JP2003057465A - Waveguide type optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a guide type optical circuit in which a deflection dependency is reduced or eliminated by reducing or eliminating birefringence. SOLUTION: A core structure is made into a multilayered structure formed by two or more kinds of layers having different refractive indexes. The birefringence of a waveguide can be decreased or eliminated when structural birefringence caused by the multilayered structure and the other birefringence among the birefringences of the waveguide cancel each other. For example, a silicon substrate is used for a substrate 501, a clad 502 and a core 503 are formed of silica glass, and the core 503 is made into the multilayered structure in which a plurality of first and second core layers 503a and 503b are alternately stacked.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical circuit which is one of the forms of optical parts used in optical communication systems and the like.

[0002]

2. Description of the Related Art With the worldwide spread of Internet use, there is an urgent need to construct a communication system capable of simultaneously transmitting a large amount of large-capacity data at high speed. As a system satisfying this demand, an optical communication system using an optical wavelength division multiplexing (WDM) technology has been attracting attention and has been introduced worldwide mainly in the United States.

In the optical WDM technology, an optical multiplexer / demultiplexer capable of multiplexing and demultiplexing a plurality of different wavelengths is indispensable. One of the modes of realization is a waveguide type in which an optical circuit is constituted by an optical waveguide on a substrate. .

A waveguide type optical circuit is an IC for light,
By applying LSI microfabrication technology and the like, optical waveguides are collectively formed on a flat substrate. Therefore, it is possible to realize a highly functional circuit having excellent integration and mass productivity and having a complicated circuit configuration. In recent years, research and development using various materials such as semiconductors, LiNbO ₃ , plastics, and silica-based glass has been progressing with increasing interest in optical communication systems. Among them, the silica-based optical waveguide formed of silica-based glass on the silicon substrate has good compatibility with the silica-based optical fiber, which is the transmission line of optical communication, and has high stability and long-term The reliability enables stable operation of the optical circuit. Also,
Since the rectangular core can be formed with good reproducibility, the theory and practice are in good agreement, and a high-performance circuit having a complicated optical circuit can be realized. Due to these characteristics, practical application is progressing in comparison with other waveguide materials.

A Mach-Zehnder interferometer and an arrayed-waveguide grating are basic configurations of an optical multiplexer / demultiplexer in a silica-based optical waveguide having such excellent characteristics. Various optical multiplexing / demultiplexing characteristics are realized by combining a Mach-Zehnder interferometer and an arrayed waveguide grating.

The Mach-Zehnder interferometer has a function of multiplexing / demultiplexing two different wavelengths or alternately distributing a plurality of periodically different wavelengths. FIG. 1 (a) shows the configuration of the Mach-Zehnder interferometer 101. It is composed of two optical couplers 102, two optical waveguides 103 that connect the two optical couplers 102, an input waveguide 104 and an output waveguide 105 that are respectively connected to different optical couplers 102. here,
The optical coupler is composed of a directional coupler in which two optical waveguides are close to each other. The optical waveguide is composed of a single mode waveguide. The distance between the wavelengths for multiplexing and demultiplexing is two optical waveguides 103
It is set by the waveguide length difference.

The arrayed waveguide grating has a function of simultaneously multiplexing and demultiplexing a plurality of different wavelengths. Compared to implementing similar functions with multiple Mach-Zehnder interferometers, miniaturization can be achieved. FIG. 1 (b) shows the configuration of the arrayed waveguide grating 111. Two slab waveguides 112 and two slab waveguides 112
A waveguide array 113 connecting the two, and an input waveguide 114 and an output waveguide 115 connected to different slab waveguides 112, respectively.
It is composed of The waveguide array 113, the input waveguide 114, and the output waveguide 115 are composed of a single mode waveguide. In the waveguide array 113, the waveguide lengths of adjacent optical waveguides are different, and the wavelength interval for optical multiplexing / demultiplexing is set by the difference in the waveguide length between the adjacent waveguides.

FIG. 2 shows a cross section of a silica-based optical waveguide forming these optical multiplexers / demultiplexers. The clad 202 formed on the substrate 201 has a structure covering the core 203, a silicon substrate, a quartz substrate, or the like is used for the substrate 101, and the clad 102 and the core 103 are made of quartz glass. The use of a silicon substrate as the substrate 201 is useful as a platform for hybrid mounting of light emitting and receiving elements and the like, and compressive stress acts on the clad 202 and the core 203, preventing cracks and cracks and improving reliability. I am letting you.

However, when silicon is used for the substrate, the optical characteristics of the Mach-Zehnder interferometer and the arrayed-waveguide grating formed by the silica-based optical waveguide show that the spectrum of the mode (TM mode) having an electric field perpendicular to the substrate is Compared with a mode having an electric field in the horizontal direction (TE mode), polarization dependence that shifts to the long wavelength side occurs. The transmission spectra of the Mach-Zehnder interferometer and the arrayed waveguide grating are shown in FIGS. 3 and 4. Figure 3 shows the transmission spectrum of a Mach-Zehnder interferometer with a wavelength separation / division wavelength of 0.8 nm.
The peak wavelength at which the loss becomes the lowest in the E mode and the TM mode is shifted by about 0.25 nm, and the loss in the other mode at the peak wavelength of each mode is increased by several dB or more. FIG. 4 is a transmission spectrum of an arrayed-waveguide grating having a demultiplexing / demultiplexing interval of 0.8 nm, and FIG. 4 (b) is an enlarged transmission spectrum near 1562 nm of FIG. 4 (a). Similar to the Mach-Zehnder interferometer, the peak wavelength at which the TE and TM modes have the lowest loss is shifted by about 0.25 nm, and the loss of the other mode at the peak wavelength of each mode is several dB.
It is getting bigger.

The polarization dependency in such an optical multiplexer / demultiplexer is that the transmission loss and crosstalk are time-varying because the polarization direction of the signal light transmitted by the optical fiber is indefinite and time-varying. Therefore, the reliability of the signal is deteriorated.

The cause of the polarization dependence of the optical multiplexer / demultiplexer is that the optical waveguide forming the circuit is larger than the effective refractive index felt by the TE mode as compared with the effective refractive index felt by the TE mode. This is because it has refraction. In the Mach-Zehnder interferometer, polarization dependence occurs due to the waveguide birefringence in the two optical waveguides 103 and in the arrayed waveguide grating in the waveguide array 113.

The waveguide birefringence of a quartz optical waveguide is 2 × 10 in the case of a single mode waveguide, where the effective refractive index of the TM mode is subtracted from the effective refractive index of the TE mode.
The value is from ^{-4 to} 3 x ^10-4 . The cause of such waveguide birefringence is that compressive stress acts on the optical waveguide, and the compressive stress is due to the residual thermal expansion due to the difference in thermal expansion coefficient between silicon, which is the substrate, and silica glass, which is the material of the optical waveguide. It is stress. In addition, since the coefficient of thermal expansion differs depending on the type and concentration of the dopant in the silica-based glass, even if the material system is of the same type,
Waveguide birefringence is different.

Eliminating the polarization dependence of the optical multiplexer / demultiplexer is an essential issue for practical use. As a method of eliminating it, (1) forming grooves on both sides of the waveguide A method for reducing such stress and reducing waveguide birefringence, (2) after forming a stress-imparting film such as a-Si on the waveguide, trimming the stress-imparting film while monitoring the optical circuit characteristics. A method of controlling the refraction to reduce the polarization dependence of the entire optical circuit. (3) Controlling the birefringence by irradiating the waveguide with ultraviolet light while monitoring the optical circuit characteristics, and controlling the polarization dependence of the entire optical circuit. Method, (4) Inserting a half-wave plate in the optical circuit and changing the polarization mode to reduce the polarization dependence of the entire optical circuit, (5) The cladding that covers the core Ge
A method of reducing birefringence by doping a material such as O ₂ , B ₂ O ₃ and P ₂ O ₅ which has a large coefficient of thermal expansion to bring it closer to the coefficient of thermal expansion of the substrate has been developed.・
The wavelength shift of the Zender interferometer or arrayed waveguide grating is set to 0.
It has been reduced to the order of 01 nm. These techniques act as a birefringence compensator for guided wave optical circuits.

[0014]

A waveguide type optical circuit in which the polarization dependence is eliminated has been realized by these methods.
There were the following problems in practical application.

In the method (1), since the birefringence strongly depends on the position and depth of the groove, it is sensitive to the manufacturing precision, the yield tends to be lowered, and the number of steps is increased. Also, (2)
In this method, the manufacturing accuracy is low, but since the trimming is performed while individually monitoring the characteristics of the optical circuit, mass productivity is poor. Also, the method (3) is also poor in mass productivity because it is necessary to adjust the optical circuit while individually monitoring the characteristics. In addition, in the method of (4), since the waveguide having a thickness of more than a few tens of microns to a few tens of microns of the wave plate is cut,
Loss increases. Further, in the method (5), when the stress applied to the clad becomes tensile stress, the clad glass is likely to be cracked, and due to the high concentration of the dopant, the weather resistance is deteriorated and the reliability is deteriorated.

An object of the present invention is to provide a waveguide type optical circuit which solves these problems of yield, mass productivity, optical characteristics and reliability, and reduces or eliminates birefringence to reduce or eliminate polarization dependence. To do.

[0017]

In order to achieve the above-mentioned object, a waveguide type optical circuit according to the present invention is a waveguide type optical circuit provided with a waveguide having a clad and a core. At least a part of the core of the waveguide has a multi-layer structure formed by a plurality of types of layers having different refractive indexes.

Here, of the birefringence of the waveguide, the value of the structural birefringence caused by the multilayer structure and the value of the other birefringence may have opposite signs. .

The waveguide type optical circuit includes a substrate,
The waveguide may be formed on the substrate, and the multilayer structure may be a multilayer structure in a direction substantially perpendicular to the surface of the substrate.

The waveguide type optical circuit includes a substrate,
The waveguide may be formed on the substrate, and the multilayer structure may have a multilayer structure in a direction substantially parallel to the surface of the substrate.

Here, the waveguide type optical circuit includes a substrate,
The substrate may be a silicon substrate or a quartz substrate, and the waveguide may be made of quartz glass.

Here, the waveguide includes a single mode waveguide, and at least a part of the core of the single mode waveguide has a multi-layer structure formed by a plurality of types of layers having different refractive indexes. It can be

Here, the waveguide includes a multimode waveguide, and at least a part of the core of the multimode waveguide has a multi-layer structure formed by a plurality of types of layers having different refractive indexes. Can be one.

Here, the waveguide includes a single mode waveguide and a multimode waveguide, and at least a part of the core of the single mode waveguide is formed by a plurality of types of layers having different refractive indexes. The multi-mode waveguide may have a multi-layer structure, and at least a part of the cores of the multi-mode waveguide may have a multi-layer structure formed by a plurality of types of layers having different refractive indexes.

Here, the structural birefringence value generated by the multilayer structure of the single mode waveguide and the structural birefringence value generated by the multilayer structure of the multimode waveguide can be different from each other.

Here, the structural birefringence value generated by the multi-layer structure of the multimode waveguide may be larger than the structural birefringence value generated by the multi-layer structure of the single mode waveguide.

Here, the refractive index and the thickness of the layers forming the multi-layer structure may be approximately symmetrical with respect to the central layer.

Here, the multilayer structure may be formed by arranging at least two types of layers having different refractive indexes substantially alternately.

Here, the refractive index of the layer forming the multi-layer structure may be increased from the layers at both ends on the clad side toward the core inner side layer.

Here, the total number of layers in the multi-layer structure may be three or more.

Here, the total number of layers of the multilayer structure is 5 to
It can be approximately 10 layers.

Here, the waveguide type optical circuit comprises an optical interference type circuit having two optical couplers and a plurality of waveguides connecting the two optical couplers and having different lengths. Regarding the waveguide having the shortest waveguide length among the waveguides, the length of the core having a multilayer structure is L, and each of the waveguides other than the waveguide having the shortest waveguide length is The length of the core having a multi-layer structure can be a length obtained by adding L to the difference between the length of the waveguide and the shortest waveguide length.

Here, the waveguide type optical circuit comprises a Mach-Zehnder interferometer having two optical couplers and two single mode waveguides connecting the two optical couplers. can do.

Here, the waveguide type optical circuit includes two slab waveguides and a waveguide array which connects the two slab waveguides and is composed of a plurality of single mode waveguides having different lengths. Arrayed waveguide having an input waveguide formed of a single mode waveguide connected to one of the two slab waveguides and an output waveguide formed of a single mode waveguide connected to the other of the two slab waveguides It may be provided with a diffraction grating.

Here, the waveguide type optical circuit may include a birefringence compensator.

Here, the birefringence compensator may be a birefringence compensator using a λ / 2 wavelength plate.

The difference between the present invention and the conventional one is that the structure of the core is
That is, a multilayer structure is formed by a plurality of types of layers having different refractive indexes. And, of the birefringence of the waveguide,
By making the structural birefringence caused by the multilayer structure and the other birefringence cancel each other, the birefringence of the waveguide can be reduced or eliminated. That is, the birefringence of the waveguide can be reduced or eliminated by making the positive and negative signs of the values of the structural birefringence caused by the multilayer structure and the values of the other birefringence opposite to each other. Here, the value of the structural birefringence caused by the multilayer structure,
By setting the magnitudes (absolute values) of the other birefringence values substantially the same and inverting the positive and negative signs, the birefringence value of the waveguide can be brought close to zero.

When the core has a multi-layer structure of layers having different refractive indexes, structural birefringence in which the effective refractive index is increased in the direction parallel to the layers is generated. The magnitude of structural birefringence is the refractive index of each layer,
And the thickness. Therefore, a layer is formed in a direction that reduces the waveguide birefringence caused by residual thermal stress, and the magnitude of structural birefringence │B _s │ caused by the multilayer structure is determined by the waveguide birefringence caused by residual thermal stress etc. Is almost equal to | B ₀ |, the birefringence of the waveguide can be eliminated and the polarization dependence of the waveguide type optical circuit can be reduced. At least │
If B _s │ <2│B ₀ │, the birefringence can be reduced as compared with the conventional case. At this time, it is not necessary that all the cores of the optical waveguides of the waveguide type optical circuit have a multi-layer structure, and the optical waveguide portion that affects the polarization dependence of the waveguide type optical circuit may be multilayered.

When silica glass is used as the optical waveguide material and silicon is used as the substrate, a compressive stress acts on the optical waveguide, so that the effective refractive index increases with respect to the electric field (TM mode) in the direction perpendicular to the substrate. Therefore, by forming the multilayer structure in the horizontal direction on the substrate, the stress birefringence due to the compressive stress and the structural birefringence due to the multilayer structure cancel each other and the birefringence of the optical waveguide can be reduced. In addition, when the substrate is made of quartz, the effective refractive index of the optical waveguide becomes larger with respect to the electric field (TE mode) in the horizontal direction to the substrate. It can be reduced.

The birefringence of the waveguide can be reduced by forming the single mode waveguide into a multi-layer structure of layers having different refractive indexes. For example, as described above, the main polarization dependence of the arrayed waveguide grating is due to the waveguide birefringence of the waveguide array composed of single mode waveguides, and each waveguide of the waveguide array is multi-layered. With the structure, the birefringence of the waveguide can be reduced and the polarization dependence can be reduced. In addition, each waveguide of the waveguide array is formed into a multilayer core with a length of a similar ratio, that is, by forming an appropriate part of the optical waveguide portion that affects the polarization dependence into a multilayer core, Polarization dependence can be reduced compared to the conventional case.

Further, the birefringence of the waveguide can be reduced by forming the multimode waveguide into a multi-layer structure of layers having different refractive indexes. For example, in a multimode interference optical coupler (hereinafter referred to as “MMI”) that uses a multimode waveguide having a width several times that of a single mode waveguide as an optical coupler, excess loss has polarization dependence, By using this as a multi-layer structure, the polarization dependence of excess loss can be reduced.

Further, not only the individual cores of the single mode waveguide and the multimode waveguide have a multi-layer structure, but also the cores of both waveguides have a multi-layer structure, so that the waveguide is compared with either one. The polarization dependence of the optical circuit can be reduced. For example,
In the Mach-Zehnder interferometer using the MMI as an optical coupler, 2
Two single-mode waveguides that connect two MMIs and a multimode waveguide of two MMIs may have a multilayer structure.

The waveguide birefringence value may differ depending on the size and shape of the optical waveguide. In that case, the polarization dependence of the waveguide type optical circuit can be appropriately reduced by applying a multilayer structure having an appropriate structural birefringence value to each of the single mode waveguide and the multimode waveguide. In a silica-based optical waveguide, the waveguide birefringence tends to increase as the waveguide width increases. Therefore, the multilayer structure may be formed so that the structural birefringence value of the multimode waveguide is larger than that of the single mode waveguide.

Regarding the multi-layer structure, the center of the electromagnetic field distribution of propagating light is set to be different from the conventional one by making the refractive index and the thickness of each layer of the multi-layer structure of the core approximately symmetrical with respect to the central layer of the layers. Similarly, it can be positioned almost at the center of the core, and the shape of the electromagnetic field distribution can be made symmetrical about the center of the core in the vertical and horizontal directions to the substrate. Can be realized.

Further, by arranging at least two layers having different refractive indexes substantially alternately, the refractive index felt by the light in the direction perpendicular to the layers can be regarded as substantially uniform, and an electromagnetic field distribution substantially similar to that of the conventional structure is maintained. Until now, the reduction of the birefringence of the optical waveguide,
Alternatively, it can be resolved.

Further, by using a graded index in which the refractive index of each layer of the multi-layer structure is increased from the layers at both ends on the clad side toward the layer on the core inner side, a spot size different from that of the conventional optical waveguide is realized. The birefringence of the optical waveguide can be reduced or eliminated.

By setting the total number of layers of the multi-layer structure to 5 to approximately 10, it is possible to easily realize the multi-layer structure as a core even in a thick film manufacturing process such as the FHD method. Further, it is possible to reduce the connection loss with the optical fiber as described in the embodiment of the present invention.

Regarding the multilayer structure, by satisfying the following expressions (1) to (3), it is possible to determine the multilayer structure without complicated analysis.

[0049]

[Equation 1]

Where | B _s | is the structural birefringence value produced by the multilayer structure, n _ave is the average refractive index of the core of the multilayer structure, N is the total number of layers in the multilayer structure, and n _i and t _i
Is the refractive index and thickness of each layer of the multilayer structure, and c ₁ and c ₂ are constants (correction coefficients) determined for each waveguide structure by actual measurement or calculation.

On the right side of equation (1),

[0052]

[Equation 2]

Is the effective refractive index in the direction parallel to the layer,

[0054]

[Equation 3]

Represents the effective refractive index in the direction perpendicular to the layer. Therefore, these differences become structural birefringence derived from the multilayer structure. Then, these calculated values are associated with the structural birefringence value | B _s | obtained by actual measurement or calculation by the correction coefficients c ₁ and c ₂ as in the formula (1).

The correction coefficients c ₁ and c ₂ mainly depend on the confinement of light due to the waveguide structure (difference in relative refractive index, size), and have substantially the same value in an optical waveguide where the confinement of light is constant.
Therefore, it is easy to design a multilayer structure that satisfies the desired structural birefringence by prototyping an appropriate multilayer structure core or performing modal analysis on an appropriate multilayer structure core to obtain the correction coefficient in advance. Is possible.

In a multi-layer structure in which at least two layers having different refractive indexes are alternately arranged, the correction coefficient c ₁ = 1 almost corresponds to the case where the light is sufficiently confined in the core, and mainly when the confinement is weak. Set to 1 or less.

The polarization dependence remarkably appears in the waveguide type optical circuit in the optical interference type circuit in which one optical coupler and the other optical coupler are connected by a plurality of waveguides having different waveguide lengths. Yes, Mach-Zehnder interferometers and arrayed-waveguide gratings are typical examples. An optical circuit with reduced polarization dependence can be provided by reducing the waveguide birefringence in the optical interference type circuit with the multilayer structure core. In the optical interference type circuit, the polarization dependency of the circuit can be reduced or eliminated by reducing or eliminating the polarization dependency of only the waveguide length difference. That is, the waveguide length difference with respect to the waveguide having the shortest waveguide length may be used as the multilayer structure core. Alternatively, the length obtained by adding a certain length to the waveguide length difference may be a multilayer structure. That is, for each waveguide, the length of the portion of the core having the multilayer structure can be set as follows. However, the entire core of the waveguide may have a multilayer structure.

[0059]

[Table 1]

Further, it is not necessary to continuously form the multi-layer structure core, and it is sufficient that the total waveguide length becomes a predetermined length. As the optical interference type circuit, an optical interference type circuit in which a plurality of optical couplers such as a lattice type filter are connected by a plurality of waveguides can be considered.

By combining with the birefringence compensator based on the technique of eliminating the polarization dependence described in the prior art, when a plurality of types of multilayer structures are required, the types of multilayer structures to be formed can be reduced. Further, when it is difficult to produce a multilayer structure for giving a predetermined structural birefringence, the waveguide birefringence can be eliminated by the structural birefringence which can be set by the producible multilayer structure. That is, it is possible to increase the degree of freedom in selecting the structural parameters of the multilayer structure.
As an example of reducing the type of multilayer structure to be formed, consider the case where a λ / 2 wave plate is used in a Mach-Zehnder interferometer using an MMI as an optical coupler. By inserting the λ / 2 wave plate into the single mode waveguide between the optical couplers, the polarization dependence due to the waveguide birefringence of the single mode waveguide can be eliminated. The multilayer structure may be formed by one type applied to the core of the multimode waveguide. Further, as a method of eliminating the waveguide birefringence by the structural birefringence that can be set by the manufacturable multilayer structure, for example, GeO ₂ , B ₂ O ₃ , and P ₂ O are added to the cladding of the conventional technique.
A material having a large coefficient of thermal expansion such as ₅ is doped to adjust the structural birefringence value due to the realizable multilayer structure. In this case, in order to secure reliability to some extent, it is necessary to dope to the extent that tensile stress is not applied.

As described above, the waveguide core does not have to have a multilayer structure as a whole, and a waveguide having a multilayer core and a uniform core can be used. In such a waveguide, the average refractive index of the core of the multilayer structure and the refractive index of the uniform core are made equal, and the value of the structural birefringence brought about by the multilayer structure and the value of the birefringence of the uniform core are Between
The birefringence of the waveguide can be reduced or eliminated by making the sizes substantially equal and reversing the positive and negative signs. Here, regarding the average refractive index of the core having the multilayer structure,
By (3) and (3), the structural birefringence value generated by the multilayer structure can be given by the equations (1) and (3).

[0063]

BEST MODE FOR CARRYING OUT THE INVENTION Specific embodiments of the present invention will be described below.

In the following description of the embodiments, the refractive index of each layer of the multilayer structure core and the average refractive index of the core are represented by the relative refractive index difference with the lower cladding and the average relative refractive index difference. The refractive index of the lower clad is matched with that of quartz glass.

(Embodiment 1) FIG. 5 shows a cross section of a single mode waveguide according to a first embodiment of the present invention.
A silicon substrate is used for 1, and the clad 502 and the core 503 are made of quartz glass. Here, the core 503 is
This is a multilayer structure in which a plurality of first core layers 503a and second core layers 503b are alternately laminated.

In the present embodiment, as shown in the cross section of FIG. 5, the core has a multi-layer structure in a direction perpendicular to the substrate surface. That is, the core 503 has a multilayer structure in which the first core layers 503a and the second core layers 503b are alternately arranged in the direction perpendicular to the substrate surface.

The relative refractive index difference Δ _i and the layer thickness t _i (i = 1, 2) between the first core layer 503a and the second core layer 503b were set by the following procedure. Procedure Create an optical circuit with an appropriate multilayer structure and estimate the structural birefringence value of the multilayer structure from its characteristics. The correction factors c ₁ and c ₂ are determined from the result and the equations (1) to (3). Procedure From the formulas (1) to (3), the relative refractive index difference Δ _i and the layer thickness t _i (i = 1,2) are set so that the structural birefringence due to the multilayer structure has a predetermined value.
Decide. The procedure is to use modal analysis instead of creating an optical circuit when the manufacturing accuracy of the multilayer structure is good and stress birefringence, etc. is not newly added in addition to the structural birefringence due to the multilayer structure in which layers with different refractive indices are laminated. May be.

In order to determine a multilayer structure in which the average relative refractive index difference Δ _ave of the core is 0.75% and the core size is 6 μm × 6 μm, the following three types of Mach-Zehnder interferometer shown in FIG. The first core layer 503a and the second core layer 503b were manufactured with the relative refractive index difference Δ _i and the layer thickness t _i (i = 1, 2). Regarding the number of layers, the total number of layers was 9, and the number of layers N ₁ of the first core layer was 5 and the number of layers N ₂ of the second core layer was 4.

First core layer 503a, Δ ₁ = 1.0%, t ₁ = 0.90 μm Second core layer 503b, Δ ₂ = 0%, t ₂ = 0.38 μm First core layer 503a, Δ ₁ = 3.0%, t ₁ = 0.29 μm 2nd core layer 503b, Δ ₂ = 0%, t ₂ = 1.14 μm 1st core layer 503a, Δ ₁ = 5.1%, t ₁ = 0.17 μm 2nd core layer 503b, Δ ₂ = 0%, t ₂ = 1.29 μm

FIG. 6 is a process drawing for manufacturing the waveguide of this embodiment. The steps will be described below in order. (A) A silicon substrate is used as the substrate 601, and a lower cladding 602, a first core layer film 603a and a second core layer film 603b are alternately laminated on the substrate 601 to form a core layer 603 by a flame deposition method. The flame deposition method is a method in which glass forming raw material gas containing SiCl ₄ as a main component is used to form glass fine particles containing SiO ₂ as a main component in a flame of an oxyhydrogen burner, and a glass fine particle layer is deposited on a substrate. The deposited glass fine particle layer is heated in an electric furnace together with the substrate to form a transparent glass film. The glass fine particle layer for the first core layer film 603a is doped with GeO ₂ in order to increase the refractive index. The doping amount was 10 mol% per relative refractive index difference Δ1%. In addition, B ₂ O ₃ and P ₂ O ₅ are added to lower the clearing temperature. (B)
Unnecessary portions of the core layer 603 are removed by reactive ion etching to form a ridge-shaped core 604. (C) An upper clad 604 having the same refractive index as the lower clad 602 is formed so as to cover the core 604. To form the upper clad 605, a glass fine particle layer was deposited again by the flame deposition method and heated in an electric furnace.

The transmission spectrum of the produced Mach-Zehnder interferometer was measured, and the TE mode and TM obtained from this were measured.
The birefringence value _Be is estimated from the wavelength shift of the mode. Structural birefringence B _s by multilayer structure was determined by subtracting the waveguide birefringence B ₀ = 2.3 × 10 ^-4 occurring in the conventional core structure from the B _e. The black circle in Fig. 7 plots the magnitude of birefringence │B _s │ due to the multilayer structure. Based on this result and the equations (1) to (3), the correction coefficient c ₁ was set to 0.8 and c ₂ was set to 0. This correction factor and (1) to (3)
The solid line indicates | B _s | calculated by the formula. │B _s │ obtained by modal analysis of the structural birefringence in the structure
^-4, which is almost in agreement with the calculation in FIG.

Next, as a procedure, the correction coefficient c₁To 0.8
From equations (1) to (3), │B_s│ = │B₀│ = 2.3 × 10^-4Becomes the first
The relative refractive index difference Δ between the core layer 503a and the second core layer 503b_i, Layer thickness t
_iFind (i = 1,2). From the solid line that is the calculation result of FIG.
│B_s│ = │B₀│ = 2.3 × 10^-4Ratio of the first core layer 503a
Folding rate difference Δ₁2.0% is obtained, and from Eqs. (2) and (3),
t ₁, T₂Is the following value.

First core layer 503a, Δ ₁ = 2.0%, t ₁ = 0.44 μm, N ₁ = 5 Second core layer 503b, Δ ₂ = 0%, t ₂ = 0.95 μm, N ₂ = 4 , Fig. 7 shows that │B _s │ is approximately ₁ of Δ ₁ .
Since it changes as a quadratic function, it is possible to obtain | B _s | for several Δ ₁ by a procedure, estimate a first-order approximation curve, and obtain Δ ₁ for a predetermined | B _s | from it.

With this multilayer structure, the Mach-Zehnder interferometer shown in FIG. 1 (a) and the arrayed waveguide grating shown in FIG. 1 (b) were produced. The slab waveguide 112 of the arrayed waveguide grating is
It is formed in the same layer structure as the single mode waveguide.

FIG. 8 shows the transmission spectrum of the Mach-Zehnder interferometer, and FIG. 9 shows the transmission spectrum of the arrayed waveguide grating. The loss spectra of the TE mode and the TM mode are almost the same. The wavelength shift Dl of the peak wavelength that causes the lowest loss in TE mode and TM mode is 0.01 nm or less in the Mach-Zehnder interferometer and 0.03 nm in the arrayed waveguide grating.
Or less, the birefringence B _e from the wavelength shift 3 × 10 ^-5
It is estimated as follows, and it is understood that the waveguide structure of the present invention can sufficiently reduce the birefringence of the waveguide. In addition, the loss difference at the peak wavelength of each mode is about 0.1 dB, which is smaller than the conventional several dB, and it can be seen that the polarization dependence of the waveguide type optical circuit can be almost eliminated.

(Embodiment 2) In the second embodiment of the present invention, the number of layers of the core in Embodiment 1 is changed within the range of about 1/2 to about 2 times.

The core 503 of the multilayer structure which is the single mode waveguide shown in FIG. 5 has an average relative refractive index difference Δ _ave of 0.75%, a core size of 6 μm × 6 μm, and a multilayer structure as in the first embodiment. The magnitude of birefringence │B _s │ is 2.3 × 10 ^-4 ,
The relative refractive index difference Δ ₁ of the first core layer 503a is 2.0%, and the second core layer 503
The relative refractive index difference Δ ₂ of b was set to 0%. Number of layers of the first core layer N ₁ is 2
The arrayed waveguide grating shown in FIG. 1 (b) was produced by changing the number of layers to 11 layers. The slab waveguide 112 of the arrayed waveguide grating is
It is formed in the same layer structure as the single mode waveguide. First
The number of layers N ₂ of the two core layers is (N ₁ -1) layers. First core layer 503a
The layer thickness t _{1 of the above} and the layer thickness t ₂ of the second core layer 503b were set as follows.

[0078] _{N 1 = 2, t 1 =} 1.10μm, t 2 = 3.79μm N 1 = 3, t 1 = 0.74μm, t 2 = 1.90μm N 1 = 4, t 1 = 0.55μm, t 2 = 1.26 μm N ₁ = 5, t ₁ = 0.44 μm, t ₂ = 0.95 μm N ₁ = 6, t ₁ = 0.37 μm, t ₂ = 0.76 μm N ₁ = 7, t ₁ = 0.32 μm, t ₂ = 0.63 μm N ₁ = 8, t ₁ = 0.28 μm, t ₂ = 0.54 μm N ₁ = 9, t ₁ = 0.25 μm, t ₂ = 0.47 μm N ₁ = 10, t ₁ = 0.22 μm, t ₂ = 0.42 μm N ₁ = 11, t ₁ = 0.20 μm, t ₂ = 0.38 μm The manufacturing process of the waveguide is the same as that of the first embodiment.

From the wavelength shift of the measured transmission spectrum
Magnitude of structural birefringence due to the estimated multilayer structure │B_s│
As shown in FIG. N₁Is 2, │B_s│ is 5 × 10 from the specified value^-5
Large, circuit loss, and connection loss with ordinary optical fiber
Is slightly larger. This is compared to the electromagnetic field distribution of the conventional structure.
This is because the distortion of the waveform is large compared to the above. That is, total
If the number of layers is 5 or more,_s│ against 2 × 10^-5Within
You can see that it is set. Also, a normal optical fiber
The connection loss with the conventional structure is about 0.4 dB, but N ₁Is 3 to 6
About 0.15 dB, N₁Is about 0.1 dB at 7, 8 N₁Is 9 to 11 and is about 0.05 dB
It is a small value, and by selecting an appropriate number of layers,
Connection loss can be reduced. Therefore, the waveguide
The birefringence can be made small enough, and the polarization dependence of the waveguide type optical circuit can be improved.
In order to eliminate it, it is desirable to set the total number of layers to 5 or more.
Yes.

(Third Embodiment) The third embodiment of the present invention has a higher average relative refractive index difference than the core of the first embodiment.

The multi-layer structure of the single mode waveguide shown in FIG.
Average relative refractive index difference Δ of the core 503_aveTo 1.5%, core dimensions
Is 4 μm × 4 μm, and the arrayed waveguide grating shown in Fig. 1 (b) is made.
Made The slab waveguide 112 of the arrayed waveguide grating is a single
The layer structure is the same as that of the mode waveguide. Multi-layered structure
Size of birefringence due to structure│B_s│ is 2.3 × 10^-4Will be
, The relative refractive index difference Δ of each layer_i, Layer thickness t_i, Number of layers N_iIs the expression
(1) to (3) and correction coefficient c ₁= 0.8, c₂Set to the following values from = 0
Decided Correction factor c₁, C₂Is the value obtained in the first embodiment
is there.

First core layer 503a, Δ ₁ = 2.25%, t ₁ = 0.56 μm, N ₁ = 5 Second core layer 503b, Δ ₂ = 0%, t ₂ = 0.30 μm, N ₂ = 4 The manufacturing process is the same as that of the first embodiment.

From the transmission spectrum of the fabricated arrayed waveguide grating, the wavelength shift shows that the TE mode is larger than the TM mode.
It is -0.02 nm, and it can be seen that the birefringence can be reduced to 2 x 10 ^-5 . In addition, the connection loss with an ordinary optical fiber was 0.2 dB smaller than that of the conventional structure.

Here, the correction coefficient c obtained in the first embodiment is used.
₁ and c ₂ are applied, the Mach-Zehnder interferometer of FIG. 1 (a) is manufactured by the following three types of multilayer structures as in the first embodiment, and the correction coefficients c ₁ and c are obtained from the transmission spectrum thereof. Estimated ₂ . Number of layers of the first core layer N ₁ is 5 layers, number of layers of the second core layer N ₂
There were 4 layers and the total number of layers was 9 layers.

First core layer 503a, Δ ₁ = 2.02%, t ₁ = 0.60 μm Second core layer 503b, Δ ₂ = 0%, t ₂ = 0.26 μm First core layer 503a, Δ ₁ = 3.05%, t ₁ = 0.39 μm Second core layer 503b, Δ ₂ = 0%, t ₂ = 0.51 μm First core layer 503a, Δ ₁ = 5.13%, t ₁ = 0.23 μm Second core layer 503b, Δ ₂ = 0%, t ₂ = 0.72 μm Structural birefringence magnitude due to the multilayer structure estimated from the transmission spectrum of the fabricated Mach-Zehnder interferometer │B _s │
Is shown by the black triangle in FIG. From the comparison between this result and Eqs. (1) to (3), it is estimated that c ₁ is 0.78 and c ₂ is 3 × 10 ^−5, and (1) to (3)
The calculation result by the formula is shown by the broken line in FIG. The values are approximately the same as c ₁ = 0.8 and c ₂ = 0 estimated in the _first embodiment, and for Δ ₁ = 2.25% produced in the present embodiment, | B _s | estimated by this calculation is 2.5 × 10 5. ^-4 , which corresponds to the wavelength shift of the fabricated arrayed waveguide grating.

Therefore, in the case of a single mode waveguide structure having different average relative refractive index differences and dimensions, it is desirable to estimate the correction coefficients c ₁ and c ₂ by the procedure in consideration of the influence of manufacturing error. With the same degree of confinement (similar V parameter) as in the first embodiment, even if the already estimated value is applied, the birefringence of the waveguide can be sufficiently reduced, It can be seen that the polarization dependence of the waveguide type optical circuit can be almost eliminated.

(Embodiment 4) In the fourth embodiment of the present invention, the magnitude of the waveguide birefringence in the conventional core structure | B ₀
│ is a half of that of the third embodiment, and the clad is formed of 1.1 × 10 ⁻⁴ silica glass. Further, the relative refractive index difference Δ ₂ of the second core layer 503b was set to a value other than 0.

Average Specific Refraction of Core 503 of Multilayer Structure shown in FIG.
Rate difference Δ_aveIs 1.5% and the core size is 4 μm × 4 μm.
Fabricate arrayed waveguide grating shown in Fig. 1 (b)
did. The slab waveguide 112 of the arrayed waveguide grating is
It is formed with the same layer structure as the single-mode waveguide described below.
It Magnitude of birefringence due to multilayer structure │B_s│ is 1.2 × 10
^-4So that the relative refractive index difference D_i, Layer thickness t_i,layer
Number N_iIs the correction factor c₁= 0.8, c₂From equations (1) to (3) where = 0
Estimated and set the following values. This correction factor c₁, C
₂Is the value obtained in the first embodiment.

First core layer 503a, Δ ₁ = 1.91%, t ₁ = 0.42 μm, N ₁ = 7 Second core layer 503b, Δ ₂ = 0.37%, t ₂ = 0.18 μm, N ₂ = 6 The manufacturing process is the same as that of the first embodiment.

From the transmission spectrum of the fabricated arrayed waveguide grating, the wavelength shift is 0.01 nm and the birefringence is 1 × 10 5.
^It can be reduced to ^{−5, and} it can be seen that the birefringence can be reduced regardless of the magnitude | B ₀ | of the waveguide birefringence in the conventional core structure. Also, a multilayer structure different from that of the third embodiment can be selected.

(Embodiment 5) The fifth embodiment of the present invention is a core structure in which three different types of relative refractive index differences are alternately laminated.

FIG. 11 shows a cross section of the single mode waveguide of this embodiment. A silicon substrate is used as the substrate 1101, and the clad 1102 and the core 1103 are made of silica glass. The core 1103, the first core layer 1103a, the second core layer 1103b,
And the third core layer 1103c from the substrate side to the first core layer 1103a,
Second core layer 1103b, third core layer 1103c, first core layer 1103a,
Second core layer 1103b, third core layer 1103c, half-thick second core layer 1103b, first core layer 1103a, half-thick second core layer 110
3b, third core layer 1103c, second core layer 1103b, first core layer 1103
a, third core layer 1103c, second core layer 1103b, first core layer 1103a
In this structure, each layer was symmetrically arranged with respect to the central layer of the core.

The average relative refractive index difference Δ _ave of the core is 0.75%, the core size is 6 μm × 6 μm, and the relative refractive index difference Δ ₁ of the first core layer 1103a is
3%, the relative refractive index difference Δ ₂ of the second core layer 1103b is 0.75%, the relative refractive index difference Δ ₃ of the third core layer 1103c is 0%, the correction coefficient c ₁ = 0.8 obtained in the first embodiment, In order to confirm that the multilayer structure can be designed by the equations (1) to (3) using c ₂ = 0, Mach-Zehnder interferometers with the following four types of layer thickness were fabricated.
The waveguide manufacturing process was performed in the same manner as in the first embodiment.

First core layer 1103a, Δ ₁ = 3.0%, t ₁ = 0.11 μm Second core layer 1103b, Δ ₂ = 0.75%, t ₂ = 0.76 μm Third core layer 1103c, Δ ₃ = 0%, t ₃ = 0.42 μm First core layer 1103a, Δ ₁ = 3.0%, t ₁ = 0.16 μm Second core layer 1103b, Δ ₂ = 0.75%, t ₂ = 0.56 μm Third core layer 1103c, Δ ₃ = 0%, t ₃ = 0.60 μm First core layer 1103a, Δ ₁ = 3.0%, t ₁ = 0.20 μm Second core layer 1103b, Δ ₂ = 0.75%, t ₂ = 0.37 μm Third core layer 1103c, Δ ₃ = 0% , T ₃ = 0.79 μm 1st core layer 1103a, Δ ₁ = 3.0%, t ₁ = 0.25 μm 2nd core layer 1103b, Δ ₂ = 0.75%, t ₂ = 0.17 μm 3rd core layer 1103c, Δ ₃ = 0 %, T ₃ = 0.97 μm Structural birefringence │B _s │ due to the multilayer structure was estimated from the wavelength shift obtained from the transmission spectrum of the fabricated Mach-Zehnder interferometer. │B _s for the thickness t ₁ of the first core layer
| Is plotted in the black circles shown in FIG. The solid line is the result calculated by the equations (1) to (3) using the correction factors c ₁ = 0.8 and c ₂ = 0, which is in good agreement with | B _s | estimated from the wavelength shift.

From this, the structural birefringence due to the multilayer structure | B _s
The correction factor c ₁ = 0 for the layer thickness of each layer where │ is 2.3 × 10 ^-4 .
8 and c ₂ = 0, set as follows from Eqs. (1) to (3).
The arrayed waveguide grating shown in (b) was prepared. The slab waveguide 112 of the arrayed waveguide grating is formed in the same layer structure as the single mode waveguide described below. The manufacturing process is similar to that of the first embodiment.

First core layer 1103a, Δ ₁ = 3%, t ₁ = 0.17 μm, N ₁ = 5 Second core layer 1103b, Δ ₂ = 0.75%, t ₂ = 0.51 μm, N ₂ = 6 (film thickness The thickness of the second core layer 1103b having a thickness of 1/2 is 0.26 μm and N ₂ = 6.
Includes two layers with a film thickness of 1/2) Third core layer 1103c, Δ ₃ = 0%, t ₃ = 0.65 μm, N ₃ = 4 From the transmission spectrum of the arrayed waveguide grating produced, the wavelength shift is It is 0.01nm, and it is confirmed that the birefringence can be reduced to 1 × 10 ⁻⁵ and that the waveguide type optical circuit that eliminates the polarization dependence can be realized even by alternately arranging three types of layers having different relative refractive index differences. did.

The arrangement of three types of layers having different relative refractive index differences alternately to form a multilayer structure means that the layers of two types of different relative refractive index difference are alternately arranged to form a multilayer structure. In comparison, the degree of freedom in selecting the relative refractive index difference and the film thickness is increased. Average birefringence difference of core and structural birefringence due to multilayer structure │B _s │
Is mainly determined by the relative refractive index difference and the film thickness of the layers constituting the multilayer structure. Here, the number of layers is fixed.

In the case of being composed of alternating layers of two kinds of layers, the parameters that define the multilayer structure are two kinds of relative refractive index difference and two kinds of film thickness. If one of them is set, the average ratio is set. Since the refractive index difference and the structural birefringence │B _s │ are fixed, the other 3
Two values are set automatically. When it is composed of alternating layers of three kinds of layers, there are six kinds of parameters that define the multilayer structure.If three of them are defined, the average relative refractive index difference and | B
_{Since s} │ has been decided, the other three values are decided.

Therefore, for example, in alternate layers of two types of layers, the relative refractive index difference of the second core layer is set first as in Embodiments 1 to 4, so that the relative refractive index of the first core layer is set. Difference, first
The film thicknesses of the core layer and the second core layer were automatically determined. In the alternating layers of three types of layers, as shown in the present embodiment, if the relative refractive index difference of each layer is set, the film thickness of each layer is automatically determined. That is, the relative refractive index difference can be appropriately selected. If there is a parameter that makes it difficult to obtain the manufacturing accuracy, for example, if it can be set only to the three types of relative refractive index difference set in the present embodiment, if the film thickness can be set to a high accuracy, then appropriate structural birefringence can be given. Therefore, birefringence can be reduced.

As an advantage of the alternating layers of the three types of layers, the photo-induced refractive index change can be efficiently generated in the silica type optical waveguide. This is because the silica-based glass to which GeO ₂ is added at a high concentration can obtain a high photo-induced refractive index, and the alternating layers of the three types of layers contain GeO ₂ more than the alternating layers of the two types of layers. This is because it is easy to set a layer added at a high concentration. The above advantages are enhanced by further increasing the number of types of alternating layers.

(Embodiment 6) In the sixth embodiment of the present invention, a quartz substrate is used as the substrate 1301, and the core is parallel to the substrate surface as in the cross section of the single mode waveguide shown in FIG. A multilayer structure was formed in the direction. The clad 1302 and the core 1303 are formed of silica glass, and the core 1303 has a multilayer structure in which the first core layers 1303a and the second core layers 1303b are alternately arranged in the direction parallel to the substrate surface. The layers are approximately parallel to the traveling direction of light, that is, substantially parallel to the side surface of the optical waveguide.

When the optical waveguide having the conventional structure shown in FIG. 2 is formed on the quartz substrate, the value of the effective refractive index of the TM mode compared with the TE mode is smaller than that of the above embodiment, and the birefringence is decreased. The value B ₀ is a negative value of −2.1 × 10 ⁻⁴ .
This is because tensile stress acts on the optical waveguide. Figure
In the transmission spectrum of the arrayed waveguide grating shown in 1 (b), the TE mode shifts to the long wavelength side as compared with the TM mode. Therefore, in order to reduce the waveguide birefringence, a multilayer structure in the vertical direction is formed in the core 1303 on the substrate that gives the structural birefringence in which the effective refractive index of TM mode is higher than that of TE mode.

The average relative refractive index difference Δ _ave of the multilayer core 1303 shown in FIG. 13 is 0.75%, the core size is 6 μm × 6 μm, and the birefringence of the multilayer structure is 2.1 × 10 ^−4. , The relative refractive index difference D _{i of} each layer, the layer thickness t _i , and the number of layers N _i are set to the following values from the equations (1) to (3) in which the correction coefficients c ₁ = 0.8 and c ₂ = 0. The correction coefficients c ₁ and c ₂ are the values obtained in the first embodiment. Further, the arrayed waveguide grating shown in FIG. 1 (b) was manufactured.
The slab waveguide 112 of the arrayed waveguide grating is formed in the same layer structure as the single mode waveguide described below, and the layers are almost in the direction connecting the center of the input / output waveguide and the center of the waveguide array. Formed in parallel.

First Core Layer 1303a, Δ ₁ = 2.0%, t ₁ = 0.75 μm, N ₁ = 3 Second Core Layer 1303b, Δ ₂ = 0%, t ₂ = 1.88 μm, N ₂ = 2 FIG. FIG. 6 is a process diagram for manufacturing the waveguide according to the present embodiment. The steps will be described below in order. (A) A quartz substrate was used as the substrate 1401, and the glass fine particle layer for the lower clad 1402 and the glass fine particles for the first core layer 1404a were deposited on the substrate 1401 by the flame deposition method, and the glass vitrified in an electric furnace.
(B) An unnecessary portion of the first core layer film 1403 is removed by reactive ion etching to form a strip-shaped first core layer 1404a. (C) An upper clad 1405 having the same refractive index as the lower clad 1402 is formed so as to cover the first core layer 1404a. To form the upper clad 1405, a glass fine particle layer was deposited again by the flame deposition method and heated in an electric furnace. By forming the upper clad 1405, the strip-shaped first core layer 1404a is embedded, the second core layer 1404b is formed, and the core 1404 has a multilayer structure.

From the transmission spectrum of the fabricated arrayed waveguide grating, the wavelength shift is 0.05 nm in the TE mode compared to the TM mode, the birefringence can be reduced to about 5 × 10 ^−5, and the sign of the birefringence is different. It can be seen that the polarization dependence of the waveguide type optical circuit can be reduced regardless of the above.

In this embodiment, the multilayer structure is applied to the structure in which the clad is formed between the substrate 1301 and the core 1303 shown in FIG. 13, but since the substrate is a quartz substrate, the substrate shown in FIG. Of course, the structure can also be applied.

(Embodiment 7) A seventh embodiment of the present invention is a multi-layered structure having a graded index in which the refractive index is increased from the layers at both ends on the cladding side toward the layer at the center of the core. FIG. 16 shows a cross section of a single mode waveguide having a multilayer structure as a graded index according to the present embodiment. A silicon substrate is used as a substrate 1601, and a clad 1602 and a core 1603 are made of silica glass. . Core 16
03 is a multilayer structure having a total number of layers of 5 in which the first core layer 1603a and the second core layer 1603b are arranged symmetrically with respect to the third core layer 1603c which is an intermediate core layer, and the refractive index is the first. Core layer 1603a, second
The core layer 1603b and the third core layer 1603c were made higher in this order.

The average relative refractive index difference Δ _ave is set to 0.7 with the following settings.
The arrayed waveguide grating shown in FIG. 1 (b) was fabricated with two types of structures, 5% and 1.5%. The slab waveguide 112 of the arrayed waveguide grating was formed in the same layer structure as the single mode waveguide. Multilayer core 160 with an average relative refractive index difference Δ _ave of 0.75%
3 has a core size of 6 μm × 6 μm, and each layer is composed of a first core layer 1603a, Δ ₁ = 0.45%, t ₁ = 1.0 μm, a second core layer 1603b, Δ ₂ = 0.75%, t ₂ = 1.2 μm The core layer 1603c had Δ ₃ = 1.1% and t ₃ = 1.6 μm. The core 1603 having a multilayer structure with an average relative refractive index difference Δ _ave of 1.5% has a core size of 4 μm × 4 μm, and each layer is a first core layer 1603a, Δ ₁ = 1.1%, t ₁ = 0.8 μm Core layer 1603b, Δ ₂ = 1.5%, t ₂ = 0.9 μm Third core layer 1603c, Δ ₃ = 2.0%, t ₃ = 1.1 μm.

The manufacturing process is the same as that of the first embodiment.
As in the case of the fourth embodiment, the clad is made of silica-based glass in which the magnitude of waveguide birefringence | B ₀ | in the conventional core structure is 1.1 × 10 ⁻⁴ .

From the transmission spectrum of the arrayed waveguide grating thus produced, the wavelength shift is the average relative refractive index difference Δ _ave 0.75%.
0.02 nm, 0.01 nm with an average relative refractive index difference Δ _ave 1.5%, birefringence │B _e │ can be reduced to 2 × 10 ⁻⁵ and 1 × 10 ⁻⁵ , respectively, and a layered structure that becomes a graded index is obtained. Even
It was confirmed that the waveguide birefringence can be reduced and the polarization dependence of the waveguide type optical circuit can be reduced.

The magnitude of the structural birefringence │B _s │ by the multilayer structure estimated by the mode analysis is the average relative refractive index difference Δ _ave 0.
It is 2.2 × 10 ⁻⁵ at 75%, and the average relative refractive index difference Δ _ave 1.5%
Is 6.3 × 10 ⁻⁵ . │B ₀ │1.1 × 10 ^-4 with conventional structure
The value is about 1/2 or less compared to. This is because the graded index structure strengthens the electric field distribution near the center of the third core layer and that there is a remarkable stress distribution in the core, and the birefringence near the center of the core is reduced. By analogy. When core layers with different refractive indexes are alternately arranged, light feels an average value of birefringence distribution due to stress, and its magnitude is almost the same as the waveguide birefringence in the conventional optical waveguide structure. Therefore, it is considered that the equation (1) can be set by correcting only the correction coefficient c ₁ as in the above-described embodiment.

Estimated from the magnitude of waveguide birefringence │B ₀ │ in the conventional core structure, the magnitude of structural birefringence │B _s │ obtained from mode analysis, and the transmission spectrum of the arrayed waveguide grating. From the birefringence │B _e │, the birefringence other than the structural birefringence │B _s │ obtained from the mode analysis is the average relative refractive index difference Δ _ave 0.75%, 7 × 10 ^-5 , the average. The relative refractive index difference Δ _{ave is} 1.5%, which is 4 × 10 ⁻⁵ . For further optimization, when adjusting in the vicinity of the above setting, the correction factors c ₁ and c ₂ in the equations (1) to (3) should be set to c ₁ = 1.15, c ₂ = 7 × 10 ⁻⁵ , c ₁ = 2.5
2, c ₂ = 4 × 10 ⁻⁵ .

(Embodiment 8) In the eighth embodiment of the present invention, a multimode waveguide 1702 which is an optical interference part of an MMI 1701 which is an optical coupler shown in FIG. This is a waveguide type optical circuit to which a multilayer structure in which two types of layers having different index differences are alternately laminated is applied.

Compared to the directional coupler, the MMI1701 has a small variation in the coupling rate due to a manufacturing error, but has a large excess loss. Further, when the waveguide has birefringence, the excess loss has polarization dependency.

The average relative refractive index difference Δ _ave of the core is 0.75%, the core height is 6 μm, and the core width of the multimode waveguide 1702 is 24.
The core width of the input and output waveguides 1703 and 1704 was 6 μm. Birefringence │B _s │ due to the multilayer structure is 5 × 10
The relative refractive index difference Δ _{i of} each layer, the layer thickness t _i , and the number of layers N _i were set to the following values by modal analysis so as to be ⁻⁴ . │ B _s │
The value of was estimated by BPM from the polarization dependence of the excess loss of MMI 1701 manufactured by the conventional waveguide structure. Also, estimating the correction coefficient of equation (1) from the result of modal analysis, c ₁ = 0.
97, c ₂ = 0. Two lengths of the multimode waveguide 1702 in the light propagation direction, 1.4 mm and 1.45 mm, were prepared.

First core layer 503a, Δ ₁ = 3.1%, t ₁ = 0.28 μm, N ₁ = 5 Second core layer 503b, Δ ₂ = 0%, t ₂ = 1.15 μm, N ₂ = 4 The same as in the first embodiment.

The excess loss difference (wavelength 1.55 μm) due to the polarization of the MMI 1701 when the lengths of the multimode waveguide 1702 in the light propagation direction are 1.4 mm and 1.45 mm is 0.1 dB in the conventional core structure,
It was 0.4 dB, but both were reduced to less than 0.02 dB due to the multilayer structure. From this, it was confirmed that the polarization dependence of the waveguide type optical circuit can be reduced by making the core of the multimode waveguide a multilayer structure.

The Mach-Zehnder interferometer 10 shown in FIG.
In the case where 1 is produced by applying MMI 1802 instead of the directional coupler 102, the optical waveguide 103 and the MMI 1702 have different birefringences.
It is desirable to apply the multilayer structure of the present embodiment to 02. As a manufacturing process, according to the process of this embodiment,
The core of the multimode waveguide may be manufactured, and then the single mode waveguide may be manufactured according to the first embodiment. When the core layer for a single mode waveguide is formed, it is also formed on the core of the multimode waveguide, but it can be removed at the same time when the core layer is processed to form a core ridge. . The characteristics of the Mach-Zehnder interferometer fabricated in this way are that the wavelength shift is
It was 0.01 nm, and the difference in peak wavelength loss between TE mode and TM mode was 0.03 dB or less. Thereby, by forming an appropriate multilayer structure in the optical waveguide having different birefringence,
It was confirmed that the polarization dependence of the waveguide type optical circuit can be eliminated.

In the present embodiment, the single mode waveguide and the multimode waveguide are realized by different multilayer structures.
The cores of both waveguides may be manufactured by a multilayer structure adapted to the multimode waveguide, and the λ / 2 wavelength plate may be inserted into the optical waveguide 103. In this configuration, the wavelength shift is 0.005 nm and T
The loss difference between the peak wavelengths of E mode and TM mode was 0.03 dB or less.

(Others) In the present invention, the average relative refractive index difference of the core, the size, and the birefringence value due to the multilayer structure may be adjusted by the relative refractive index difference of each layer, the thickness, and the number of layers. Therefore, the average relative refractive index difference, size, birefringence value due to the multilayer structure, refractive index of each layer of the core, thickness, and number of layers are not limited to those in the above-described embodiments.

The single-mode waveguide in the present invention is a waveguide of about two modes, and includes a pseudo single-mode waveguide that functions as a single-mode waveguide in a circuit.

Further, although the Mach-Zehnder interferometer, the arrayed-waveguide grating, and the waveguide type optical circuit of the MMI alone are used in the above-mentioned embodiments, the present invention is not limited to this.
Any optical circuit such as a ring resonator can be applied as long as it can be configured by an optical waveguide.

In addition, in the above-described embodiment, a multi-layer structure is adopted.
Structural birefringence value │B_S│ is the waveguide birefringence in the core structure
Size │B₀I made it almost equal to │, but │B₀│
If it is less than 2 times, the birefringence will be at least lower than before.
Can be reduced. In addition, the Mach-Zehnder of the above embodiment
-For interferometers and arrayed waveguide gratings, the
^-5By reducing to below, loss fluctuation due to polarization can be reduced
It can be suppressed within 0.1 dB.

Although it has been described in the second and third embodiments that the connection loss with the optical fiber is reduced as compared with the conventional one, the optical fibers of other embodiments are also used when the number of layers is small. The connection loss with is further reduced. The excess loss of the fabricated optical circuit as a circuit has not increased compared to the conventional one, and by appropriately selecting the number of layers, the spot of light propagating through the input / output waveguide can be maintained without impairing the circuit characteristics. The size can be made closer to the optical fiber, and the connection loss with the optical fiber can be reduced.

Although the cladding and the core are formed by the flame deposition method in the above-mentioned embodiment, the method is not limited to the manufacturing method, and means for forming another multilayer structure of quartz glass, such as ECR-CVD (Electron). Cyclotron R
esonance Chemical Vapor Deposition), spatter,
The waveguide birefringence can be reduced or eliminated even if it is formed by plasma CVD or the like. Although GeO ₂ was used to adjust the refractive index, it is sufficient if the refractive index can be set to a desired value.
Other dopants such as TiO ₂ may be applied. Furthermore, although silica-based glass is used as the waveguide material, it is sufficient if layers having different refractive indices can be formed into multiple layers, and other glass-based materials, polymer materials, and the like can be applied.

In the above-mentioned embodiment, the waveguide birefringence generated in the conventional structure is reduced only by the multi-layer structure. However, the waveguide in which the cladding material covering the core and the doping amount are adjusted as described in the prior art. It can also be combined with the method for reducing birefringence.

[0127]

As described above, according to the present invention, the waveguide birefringence can be eliminated or reduced, and the polarization dependence of the optical circuit can be eliminated or reduced. Further, as described above, in the present invention, the conventional waveguide manufacturing method can be applied only by forming the core into a multi-layer structure, which is a problem of the other technology for eliminating the polarization dependence described above. Manufacturing yield, mass productivity, optical characteristics,
An optical circuit can be manufactured without reducing reliability, and a practical optical circuit with low cost, high performance, and high reliability can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a Mach-Zehnder interferometer ((a)) and an arrayed waveguide grating ((b)).

FIG. 2 is a diagram showing an example of a cross section of a conventional optical waveguide.

FIG. 3 is a diagram showing an example of a transmission spectrum of a conventional Mach-Zehnder interferometer.

FIG. 4 is a diagram showing an example of a transmission spectrum of a conventional arrayed waveguide grating.

FIG. 5 is a diagram showing an example of a cross section of an optical waveguide in the first, second, third, fourth and eighth embodiments of the present invention.

FIG. 6 is a diagram for explaining an example of a manufacturing process of an optical waveguide in the first, second, third, fourth, fifth and eighth embodiments of the present invention.

FIG. 7 is a graph showing an example of the birefringence magnitude | B _S | of the multilayer structure with respect to the relative refractive index difference Δ ₁ of the first layer of the core of the multilayer structure according to the first and third embodiments of the present invention. Is.

FIG. 8 is a diagram showing an example of a transmission spectrum of the Mach-Zehnder interferometer according to the first embodiment of the present invention.

FIG. 9 is a diagram showing an example of a transmission spectrum of the arrayed waveguide grating in the first embodiment of the present invention.

FIG. 10 is a graph showing an example of the birefringence magnitude | B _S | of the multilayer structure with respect to the number N ₁ of layers of the first layer of the core of the multilayer structure according to the second embodiment of the present invention.

FIG. 11 is a diagram showing an example of a cross section of an optical waveguide according to a fifth embodiment of the present invention.

FIG. 12 is a graph showing an example of the magnitude | B _S | of birefringence due to the multilayer structure with respect to the thickness t ₁ of the first layer of the core of the multilayer structure according to the fifth embodiment of the present invention.

FIG. 13 is a diagram showing an example of a cross section of an optical waveguide according to a sixth embodiment of the present invention.

FIG. 14 is a diagram for explaining an example of a manufacturing process of the optical waveguide in the sixth embodiment of the present invention.

FIG. 15 is a diagram showing another example of the cross section of the optical waveguide in the sixth exemplary embodiment of the present invention.

FIG. 16 is a diagram showing another example of the cross section of the optical waveguide according to the seventh embodiment of the invention.

FIG. 17 is a diagram showing a configuration example of an MMI in the eighth embodiment of the present invention.

[Explanation of symbols]

101 Mach-Zehnder interferometer 102 directional coupler 103 Optical waveguide 104 Input waveguide 105 Output waveguide 111 arrayed waveguide grating 112 slab waveguide 113 Waveguide array 114 Input waveguide 115 Output waveguide 201 substrate 202 clad 203 core 501 board 502 clad 503 core 503a 1st core layer 503b 2nd core layer 601 board 602 lower cladding 603 core layer 603a 1st core layer film 603b Second core layer film 604 core 604a 1st core layer 604b 2nd core layer 605 Upper cladding 1101 PCB 1102 Clad 1103 core 1103a 1st core layer 1103b Second core layer 1103c Third core layer 1301 PCB 1302 Clad 1303 Core 1303a 1st core layer 1303b 2nd core layer 1401 PCB 1402 lower cladding 1403 1st core layer film 1404 core 1404a 1st core layer 1404b 2nd core layer 1405 upper cladding 1501 PCB 1502 Upper cladding 1503 core 1503a 1st core layer 1503b Second core layer 1601 PCB 1602 clad 1603 core 1603a 1st core layer 1603b 2nd core layer 1603c 3rd core layer 1701 MMI 1702 Multimode waveguide 1703 Input waveguide 1704 Output waveguide

Continued front page    (72) Inventor Mikitaka Ito             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation F-term (reference) 2H047 KA01 KA04 KA12 KB01 LA00                       LA12 LA18 QA02 QA04 TA22

Claims (20)

[Claims] 1. A waveguide type optical circuit including a waveguide having a clad and a core, wherein at least a part of the core of the waveguide is a multi-layer structure formed by a plurality of types of layers having different refractive indexes. A waveguide type optical circuit characterized in that 2. The waveguide type optical circuit according to claim 1, wherein, of the birefringence of the waveguide, a value of structural birefringence caused by the multilayer structure and a value of other birefringence are different. ,
A guided-wave optical circuit having positive and negative signs reversed. 3. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit includes a substrate, and the waveguide is formed on the substrate. Is a waveguide type optical circuit having a multilayer structure in a direction substantially perpendicular to the surface of the substrate. 4. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit includes a substrate, and the waveguide is formed on the substrate. Is a waveguide type optical circuit having a multilayer structure in a direction substantially parallel to the surface of the substrate. 5. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit includes a substrate, and the substrate is a silicon substrate or a quartz substrate. Is a waveguide type optical circuit characterized by being made of quartz glass. 6. The waveguide type optical circuit according to claim 1, wherein the waveguide includes a single mode waveguide, and a core of at least a part of the single mode waveguide. Is a waveguide type optical circuit having a multi-layer structure formed by a plurality of types of layers having different refractive indexes. 7. The waveguide type optical circuit according to claim 1, wherein the waveguide includes a multimode waveguide, and at least a part of cores of the multimode waveguide is A waveguide type optical circuit having a multi-layer structure formed by a plurality of types of layers having different refractive indexes. 8. The waveguide type optical circuit according to claim 1, wherein the waveguide includes a single mode waveguide and a multimode waveguide. At least some of the cores have a multilayer structure formed by a plurality of types of layers having different refractive indices, and at least some of the cores of the multimode waveguide are also made of a plurality of types of layers having different refractive indices. A waveguide type optical circuit having a formed multilayer structure. 9. The waveguide type optical circuit according to claim 8, wherein a structural birefringence value generated by the multilayer structure of the single mode waveguide and a structural birefringence generated by the multilayer structure of the multimode waveguide. Waveguide type optical circuits characterized by different values. 10. The waveguide type optical circuit according to claim 9, wherein the structural birefringence value generated by the multilayer structure of the multimode waveguide is the structural birefringence value generated by the multilayer structure of the single mode waveguide. A waveguide type optical circuit characterized by being larger than a value. 11. The waveguide type optical circuit according to claim 1, wherein the layers forming the multilayer structure have a refractive index and a thickness that are substantially symmetrical with respect to the central layer. A waveguide type optical circuit characterized in that 12. The waveguide type optical circuit according to claim 1, wherein the multilayer structure is formed by arranging at least two types of layers having different refractive indexes substantially alternately. A waveguide type optical circuit characterized by the above. 13. The waveguide type optical circuit according to claim 1, wherein a layer forming the multilayer structure has a refractive index from a layer at both ends on a clad side toward a layer on an inner side of a core. Guided-type optical circuit characterized by high height. 14. The waveguide type optical circuit according to claim 1, wherein the total number of layers of the multilayer structure is three.
A waveguide type optical circuit characterized in that it is composed of more layers. 15. The waveguide type optical circuit according to claim 14, wherein the total number of layers of the multilayer structure is 5 to approximately 10 layers. 16. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit comprises two optical couplers and a long optical coupler. An optical interference type circuit having a plurality of waveguides having different lengths is provided, and the length of the core having a multilayer structure is L for the waveguide having the shortest waveguide length among the plurality of waveguides. At this time, for each of the waveguides other than the waveguide having the shortest waveguide length, the length of the core having the multilayer structure is L, which is the difference between the length of the waveguide and the shortest waveguide length. A waveguide type optical circuit characterized by having an added length. 17. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit includes two optical couplers and two optical couplers connecting the two optical couplers. And a single mode waveguide of Mach-Zehnder interferometer. 18. A waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit connects two slab waveguides and a long slab waveguide. A waveguide array composed of a plurality of single mode waveguides having different heights,
Array having an input waveguide formed of a single mode waveguide connected to one of the two slab waveguides and an output waveguide formed of a single mode waveguide connected to the other of the two slab waveguides A waveguide-type optical circuit comprising a waveguide diffraction grating. 19. The waveguide type optical circuit according to claim 1, wherein the waveguide type optical circuit includes a birefringence compensator. . 20. The guided-wave optical circuit according to claim 19, wherein the birefringence compensator is a birefringence compensator using a λ / 2 wavelength plate. circuit.