.

The reason why the above problems can be solved by the present invention will be described below.
In the following description, when the core has a laminated structure of a plurality of glass layers, the thickness of the core means the sum of the thicknesses of the glass layers.

First, the reason why the connection loss between the optical fiber and the optical waveguide can be reduced will be described.
When the etching depth of the core is made smaller than the thickness of the core, the core remains in a slab (flat plate) shape even at the etched portion. Since light is also transmitted to the slab-like core (flat plate portion), the mode field is widened and the connection loss with the optical fiber is reduced. However, if the thickness of the slab-like core (the remaining thickness of the core) exceeds 30% of the core thickness, the light confinement in the horizontal direction becomes weak, and the mode field spreads too much to the slab-like core portion. On the other hand, connection loss increases. Therefore, if a slab-like core is formed under the core and the thickness of the slab-like core is set appropriately, the connection loss with the optical fiber can be reduced.

Next, the reason why the in-plane variation of the coupling rate of the directional coupler can be reduced will be described.
In the present invention, the core remains in a slab shape even in the etched portion, and the deformation temperature of the core itself is higher than the sintering temperature of the over clad (upper clad layer). It does not deform even at high temperatures. That is, the core itself is not easily affected by the deformation of the underclad (lower clad layer) below the core, and therefore the core is less likely to collapse. As a result, the gap between the cores is properly maintained, and the variation in the wafer surface of the coupling ratio of the directional coupler can be reduced. However, when the thickness of the slab-like core (the remaining thickness of the core) is less than 5% of the thickness of the core, the remaining thickness of the core is too small, and the effect of suppressing the collapse of the core cannot be obtained. From the viewpoint of stabilizing the coupling ratio of the directional coupler, it is desirable that the remaining thickness of the core is large. However, if the remaining thickness of the core exceeds 30% of the thickness of the core, the optical Since the loss in the waveguide is increased, the remaining thickness of the core needs to be 30% or less of the thickness of the core.

  In addition, when the core has a laminated structure of a plurality of glass layers, if the deformation temperature of all of the glass layers is higher than the sintering temperature of the over clad, the effect of suppressing the collapse of the core is obtained. can get. For example, when the overcladding sintering temperature is 1200 ° C., a material having a deformation temperature of 1300 ° C. or higher, preferably 1400 ° C. or higher is used as the glass layer for forming the core. Thus, the most practical method for preventing the collapse of the core is to use a core having a deformation temperature higher than the sintering temperature of the overclad. In addition, when making a core into a laminated structure, it is good to make the layer with the smallest refractive index into the lowest layer of a core from the viewpoint of optical confinement.

  In addition, as a method for preventing the collapse of the core, the following two methods are conceivable in addition to the method of using the core having a deformation temperature higher than the sintering temperature of the over clad described above.

The first method is to use a film having a deformation temperature higher than the sintering temperature of the over cladding not only for the core but also for the under cladding.
As a method for producing a glass film having a deformation temperature higher than the sintering temperature of the over clad produced by the flame deposition method, for example, there are a thermal oxidation method and a chemical vapor deposition method. However, the thickness of the glass film that can be produced by these methods is about 10 to 20 μm at the maximum, and the production time becomes long when a thick film is produced. For example, in the case of a silica-based optical waveguide, the required underclad thickness varies depending on the type of circuit and the relative refractive index difference Δ, but 15 to 20 μm for an AWG, a circuit that uses the thermo-optic effect like an optical switch In this case, 35 μm or more is necessary to suppress the power consumption of the thin film heater for inducing the thermo-optic effect. Therefore, the method of increasing the deformation temperature of the underclad is suitable when the underclad may be thin like AWG.

The second method is to lower the overcladding fabrication temperature below the undercladding deformation temperature.
Unlike the flame deposition method, the chemical vapor deposition method does not go through the process of soot deposition and subsequent sintering, so that a glass film can be deposited at a heating temperature of about several hundred degrees Celsius. Moreover, the deformation temperature of the deposited glass film is high. However, in the chemical vapor deposition method, in general, it is difficult to flatten the surface on which the unevenness is formed without gaps. For example, a special mechanical method such as CMP (Chemical Mechanical Polish) for planarization is used. A process is required. Note that the thermal oxidation method is a method of forming an oxide film by oxidizing the surface of silicon, and thus cannot be applied to the production of an over clad.

  According to the present invention, since the slab-like core (flat plate portion) is provided under the core of the optical waveguide, the connection loss with the optical fiber can be reduced, and the variation of the coupling ratio of the directional coupler in the substrate plane can be reduced. The load on the manufacturing process can be reduced.

  The present invention relates to an optical waveguide and a method for manufacturing the same, and by using a core having a convex (inverted T-shaped) structure, the mode field of the optical waveguide is brought close to the mode field of the optical fiber, so that By reducing the connection loss and suppressing the collapse during the formation of the overcladding, the variation in the coupling ratio of the directional coupler is reduced, thereby improving the yield.

Hereinafter, embodiments of the present invention will be described with reference to FIGS.
In the following examples, from the viewpoint of practicality, the case where both clads are produced by flame deposition and the core is produced by chemical vapor deposition will be described as an example, but the deformation temperature of the core is higher than the sintering temperature of the over clad. If it is higher, the method for producing each glass layer in the present invention is not limited to the above method in principle.

  FIG. 1 shows an example of an embodiment of an optical waveguide according to the present invention, and is a schematic diagram showing a cross section of the structure of the optical waveguide.

  As shown in FIG. 1, the optical waveguide of the present embodiment is composed of an underclad 12 formed on a silicon substrate 11, and a slab-like core (flat plate part) formed on the underclad 12 and having a predetermined thickness D. The core 13 includes a lower core 13 b and an upper core 13 a having a rectangular cross section projecting from the lower core 13 b, and an overcladding 14 formed around the core 13.

The manufacturing method of the optical waveguide shown in FIG. 1 will be described. First, an underclad 12 containing SiO ₂ as a main component is deposited on a silicon substrate 11 by a flame deposition method, and then transparent glass is formed in an electric furnace. Next, a core layer mainly composed of SiO ₂ doped with GeO ₂ as a dopant is deposited by chemical vapor deposition. Next, the core layer is patterned by photolithography and reactive ion etching to form a desired circuit pattern (core 13). Finally, an over clad 14 having the same refractive index as that of the under clad 12 is deposited by a flame deposition method, sintered, and an optical waveguide is fabricated so that the core 13 is embedded. Here, in the optical waveguide manufacturing method according to the present invention, when etching the core layer, the etching depth of the core layer is made smaller than the thickness of the core 13 (or the core layer), and the slab is formed below the core 13. It is characterized in that the lower core 13b is left. That is, in the core layer, the portion patterned by etching becomes the upper core 13a, the core layer remaining after the etching becomes the slab-like lower core 13b, and the thickness of the lower core 13b becomes D. In this embodiment, the core 13 including the lower core 13b has a single layer structure.

  Here, for comparison experiments, the thicknesses of the underclad 12, the core 13, and the overclad 14 are 40 μm, 6 μm, and 30 μm, respectively, and the etching depth of the core layer is 7.0 μm, 6.0 μm, 5.5 μm, The optical waveguide was formed by changing the thickness in five ways of 5.0 μm, 4.5 μm, and 4.0 μm. At this time, the remaining etching thickness D of the core 13 (thickness of the lower core 13b) is −1.0 μm, 0.0 μm, 0.5 μm, 1.0 μm, 1.5 μm, and 2.0 μm, respectively. As a matter of course, when D ≦ 0 μm, the lower core 13b does not actually exist. The width of the optical waveguide in the upper core 13a is 7 μm, the relative refractive index difference Δ between the core 13 and the clads 12 and 14 is 0.75%, and the refractive index of the core 13 including the lower core 13b is It was set to be larger than both clads 12 and 14.

  The dependence of the connection loss per connection point between the optical waveguide and the optical fiber on the D (the remaining thickness of the core) is shown by the solid line in FIG.

Here, when measuring the connection loss, TE polarized light having a wavelength of 1.55 μm was used as incident light.
As can be seen from FIG. 2, in the range of 0 μm <D ≦ 1.0 μm, the connection loss gradually decreases as D increases, and is about 0.4 dB when D = 0 μm. The loss is 0.3 dB or less. However, in the range of D> 1.0 μm, the connection loss increases rapidly as D increases, and is approximately 2.0 dB when D = 2.0 μm. This is because, as described above, in the range of 0 μm <D ≦ 1.0 μm, as D increases, the mode field of the core 13 of the optical waveguide increases and approaches the mode field in the optical fiber. This is because the mode field is too wide at D = 2.0 μm. From this result, it was shown that the connection loss can be reduced if the remaining thickness of the core is within 1.0 μm in this example.

  In this example, an AWG with 32 channels and a frequency interval of 100 GHz was produced using an optical waveguide produced under the same manufacturing method and under the same conditions as in Example 1, and incident light under the same conditions as in Example 1 was used. The D dependence of insertion loss (sum of connection loss and propagation loss) was investigated.

The solid line in FIG. 3 is a graph showing the dependency between the insertion loss of the AWG and the remaining thickness D of the core.
As shown in FIG. 3, the insertion loss gradually decreases as D increases in the range of 0 μm <D ≦ 1.0 μm as in the connection loss result of the first embodiment, whereas it rapidly increases at D = 2.0 μm. It was shown that the insertion loss can be reduced if the remaining core thickness is within 1.0 μm.

  In the first embodiment and the second embodiment, the loss is reduced simply by reducing the etching depth. Unlike the conventional techniques (for example, Patent Documents 1 and 2), an additional process is performed. Is not necessary at all. In other words, the core layer to be etched is not completely etched, and the core layer having a predetermined thickness (that is, the lower core 13b in FIG. 1) is left on the entire surface of the substrate, thereby reducing loss. Rather, since the core layer to be etched is not completely etched, the time required for etching can be shortened by reducing the etching depth, and the process load can be reduced.

  In this embodiment, the same manufacturing method as in Embodiments 1 and 2 is used, and the remaining thickness D of the core is changed from −1.0 μm to 2.0 μm for each wafer, and the same directional coupler pattern is formed on one surface. An optical waveguide constituting a directional coupler was manufactured using the arranged masks. The gap between the optical waveguides of the directional coupler is 2 μm. Also, TE polarized light having the same wavelength of 1.55 μm as in Examples 1 and 2 was used as incident light, and 100 directional couplers were measured for each wafer, and the in-wafer distribution of directional coupler coupling rates was measured. (Variation) was examined.

  Among the measurement results, FIG. 4A shows a histogram of the coupling rate distribution when D = −1.0 μm (no remaining core thickness) and FIG. 4B shows the coupling rate distribution when D = 1.0 μm. Further, with respect to the conditions of the remaining thickness D of the other cores, a similar histogram was created, and the relationship between the remaining thickness D of the core and the standard deviation of the coupling rate was examined and shown in FIG.

  From the comparison of the histograms of FIG. 4A and FIG. 4B, when D = 1.0 μm shown in FIG. 4B, the variation in the in-wafer distribution of the coupling ratio of the directional coupler is small. Recognize. Further, it can be seen from the graph of FIG. 5 that when the remaining thickness D of the core is 0 or more, the standard deviation is rapidly reduced, and when it is 0.3 μm or more, it is stable in a reduced state. That is, by leaving the core at the time of etching, the variation in the in-wafer distribution of the coupling ratio of the directional coupler can be reduced, and the effectiveness by leaving the core was recognized.

FIG. 6 shows another example of the embodiment of the optical waveguide according to the present invention, and is a schematic view of a cross section when the core of the optical waveguide has a two-layer structure.
In FIG. 6, the same components as those shown in FIG.

  As shown in FIG. 6, the optical waveguide of the present embodiment has a configuration substantially equivalent to that of the first embodiment shown in FIG. 1, but the core 13 is a single layer in the first embodiment. The present embodiment is different in that it has a laminated structure of two or more layers (two glass layers in this embodiment). Specifically, in the core 13, of the layers constituting the core 13, the lower layer core 13 d in contact with the underclad 12 has a flat plate portion having a predetermined thickness D, and a part of the lower layer core 13 d constituting the core 13 and the upper layer By forming the core 13c so as to protrude from the flat plate portion, a rectangular cross section is configured. In this embodiment, the flat plate portion is constituted by one lower core 13d, but this flat plate portion may also be constituted by a plurality of laminated layers.

The optical waveguide having the configuration shown in FIG. 6 was produced under the same manufacturing method and under the same conditions as in Examples 1 to 3, and the same examination as in Examples 1 to 3 was performed. In this example, both the upper and lower layer cores were fabricated by chemical vapor deposition. Further, the relative refractive index difference and the thickness of the lower core 13d were 0.45% and 2 μm, respectively, and the upper core 13c was 0.75% and 4 μm, respectively. Refractive index of the lower layer core 13d having a flat plate portion, by varying the concentration of the dopant GeO _2, configured to be smaller than the refractive index of the upper core 13c.

  Specifically, the optical waveguide is formed by changing the etching depth from the upper surface of the upper layer core 13c, the connection loss between the optical waveguide of this embodiment and the optical fiber, and the insertion of the AWG using the optical waveguide of this embodiment. Loss was measured. As a result, when the core 13 has a laminated structure, the loss reduction effect is achieved until the remaining thickness D of the core 13 is 1.8 μm, that is, the remaining thickness of the lower core 13d is 1.8 μm. It was seen (refer to the dotted line graphs in FIGS. 2 and 3).

  From the measurement results of Examples 1 to 3, when the core 13 has a single layer structure, the effective range of the remaining thickness D of the core is 17% or less of the thickness of the core 13 (when the core thickness is 6 μm, D = If it is equivalent to 1 μm or less), the effect of the present invention can be obtained. In addition, from the results of Example 4, it is possible to increase the allowable value of the remaining thickness of the core by forming the core 13 with a multilayer structure. Actually, in an optical waveguide having a multi-layered core 13, a wafer of a directional coupler having a core remaining thickness D of 1.8 μm and a directional coupler having a core remaining thickness D of −1.0 μm. When wafers were produced and the in-plane distribution of the coupling rate of each wafer's directional coupler was measured, the former had less variation in coupling rate, so in this example as well, by leaving the core, The effect of reducing the variation in the bonding rate distribution was observed.

  That is, from the results of Examples 1 to 4, when the core thickness is 6 μm and the relative refractive index difference Δ between the core and both clads is 0.75%, the remaining thickness of the single-layer core is 1.0 μm or less. In this case, the connection loss can be reduced when the remaining thickness of the multi-layer core is 1.8 μm or less, and the in-plane variation of the coupling ratio of the directional coupler can be reduced when the remaining thickness is 0.3 μm or more. It was. That is, when a single-layer core is used, if the ratio of the remaining core thickness to the total core thickness is in the range of 5% to 17%, the connection loss is reduced and the coupling of the directional coupler is reduced. It was shown that the in-plane variation of the rate can be reduced. Furthermore, when a multi-layer core is used, the ratio of the remaining core thickness to the total core thickness can be expanded to 30% or less.

  In Examples 1 to 4, the optical waveguide has a relative refractive index difference Δ = 0.75%. However, for the reason that the effect of the present invention can be obtained, the core remaining thickness is appropriately set. If set, it can be easily inferred that the present invention is effective for other optical waveguides of Δ.

  In the first to fourth embodiments, the silica-based optical waveguide is used as an example of the optical waveguide. However, the present invention is not limited to the silica-based optical waveguide, and can be applied to, for example, a polymer-based optical waveguide. Is.

An example of embodiment of the optical waveguide which concerns on this invention is shown, and it is a schematic diagram of the cross section of the structure. It is a graph which shows the dependence of the connection loss per connection point in the connection of an optical waveguide and an optical fiber, and the remaining thickness of a core. It is a graph which shows the dependence of the insertion loss of AWG, and the remaining thickness of a core. FIG. 4A is a graph showing the variation of the coupling ratio of the directional coupler in the wafer plane, and FIG. 4A shows the coupling ratio of the directional coupler by the conventional optical waveguide (core remaining thickness: −1.0 μm). FIG. 4B shows the variation in the coupling ratio of the directional coupler due to the optical waveguide according to the present invention (remaining core thickness: 1.0 μm). It is a graph which shows the relationship between the standard deviation of the coupling rate of a directional coupler, and the remaining thickness of a core. The other example of embodiment of the optical waveguide which concerns on this invention is shown, and it is a schematic diagram of the cross section of the structure. FIGS. 7A and 7B are diagrams for explaining the collapse of the core in the directional coupler. FIG. 7A is a cross-sectional view when no collapse occurs, and FIG. 7B is a cross-section when the collapse occurs. FIG.

Explanation of symbols

11 Substrate 12 Underclad 13 Core 14 Overclad

Claims (20)

A lower cladding layer formed on the substrate;
A core formed of a lower core formed of a flat plate portion of a predetermined thickness and an upper core of a rectangular cross section projecting from the lower core, formed on the lower cladding layer;
An optical waveguide comprising an upper clad layer formed around the core. A lower cladding layer formed on the substrate;
A core composed of a laminate of two or more layers formed on the lower clad layer, wherein the layer in contact with the lower clad layer has a flat plate portion having a predetermined thickness, and has a rectangular cross section protruding from the flat plate portion When,
An optical waveguide comprising an upper clad layer formed around the core. The optical waveguide according to claim 1,
When the core is composed of a single layer, the thickness of the flat plate portion is 17% or less of the thickness of the core. The optical waveguide according to claim 2, wherein
An optical waveguide characterized in that the thickness of the flat plate portion is 30% or less of the thickness of the core. The optical waveguide according to any one of claims 1 to 4,
An optical waveguide characterized in that the thickness of the flat plate portion is at least 5% of the thickness of the core. The optical waveguide according to any one of claims 1 to 5,
The optical waveguide according to claim 1, wherein the core is made of a material whose deformation is induced higher than a sintering temperature of the upper clad layer. The optical waveguide according to any one of claims 1 to 6,
The optical plate, wherein the flat plate portion and the vicinity thereof are made of a material having a smaller refractive index than other portions of the core. The optical waveguide according to any one of claims 1 to 7,
The optical waveguide, wherein the flat plate portion is made of a material having a higher refractive index than the lower clad layer and the upper clad layer. The optical waveguide according to any one of claims 1 to 8,
The optical waveguide, wherein the flat plate portion is disposed on the entire surface of the substrate. The optical waveguide according to any one of claims 1 to 9,
An optical waveguide, wherein the lower clad layer, the upper clad layer, and the core are made of a quartz material. The optical waveguide according to claim 10, wherein
When the relative refractive index difference between the core having the highest refractive index and the lower cladding layer and the upper cladding layer is 0.75% and the core thickness is 6 μm, the thickness of the flat plate portion Is 0.3 μm or more and 1.0 μm or less in the case of a single-layer core, or 1.8 μm or less in the case of a multi-layer core. Forming a lower cladding layer on the substrate;
Forming a core layer composed of one or more layers on the lower cladding layer; and
Forming a core from the core layer by photolithography and etching;
A step of forming an upper cladding layer for embedding the core by sintering soot deposited by a flame deposition method,
When processing the core layer by etching, the etching depth of the core layer is made smaller than the thickness of the core layer, and a flat plate portion is left under the core, and a method of manufacturing an optical waveguide . In the manufacturing method of the optical waveguide according to claim 12,
When the core is formed of a single layer, the thickness of the flat plate portion is set to 17% or less of the thickness of the core. In the manufacturing method of the optical waveguide according to claim 12,
When the said core consists of multiple layers, the thickness of the said flat plate part shall be 30% or less of the thickness of the said core, The manufacturing method of the optical waveguide characterized by the above-mentioned. In the manufacturing method of the optical waveguide in any one of Claims 12 thru / or 14,
The method of manufacturing an optical waveguide, wherein the thickness of the flat plate portion is at least 5% of the thickness of the core. In the manufacturing method of the optical waveguide in any one of Claims 12 thru / or 15,
The method for manufacturing an optical waveguide, wherein a temperature at which the deformation of the core is induced is higher than a sintering temperature of the upper clad layer. In the manufacturing method of the optical waveguide according to any one of claims 12 to 16,
The flat plate portion and the vicinity thereof are made of a material having a smaller refractive index than other portions of the core. In the manufacturing method of the optical waveguide in any one of Claims 12 thru / or 17,
The method of manufacturing an optical waveguide, wherein the flat plate portion is made of a material having a higher refractive index than the lower clad layer and the upper clad layer. In the manufacturing method of the optical waveguide according to any one of claims 12 to 18,
A method of manufacturing an optical waveguide, wherein the lower cladding layer, the upper cladding layer, and the core are made of a quartz-based material. In the manufacturing method of the optical waveguide according to claim 19,
When the relative refractive index difference between the core having the highest refractive index and the lower cladding layer and the upper cladding layer is 0.75% and the core thickness is 6 μm, the thickness of the flat plate portion Is 0.3 μm or more and 1.0 μm or less in the case of a single-layer core, or 1.8 μm or less in the case of a multi-layer core.

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