JP2007047326A - Thermo-optic optical modulator and optical circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermo-optic optical modulator and an optical circuit that can reduce power consumption and suppress loss. <P>SOLUTION: The modulator comprises a core 3 made of a quartz material, a core 4 formed on the core 3, the core 4 made of a material having a refractive index higher than that of the core 3 and having a temperature coefficient of the refractive index higher than that of the core 3, a tapered portion 5 as a spot size converter connecting the core 3 and the core 4, and a heater 6 fabricated on the clad 2 to heat the core 4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a thermo-optic light modulator and an optical circuit, and more particularly, a thermo-optic light that is an optical waveguide circuit used in the field of optical communication and realizes low loss and low power consumption using a thermo-optic effect. The present invention relates to a modulator and an optical circuit.

  Devices using planar optical circuit technology have become key devices in the optical communication field. Among them, an optical switch and an optical attenuator using a thermal phase modulator are particularly attracting attention. In the thermo-optic light modulator used in these devices, the phase of light is controlled by increasing the temperature of the core by applying electric power to the thin film heater formed on the waveguide. By increasing the temperature of the modulation region by the heat from the heater, the refractive index of the modulation region is increased, and the optical path length corresponding to the amount and length of the change is changed to modulate the phase at the output end. The

  In the circuit using the thermal phase modulator, the circuit configuration most used is a thermal circuit configured by branching light incident on the waveguide by a 3 dB coupler and providing a heater on at least one of the branched waveguides. This is a Mach-Zehnder interferometer circuit (MZI circuit) that is connected to an optical light modulator and couples light emitted from the thermo-optic light modulator by a coupler again. By connecting at least one of the branches to a phase modulator and giving a phase difference corresponding to a half wavelength of the wavelength compared to the other, light intensity modulation at the output end is performed.

  The biggest problem of a thermo-optic light modulator using this planar optical circuit technology is power consumption. The power consumed by one thermo-optic light modulator is about 450 mW in a typical structure. If this is used to realize a large-scale switch of 1 × 32, 16 × 16, etc., the power consumption of the entire circuit becomes very large. Therefore, there is a strong demand to reduce power consumption.

Among various methods proposed, as one method for reducing power consumption, it is conceivable to use a material having a large refractive index temperature coefficient (also referred to as a refractive index temperature coefficient) in the modulation region.
In the MZI circuit, assuming that the phase difference between two coupled waveguides is Δψ, Δψ is
Δψ = 2π · L · dn / dT · ΔT / λ (1)
Given in.

Here, L is the length of the connected waveguide under the heater, dn / dT is the temperature coefficient of the refractive index of the connected waveguide, ΔT is the temperature difference between the two connected waveguides, and λ is the wavelength of light. In the thermo-optic light modulator, the phase difference is given by increasing ΔT in the equation (1) by the heat applied by the heater. Here, it can be said that increasing dn / dT is effective in reducing power consumption. In the case of a general quartz material, dn / dT is about 1 × 10 ⁻⁵ / ° C., whereas in the case of crystalline Si, for example, dn / dT is about 16 × 10 ⁻⁵ / ° C. large. In a thermo-optic light modulator having a structure composed only of a quartz-based material, assuming that the power required to rotate the phase by π at a certain wavelength is 100 mW, it is about 6 mW if the material is replaced with Si by simple calculation. Therefore, the power consumption can be significantly reduced.

  In recent years, arrayed waveguide diffraction gratings have become key devices for realizing wavelength division multiplexing transmission. In this device, in recent years, it is essential to flatten the light intensity of each channel, and it is often integrated with a multi-channel optical variable attenuator. In particular, a thermo-optic variable optical attenuator fabricated using a planar optical circuit is a device that adjusts the output light intensity using a thermo-optic light modulator and is easy to integrate with an arrayed waveguide grating. Widely used. Further, in these integrated circuits, in order to reduce the cost, it is strongly required to simultaneously reduce the cost and reduce the size of the circuit by simultaneously manufacturing them in a wafer.

  As described above, if a thermo-optic light modulator is manufactured using a material having a large absolute value of dn / dT, there is a great merit for power consumption. Simply, if a material having a dn / dT of n times is used, the power consumption can be reduced to 1 / n times if the thermo-optic light modulator has the same structure. When Bi oxide is used as the waveguide, the dn / dT is about 3 times larger than that of quartz. Therefore, it is possible to simply reduce the power consumption to 1/3.

  However, although it exists at the research level, there have been few examples of application to a device in which a thermo-optic light modulator using a material having a large absolute value of dn / dT is commercially available. This is because if all the circuits are manufactured using a material having a large absolute value of dn / dT, there is a problem that propagation loss increases and circuit loss increases. In addition, a quartz-based planar optical circuit is generally excellent in connection with a single mode fiber, but a material having a large dn / dT generally has a high refractive index and a large coupling loss with the single mode fiber. There is a problem that reflection occurs. A high refractive index means that the size of the waveguide core that achieves the single mode is also reduced.

  For this reason, there is a problem that processing tolerance becomes strict and stable production is impossible. When the core size is small, there is a problem that it is difficult to stably manufacture the 3 dB coupler constituting the MZI circuit. A quartz planar optical circuit is often used that uses a directional coupler as a 3 dB coupler. This is because there is no loss of principle, and a core of a dimensional size made of quartz material is easy to manufacture. On the other hand, a multimode interference waveguide (MMI) has been considered, but there is a problem that it has a principle loss and is difficult to manufacture stably.

  In addition, if the circuit is composed only of a material having a high refractive index even if dn / dT is large, reflection at the fiber connection point occurs, which becomes a problem. In recent years, as integration of planar optical circuits using quartz-based materials has progressed, the absolute value of dn / dT when a thermo-optic light modulator is integrated with a circuit such as an arrayed-waveguide diffraction grating and manufactured together. However, it is very difficult to collect and collect using only a large material.

Considering a waveguide with a circuit length of l and an effective refractive index of the waveguide of n, the optical path length L ₁ is
L ₁ = n × l (2)
Given in. The amount of change of the optical path length with respect to temperature can be obtained by differentiating equation (2) with respect to temperature T.
dL ₁ / dT = n · dl / dT + l · dn / dT (3)
It becomes. When a quartz-based waveguide is fabricated on a Si substrate, the amount of change of the optical path length with respect to temperature becomes a positive value, and when an arrayed waveguide grating is fabricated, the center wavelength varies with changes in environmental temperature.

  Considering only the optical variable attenuator, as described above, it is preferable to manufacture the optical variable attenuator using a material having a large dn / dT from the viewpoint of reducing power consumption. However, for the arrayed waveguide grating, it is possible to suppress fluctuations in the center wavelength by fabricating the arrayed waveguide grating using a material having a small dn / dT that contradicts the variable optical attenuator from Equation (3). preferable.

  As described above, it is preferable to use a material having a small absolute value of dn / dT, which is contrary to the thermo-optic light modulator, as a material for forming the arrayed waveguide grating. For example, an arrayed waveguide grating and a thermo-optic light modulator can be integrated on the same substrate using only that material even if the material has a small loss due to absorption, scattering, etc. and a large dn / dT. Is difficult. In the case of an arrayed waveguide grating made of a quartz material, the center wavelength changes by about 0.01 nm when the temperature changes by 1 ° C. Therefore, the array waveguide circuit grating is normally used while controlling the temperature using a Peltier element so that the temperature does not change. However, the array waveguide diffraction grating is used without temperature control because it is temperature independent. However, it is also desirable from the viewpoint of power consumption and device cost.

  Therefore, as a method for making temperature independence, a silicone resin having a large dn / dT having a reverse sign compared to quartz as shown in Non-Patent Document 1 is poured into a groove provided in the arrayed waveguide portion, and the optical path Techniques for compensating for long temperature changes are known. However, when this conventional method is applied to an arrayed waveguide grating made of a material having a large dn / dT for the purpose of reducing the power consumption of the thermo-optic light modulator, the loss increases. In addition to material loss, more silicone resin is required to cancel out the large dn / dT, so that the width of the silicone resin groove is widened and the loss at that location is significantly increased. As described above, many of the materials having a large dn / dT have a large refractive index, and the diffraction loss when radiated from the waveguide to the resin groove portion is further increased.

Patent Document 1 discloses an athermal array waveguide diffraction grating using a TiO ₂ / SiO ₂ based material as a method for making temperature independence. This method cancels the temperature dependence of the center wavelength of a quartz-based arrayed waveguide grating fabricated on a Si substrate because the TiO ₂ / SiO ₂ -based material has a negative refractive index temperature coefficient. The focus is on what is possible.

  The first item on the right side of Equation (3) shown above corresponds to the elongation of the substrate with temperature. The second term corresponds to the refractive index changing with temperature. In the silica-based waveguide formed on the Si substrate, the first term is controlled by the thermal expansion of the Si substrate and has a positive value. Therefore, the second term is set to a negative value, the change in the optical path length due to expansion is canceled, and the right side of Equation (3) is brought to 0 or close to 0, thereby forming a temperature-independent arrayed waveguide diffraction grating.

TiO ₂ / SiO ₂ glass has negative dn / dT, but its absolute planting is larger than quartz. Therefore, if a thermo-optic light modulator is manufactured using the material, a circuit that achieves both a reduction in power consumption of the thermo-optic light modulator and temperature independence of the arrayed waveguide grating on a single substrate. It can be produced. Therefore, it can be said that the TiO ₂ / SiO ₂ material is a very powerful material system.

However, if an arrayed waveguide grating is fabricated using only TiO ₂ / SiO ₂ glass as a single material, the problem of increased material loss and increased reflection is inevitably avoided as compared with the case of fabricating a quartz material. Difficult to do. In addition, the refractive index of the TiO ₂ / SiO ₂ material is 1.7 or more in a concentration range where dn / dT is negative, and the waveguide core size is reduced. Therefore, there is a problem that a high processing technique is required and it is difficult to manufacture a coupler that ensures stability. Furthermore, it has been difficult to integrate an athermal array waveguide diffraction grating using a TiO ₂ / SiO ₂ based material and an optical attenuator using a thermo-optic light modulator on a single substrate.

  As described above, in a conventional thermo-optic light modulator using a material having a large dn / dT, 1) material absorption and scattering loss increase by using the material, and 2) refraction of the material. In general, the coupler is not able to be stably manufactured because the waveguide core size is small due to the high ratio of the waveguide. 3) A thermo-optic variable optical attenuator using a thermo-optic light modulator and an arrayed waveguide diffraction grating When integrated on a single substrate, it is possible to reduce the power consumption of the variable optical attenuator taking advantage of the large absolute value of dn / dT and to make the temperature of the arrayed waveguide diffraction grating independent of each other while suppressing loss. There was a problem that it was difficult to realize.

  The present invention has been made in view of such problems, and an object thereof is to provide a thermo-optic light modulator and an optical circuit that can reduce power consumption and suppress loss.

  In order to achieve the above object, the present invention provides a thermo-optic light modulator including a buried optical waveguide having a shape in which a core is buried with a cladding layer, The second core is formed of a material having a refractive index different from that of the first core and having a higher temperature coefficient of refractive index than that of the first core, which is formed on or in the first core. A spot size converter that connects the first core and the second core, and a heater formed on the cladding layer to apply a temperature to the second core, The first core is made of a material having a propagation loss smaller than that of the second core, and the core includes the first core and the second core, and the first core and the second core Of the core with the higher refractive index, the core with the lower refractive index A optical wherein the connecting portion has a spot size converter is formed with a lower portion of the heater formed in the cladding layer is characterized in that at least the second core is formed.

  The invention according to claim 2 is the invention according to claim 1, further comprising a first 3 dB coupler formed on the input side and a second 3 dB coupler formed on the output side, The 3 dB coupler and the second 3 dB coupler are connected by the two buried optical waveguides.

  According to a third aspect of the invention, in the first or second aspect of the invention, the first core is made of a quartz-based material containing B, P, and Ge.

The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein the second core is made of Si, amorphous Si, SiO ₂ , SiO ₂ (0 <x <2), TiO ₂ , It consists of the material containing any one of SiON, It is characterized by the above-mentioned.

  The invention according to claim 5 is the invention according to any one of claims 1 to 3, wherein the second core is made of a material containing at least 50 mol% of Ti oxide.

  According to a sixth aspect of the present invention, there is provided the thermo-optic light modulator according to any one of the first to fifth aspects, and a temperature of a refractive index smaller than that of the second core formed on the same substrate as the thermo-optic light modulator. And an arrayed waveguide diffraction grating having an arrayed waveguide including a core made of a material having a coefficient.

  The invention described in claim 7 is a thermo-optic light modulator according to any one of claims 1 to 5 and larger than the first core formed on the same substrate as the thermo-optic light modulator, and is negative. An arrayed waveguide diffraction grating having an arrayed waveguide including a core made of a material having a temperature coefficient of refractive index, and spot size converters are formed at both ends of the arrayed waveguide. To do.

  As described above, according to the present invention, the first core is made of a quartz-based material that has been proven so far, and the second core has a higher temperature coefficient of refractive index than the first core. Since it consists of material and the 1st core and the 2nd core are connected by the spot size converter, it is possible to reduce power consumption and to suppress loss.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof is omitted.
In one embodiment of the present invention, a waveguide core is made of a material having a large absolute value of the refractive index temperature coefficient (dn / dT) at least at a necessary portion, preferably only at a necessary portion. It is made of a material having a smaller propagation loss than a waveguide core made of a material having a large absolute value of dn / dT, such as a quartz-based material that has a proven record. In order to achieve a low loss connection between the two, a spot size converter is provided at the optical connection between the waveguide core made of a material having a large absolute value of the refractive index temperature coefficient and the waveguide core made of a silica-based material. Provide a circuit with low loss and low reflection.

  In the case of a circuit in which an optical variable attenuator using a thermo-optic optical modulator and an arrayed waveguide diffraction grating are integrated on the same substrate, at least the modulation region of the optical variable attenuator has an absolute value of dn / dT. If it is made of a large material, it is effective for the problem that the loss increases. That is, only the lower part of the heater (modulation region) of the optical variable attenuator is provided with a core made of a material having a large dn / dT (also referred to as a “second core” in this specification), and the first through the spot size converter. The core made of a quartz-based material (also referred to as “first core” in this specification) connected to the second core and the arrayed waveguide diffraction grating are made of a quartz-based material. In this way, loss can be suppressed and the power consumption of the optical variable attenuator can be reduced. Further, if the arrayed waveguide portion of the arrayed waveguide diffraction grating is made of a material having a large absolute value of dn / dT, not only will the loss increase, but it will be difficult to make the wavelength temperature independent. It is also effective for problems. In other words, a conventional method can be used to make the temperature of the arrayed waveguide grating independent of temperature.

Furthermore, using TiO ₂ / SiO ₂ glass with negative refractive index temperature coefficient and large absolute planting, low power consumption of variable attenuator and temperature independence of arrayed waveguide grating The above circuit is realized by making the above circuit in a batch on a single substrate, and the loss can be reduced by making most of the circuit with a low-loss quartz-based material. And

(First embodiment)
In this embodiment, a conventional MZI circuit using a quartz-based material is manufactured, and a refractive index temperature coefficient (dn / n) made of SiO _x (0 <x <2) is formed on an arm waveguide which is a connecting portion thereof. A waveguide made of a material having a large absolute value of dT) is connected through a spot size converter.

  FIG. 1 is a bird's-eye view showing a thermo-optic light modulator according to this embodiment, FIG. 2 (a) is a cross-sectional view taken along the line AA ′ of FIG. 1, and FIG. FIG. 2C is a cross-sectional view taken along the line BB ′ of FIG. 1. FIG. 3 is a configuration diagram of a 1 × 2 optical switch using the thermo-optic light modulator according to the present embodiment. In this embodiment, an optical switch is manufactured. However, it is possible to manufacture an optical attenuator with the same configuration. That is, the thermo-optic light modulator according to this embodiment can be applied to any circuit as long as the circuit uses the thermo-optic light modulator.

In FIG. 1, a Si substrate 1 includes a core (first core) 3 made of a quartz-based material, and a core made of SiO _x having a higher refractive index and a higher refractive index temperature coefficient (second core) than the first core. 4) and the taper portion 5 formed of the same material as the core 4 formed at both ends of the core 4 and having a width gradually narrowing toward the opposite side of the core 4 is embedded. A clad 2 is formed. In the present embodiment, SiO _x is used as the material of the core 4, but the present invention is not limited to this. For example, SiO ₂ , TiO ₂ , SiON, a mixture of SiO _x and TiO ₂ , or the like may be used. good. The material of the core 4 may be a material made of glass containing at least one of oxides such as Ta, Bi, Na, Al, Ce, Ga, and Li. That is, as the material of the core 4, a material having a refractive index temperature coefficient larger than the refractive index temperature coefficient of the quartz-based material used for the core 3 may be used.

  In this embodiment, a quartz-based material is used as the core 3 as the first core. However, the present invention is not limited to this, and the propagation loss is smaller than the propagation loss of the core 4 as the second core to be used. Any material may be used as long as it is a material.

  1 and 2A to 2C, a core 4 and a tapered portion 5 are formed in a predetermined portion on the core 3. Tapered portions 5 are formed at both ends of the core 4. The light propagating through the core 3 (core 4) shifts through the taper portion 5 while converting the spot size to the core 4 (core 3), so the virtual line portion 8 becomes a spot size converter. Therefore, the light propagating through the core 3 and the core 4 is optically connected via the spot size converter 8. As shown in FIG. 1 and FIG. 2C, a heater 6 is formed on the clad 2 on the top of the core 4, and connection wires 7 made of metal or the like are formed on both ends of the heater 6. . With this connection wiring 7, electric power from the outside can be supplied to the heater 6. With such a configuration, the heater 6 modulates light propagating through the core 4 by applying heat to at least the core 4 serving as a modulation region. At this time, taper portions 5 as spot size converters are formed at both ends of the core 4 serving as a modulation region, that is, portions of the core 4 that are optically connected to the core 3, so that the mode field is Different cores 3 and 4 can be connected well.

FIG. 3 shows a configuration diagram of the 1 × 2 optical switch using the thermo-optic light modulator according to this embodiment shown in FIG. In FIG. 3, heaters and wiring (electrodes) are omitted in order to prevent the figure from becoming complicated. In FIG. 3, the core is indicated by a thin line for easy viewing of the drawing.
In FIG. 3, a core 3 as an arm waveguide is connected to each output port of the 3 dB coupler 31A. A thermo-optic light modulator is formed on each of the cores 3 as the arm waveguides by the core 4, the taper portion 5, the heater 6, and the like described with reference to FIGS. 1 and 2A to 2C. The other end of the core 3 as an arm waveguide is connected to the input port of the directional coupler 31B.

A method for manufacturing the optical switch shown in FIG. 3 will be described below. The numerical values and manufacturing methods shown here are examples, and other values and manufacturing methods may be used.
First, a lower cladding 50 μm and a core layer 4.5 μm serving as a first core layer (core 3) are sequentially deposited on a Si substrate using a flame deposition method. Here, the flame deposition method is a method in which a chloride film such as SiCl ₄ is burned in an oxyhydrogen flame to form a glass film on the substrate at a high speed. This flame deposition method is suitable for depositing a relatively thick film and is widely used for the production of optical waveguides because of its excellent embedding characteristics. Immediately after deposition, the formed film is a collection of fine particles, and thus scatters visible light and shows white. However, a transparent film can be obtained by performing heat treatment. In general, in order to lower the clearing temperature, for example, glass to which an appropriate amount of P ₂ O ₅ , B ₂ O ₃ or the like is added is deposited, and GeO ₂ or the like is added to the core layer to increase the refractive index. Here, the relative refractive index difference Δ of the deposited core layer is set to 0.75% with respect to the cladding layer.

Next, a SiO _x (0 <x <2) film to be the second core layer (core 4) is formed on the first core layer by using SiH ₄ and O _2, and ECR-CVD (electron-cyclotron-resonance). Deposition using plasma-enhanced chemical vapor deposition. Here, the SiO _x film is a SiO ₂ film in which Si is excessively added, and is a film in which Si nanocrystals having a particle size of several nm are dispersed and embedded in a SiO ₂ matrix. As the film, a film obtained by adding the property of Si to SiO ₂ is obtained. Usually, dn / dT of Si = 16 × 10 ⁻⁵ / ° C., and dn / dT of SiO ₂ is about 1 × 10 ⁻⁵ / ° C. Accordingly, dn / dT takes a value in the meantime with the excess of Si as a function. That is, when x changes in the range of 0 <x <2, the refractive index temperature coefficient of SiO _x changes between 1 × 10 ⁻⁵ / ° C. and 16 × 10 ⁻⁵ / ° C. Further, the SiO ₂ of the matrix is the same material as the planar optical circuit and the optical fiber, and has the highest transmittance in the wavelength region of the communication band of 1.5 μm. Further, in the Si nanocrystal, the wavelength of 1.5 μm is a wavelength equal to or less than the band gap energy, and the Si nanocrystal is transparent to the light in this region. Further, since the particle size is smaller than the wavelength of the propagating light, it does not work as a scattering source. Therefore, there is an advantage that the transmittance is good.
The same can be said for TiO ₂ .

In this embodiment, the excess Si concentration is set to 40 at. %. The dn / dT at this time is 6 × 10 ⁻⁵ / ° C., which is about 6 times larger than that of normal quartz.
Next, a resist is applied, and the SiO _x film is processed into a circuit shape by using a general photolithography technique and reactive ion etching. A taper having a triangular shape of 500 μm is applied to both ends of a linear waveguide having a length of 2 mm and a width of 2 μm as a second core. This taper section adiabatically converts the mode field of light propagating in the first waveguide (first core) to that of the second core. Therefore, the loss accompanying the conversion of the mode field can be made very small (0.1 dB or less per place).

Again, resist is applied, alignment is performed, and photolithography and etching of the first waveguide is performed. The optical circuit fabricated as the first waveguide is an MZI circuit composed of two 3 dB directional couplers connected to both ends of a linear arm waveguide 3 mm as shown in the figure. The radiation of the waveguide is 6.0 μm. These etched circuits are further filled with 15 μm of glass serving as an overcladding, a heater made of Cr is formed on the cladding, and then a gold wiring is formed. Here, the heater has a width of 3 μm and a length of 2 mm, and is formed on the surface of the clad on the second core made of SiO _x .

In the present embodiment, a SiO _x film is used as the second core (core 4). This is because the SiO _x film is close to the composition of quartz SiO ₂ , and has a high refractive index (about 1.6) and high transmittance at a communication band of 1.5 μm. Of course, a material having a large dn / dT (for example, a glass containing single crystal Si, amorphous Si, or other metal oxide and having a large dn / dT) may be used as the second core. For example, in the case of a mixed film with TiO ₂ or its oxide, dn / dT is a negative value, but its absolute value is large and suitable for the application of this embodiment.

  As described above, when a thermo-optic light modulator is manufactured using only a conventional material having a large dn / dT, there has been a problem that a material loss is large and a coupling loss with a fiber is large. Further, when the absolute value of dn / dT is large, the refractive index is often high, and high-accuracy processing is required to achieve a single mode waveguide, resulting in a problem of poor yield. Furthermore, integration with planar optical circuits having other functions has been difficult.

  However, in the present embodiment, a spot size converter is used to form a 3 dB coupler with an existing proven low-loss silica-based optical planar circuit, and a waveguide made of the silica-based material is also coupled to an optical fiber. To implement. Then, a waveguide made of a material having a large dn / dT is collectively produced only in a place where a thermo-optic effect is required (a region where a heater is formed in the upper part), and a waveguide made of a quartz-based material, A waveguide made of a material having a refractive index temperature coefficient larger than the refractive index temperature coefficient of the quartz material constituting the waveguide is connected by a spot size converter.

  In this case, most of the circuit is composed of a proven quartz-based material. Therefore, the propagation loss due to the quartz-based material is very small, and the propagation loss of the entire circuit can be suppressed. The length of the second waveguide (second core) made of a material having a large dn / dT is only about 3 mm only in the spot size conversion section and the lower part of the heater. In this case, even if the second waveguide is made of a material having a large dn / dT and a material with a slight material loss, the propagation distance is very short, so that the loss does not increase greatly.

In the case of a material having a large dn / dT, the refractive index itself is often larger than that of quartz. In the case of the SiO _x film of this embodiment, the refractive index n is about 1.6, which is considerably larger than the refractive index of quartz of about 1.45. Furthermore, a building using single crystal Si has a refractive index n of about 3.4. That is, the waveguide dimension at which the second waveguide achieves the single mode is smaller than the above-described dimension of the first waveguide. Therefore, if they are simply connected, a loss occurs due to mode field mismatch. Therefore, a tapered spot size converter is provided in a part of the second waveguide, and the light propagating through the first waveguide is gradually transferred to the second waveguide in an adiabatic manner.

  FIG. 4 is a diagram for explaining the operation of the tapered spot size converter unit according to the present embodiment. In FIG. 4, only one arm waveguide is shown enlarged. The circle in FIG. 4 simply shows the size of the mode field of propagating light.

  When the light propagating through the core 3 serving as the first waveguide reaches the tapered portion 5, it shifts to the core 4 serving as the second waveguide having a high refractive index while gradually reducing the mode field. The light that has shifted to the core 4 while reducing the mode field propagates through the core 4 and then again is a spot size converter (tapered portion) formed at a position opposed to the tapered portion 5 formed at the incident end of the core 4. 5) Return to the core 3 while expanding the mode field.

  The tapered portion 5 functions as a spot size converter as described above. By providing spot size conversion at this location, waveguides of different materials having a large dn / dT are connected to each other with low loss and low reflection. As a result of measuring the loss due to spot size conversion using a separate test circuit, it was confirmed that the loss was very small at 0.1 dB or less per location. In other words, conventionally, when a thermo-optic light transflector is manufactured using a waveguide manufactured only with a material having a large refractive index temperature coefficient, an increase in loss has been a problem. It becomes possible to produce the thermo-optic light modulator made of only the system material.

  That is, according to the present embodiment, a material having a large refractive index temperature coefficient is used as the material of the core (second core) in at least the modulation region (region where heat can be applied by the heater), thereby reducing power consumption. It becomes possible to do. At this time, considering the reduction of propagation loss, it is preferable to apply the material having a large refractive index temperature coefficient only to the core of the modulation region. Furthermore, since the core (first core) other than the modulation area (necessary area) uses a core made of a quartz-based material that has been proven in the past, it reduces power consumption and loss. Can be compatible. Also, the connection between the optical circuit including the core and the optical fiber is good. Furthermore, since the connection between the first core and the second core is performed via a spot size converter, loss due to mode field mismatch between the first core and the second core is reduced. Is possible. Therefore, it is possible to provide a thermo-optic light modulator that can realize reduction in power consumption and loss.

  In this embodiment, as shown in FIG. 5A, a second core 54 made of a material having a large absolute value of refractive index temperature coefficient is formed on a first core 53 made of a quartz-based material. Are stacked. The first core 53 and the second core 54 are surrounded by a lower clad 51 and an upper clad 52. Similarly, the same effect as described above can be expected even if the second core is provided inside the first core. However, in the thermo-optic light modulator using the thermo-optic effect, it is possible to reduce power consumption when the distance between the core and the heater is shorter. This is because when the same electric power is applied, the temperature of the core increases as the temperature is closer to the heater. Therefore, the shape in which the second core is laminated on the first core can bring the second core, which is a core to which heat should be applied, closer to the heater, so that power consumption can be further reduced. .

  On the other hand, when the second core is included in the first core as shown in FIGS. 5B and 5C, the mode field conversion loss in the spot size converter unit can be further reduced. There are benefits. FIG. 5B shows a shape in which the second core 56 is formed in the first core 55. 5C shows a shape in which the second core 57 is formed in the first core 57 as in FIG. 5B, but the second core 57 is formed on the ridge portion 51A of the lower clad 51. The core 58 is formed, and the first core 57 is formed so as to wrap the ridge 51 </ b> A and the second core 58.

  Further, in this embodiment, a spot size converter having a simple triangular taper structure is used as in the tapered portion 5, but the shape of the spot size converter is not necessarily an isosceles triangle, and there are many cases. It may be gradually narrowed (spread) in stages, or narrowed (spread) in a curve. That is, in the spot size converter according to the present embodiment, the mode field gradually changes from the first core to the second core, or from the second core to the first core, and the mode field after the transition. It suffices if it has a function that becomes an appropriate mode field.

  In the present embodiment, the second core is described on the premise that the refractive index is higher, but the second core has a larger dn / dT than the first core (other couplers, etc.). The second core may have a refractive index lower than that of the first core. In that case, the tapered portion, that is, the spot size converter may be provided not in the second core but in the first core.

  As a result of measuring the power consumption of the actually produced optical switch according to the present embodiment, the power required to rotate the phase by π is only 140 mW, which is 1 of the power consumption of 450 mW of the conventional switch having the same structure. The power was reduced to / 3. In addition, the insertion loss was reduced only by an increase of 0.2 dB from that produced by the prior art. This indicates that the problem of loss, which has been a problem in the prior art, can be solved by using this embodiment.

In the present embodiment has described a form using SiO _x as the second core, it was subjected to the experiments using SiON a oxynitride of TiO ₂ and silicon as the second core, the SiO _x The same effect as the case was obtained.

(Second Embodiment)
In the present embodiment, a conventional MZI circuit using a quartz-based material is manufactured, and a second core is manufactured using a mixed film of TiO ₂ and SiO ₂ on the arm waveguide which is the connecting portion. And the thermo-optic switch which connected the 1st core and the 2nd core via the spot size converter is produced. In the present embodiment, the second core is a mixed film of two materials of TiO ₂ and SiO ₂ , but even if other materials are added as additives, the same effect can be obtained.

Here, the mixed film of TiO ₂ and SiO ₂ refers to a material in which TiO ₂ fine particles having a crystalline particle size of nanometer size are dispersed and embedded in a SiO ₂ matrix. The size of the TiO ₂ crystal is preferably smaller in order to reduce scattering loss. In the above-described SiO ₂ film, the crystallinity of excessively added Si is not particularly limited, and an effect of increasing the absolute value of dn / dT can be obtained even in an amorphous state. However, the effect cannot be obtained simply by mixing. In the case of a mixed film of TiO ₂ and SiO ₂ , the TiO ₂ fine particles need to have a crystal structure in order to obtain the effect. As a method for forming such a film, TiO ₂ and SiO ₂ are attached to the substrate at the same time, and heat treatment is performed so that the crystalline TiO ₂ region and the amorphous SiO ₂ are phase-separated. be able to. Of course, this method is an example, and other methods may be used.

First, the refractive index temperature coefficient of the mixed film of TiO ₂ and SiO ₂ used in this embodiment will be described.
FIG. 6 shows the results of measuring the refractive index temperature coefficient (dn / dT) by preparing thin films with different TiO ₂ concentrations on the Si substrate. The mixed film is prepared by forming a mixed film on a Si substrate using a target of TiO ₂ and SiO ₂ at the same time using a sputtering method and then performing a heat treatment at 1000 ° C. The concentration of each sample is a value measured using an X-ray microanalyzer (EPMA: Electron Probe Micro-Analysis). The refractive index temperature coefficient was measured by measuring the refractive index of the sample from room temperature to 150 ° C. using a refractive index measuring device using a prism couple, and calculating the gradient. The slope was determined by fitting with a linear function.

According to the measurement results, as shown in FIG. 6, the dn / dT of normal SiO ₂ was 8.5 × 10 ⁻⁶ / ° C. (TiO ₂ = 0 mol%). Until the TiO ₂ concentration is about 50 mol%, there is almost no difference from the refractive index temperature coefficient of SiO ₂ , but when the TiO ₂ concentration increases to 50 mol% or more, the absolute value becomes negative, and when the concentration is further increased, the absolute value The value is also increasing. In 80 mol%, it was -60x10 < ^-6 > / degreeC. The absolute value of the refractive index temperature coefficient is 7 times larger than that of normal quartz.
That is, if a mixed film of 80 mol% TiO ₂ and SiO ₂ (also referred to as a TiO ₂ / SiO ₂ film) is used for the second core of the present invention, the power consumption is simply reduced to about 1/7. Is possible.

FIG. 7 is a schematic diagram of a thermo-optic switch using a TiO ₂ / SiO ₂ film as the second core according to the present embodiment. FIG. 7A is a top view of one MZI circuit, and FIG. 7B is an enlarged view of an arm waveguide portion indicated by a broken line in FIG. In both figures, the heater and wiring (electrode) are omitted in order to prevent the figure from becoming complicated. In FIG. 7A, the core is indicated by a thin line to make the drawing easier to see.

The TiO ₂ concentration of the film used for the core 4 as the second core is 72 mo1. The core 4 has a thickness of 0.8 μm and a width of 0.6 μm. The refractive index is 2.1. FIGS. 7A and 7B are almost the same as the structure shown in FIG. 3 of the first embodiment. In this embodiment, instead of the SiO _x film, the core 4 is made of TiO ₂ / A SiO ₂ film is used. Furthermore, the circuit fabricated in this embodiment is provided with heat insulating grooves 71 not used in the first embodiment on both side surfaces of the heater.

  The manufacturing method of the MZI circuit according to the present embodiment is almost the same as that of the first embodiment, and will not be described in detail here. In addition, the core 3 as the first core has the same dimensions as those of the first embodiment. In the present embodiment, the heat insulating groove 71 may be formed by manufacturing the MZI circuit by the manufacturing method described in the first embodiment and then removing the clad where the heat insulating groove 71 is to be formed by etching. . Here, the width of the heat insulating groove 71 is 50 μm, and the distance between each is 75 μm. The depth is 65 μm up to the substrate. For comparison, a similar heat insulating groove was also formed in the circuit manufactured in the first embodiment.

From FIG. 6, when the concentration of TiO ₂ is 72 mol%, dn / dT = −39 × 10 ⁻⁶ / ° C. is obtained as the refractive index temperature coefficient of the core 4. The problem is a phase difference generated between both arm waveguides constituting the MZI circuit, and the sign of dn / dT is not a problem for the MZI circuit.

The measurement results of the manufactured circuit according to this embodiment will be described below.
When an optical switch made only of a quartz-based material was measured for comparison, the effect of low power consumption was great even if only a heat insulating groove was formed, and the power required to rotate the phase by π was 52 mW. (In the case where the heat insulating groove is not formed (first embodiment), 100 mW or more). However, when a TiO ₂ / SiO ₂ film having a large dn / dT is used for the second core produced in this embodiment, the power consumption can be further reduced to 9 mW. The power consumption could be reduced to 17% compared to the case where no TiO ₂ / SiO ₂ film was used. At this time, the recognized excess loss was a very small value of about 0.3 dB. This is considered to be a loss in the spot size conversion unit.

As described above, by using a mixed film of TiO ₂ / SiO ₂ as the second core, an optical switch with low loss and low power consumption can be realized. At this time, in order to obtain an effect of reducing power consumption, the concentration of TiO ₂ / SiO ₂ needs to be at least 50 mol% or more. The refractive index of the glass containing 50 mol% or more and TiO ₂ becomes 1.8 or more and becomes high. Therefore, the core size becomes submicron size. Although it is possible to fabricate simple linear and curved waveguides with submicron size, producing a 3 dB coupler with a submicron core with a single material as in the conventional structure increases loss and increases manufacturing tolerance. However, there was a problem that the yield was low. As in this embodiment, by using only a necessary portion (lower part of the heater) as a TiO ₂ / SiO ₂ material using a spot size converter, not only can the coupler part be stably manufactured, but also the propagation loss of the circuit It can be made smaller.

(Third embodiment)
In the present embodiment, a case where an optical variable attenuator and an arrayed waveguide diffraction grating are integrated will be described.
In the present embodiment, the refractive index temperature coefficient (dn / dT) of the variable optical attenuator using the thermo-optic light modulator is at least a place where the thermo-optic effect is used, preferably only through the spot size converter. The second core using a large material is formed, and the 3 dB cobbler, arrayed waveguide diffraction grating, and single mode fiber constituting the other MZI circuit are connected to the first core made of the existing quartz-based material. It is made using.

  FIG. 8 is a diagram showing an optical circuit in which a thermo-optic light modulator and an arrayed waveguide diffraction grating are integrated on the same substrate according to the present embodiment. In FIG. 8, heaters and wiring (electrodes) are omitted in order to prevent the figure from becoming complicated. In FIG. 8, the core is indicated by a thin line for easy viewing of the drawing.

  In FIG. 8, a thermo-optic light modulator 81, a 5% tap circuit 82, and an arrayed waveguide diffraction grating 83 are formed on a substrate 80. In FIG. 8, each output terminal of the thermo-optic light modulator 81 is connected to each input terminal of the 5% tap circuit 82, and each output terminal of the 5% tap circuit 82 is an arrayed waveguide grating. 83 are connected to respective input terminals 84. The input end 84 is connected to one end of the slab waveguide 85A, and one end of a 16-channel array waveguide 86 is connected to the other end of the slab waveguide 85A. One end of the slab waveguide 85B is connected to the other end of the arrayed waveguide 86, and the output end 87 of the arrayed waveguide diffraction grating 83 is connected to the other end of the slab waveguide 85B. At this time, the material of the core (also referred to as a third core) included in the arrayed waveguide 86 is a material having a refractive index temperature coefficient smaller than that of the core 4 as the second core. The third core may be the same material as the first core or a different material.

The manufacturing method of the thermo-optic light modulator according to this embodiment is the same as the method shown in the first embodiment. First, on the Si substrate, a lower cladding 40 μm made of SiO ₂ , P ₂ O ₅ , B ₂ O ₃ and a core layer 6 μm serving as the first core (core 3) are deposited by using a flame deposition method. Here, the relative refractive index of the first core with respect to the cladding is set to 0.75% by adding GeO ₂ to the cladding. Next, a SiO ₂ (0 <x <2) film to be a second core (core 4) is deposited on the first core layer by using ECR-CVD. In this embodiment, the excess Si concentration is set to 40 at. %.

Then, by applying a resist on the SiO ₂ film, processing the SiO ₂ film to a circuit shape by using a general photolithography technique and reactive ion etching. As a second core, a waveguide having a tapered portion 5 having a triangular shape of 500 μm at both ends of a linear waveguide having a length of 2 mm and a width of 2 μm is later placed on the arm waveguide of the MZI circuit in the first core. It is produced so as to be a predetermined position.
Again, a resist is applied to the surface where the second core is etched, alignment is performed, photolithography is performed on the first core, and then etching is performed.

  Here, the optical circuit produced as the first core is an optical variable attenuator comprising an MZI circuit comprising two 3 dB directional connectors connected to both ends of a linear arm waveguide 3 mm as shown in FIG. And an arrayed waveguide diffraction grating. Here, an arrayed waveguide diffraction grating having 16 channels and 100 GHz intervals is manufactured. The channels of each arrayed waveguide grating are connected to a 16-channel variable optical attenuator via a tap circuit used for controlling the variable optical attenuator. These etched circuits are further embedded with 15 μm of overcladding glass, and a heater is formed on the clad above the portion where the second core of the arm portion of the optical variable attenuator exists using Cr on the clad. , Forming a gold wiring.

  In this embodiment, the heat insulation groove used as a method for reducing power consumption is not formed. This heat insulating groove forms a deep groove on both sides of the heater, has a function of suppressing heat diffusion in the horizontal direction of the substrate, and can reduce power consumption. Of course, it is possible to realize a thermo-optic position changer with lower power consumption by forming a heat insulating groove.

Further, here, a SiO _x film is used as a material having a large dn / dT used for the second core, but a material having an absolute value dn / dT several times larger than the quartz-based material used for the first core. If so, the effect is the same.

  If such an integrated circuit is manufactured only with a material having a large dn / dT as in the prior art, there are problems such as an increase in coupling loss with fibers, an increase in reflection, and an increase in propagation loss due to material loss. There was a problem that it could not be applied to. However, by manufacturing an integrated circuit as in the present embodiment and introducing a material having a large absolute value of dn / dT only through the spot size converter to the location where the thermo-optic effect of the optical variable attenuator works, An optical variable attenuator with low loss and low power consumption can be realized. Furthermore, the integrated arrayed waveguide multiplexer / demultiplexer can be integrated while maintaining the conventional characteristics.

  Further, in the present embodiment, not only the loss of the slab-array connection portion of the arrayed waveguide multiplexer / demultiplexer that occurs when the dn / dT absolute value is integrated with only a large material can be suppressed, but the 3 dB coupler can be stably provided. Etc., and even if they are integrated, they can be manufactured with a high yield. In the first place, the cost can be reduced without the need to separately manufacture and later connect the thermo-optic light modulator and the arrayed waveguide diffraction grating. In addition, since the arrayed waveguide diffraction grating portion is made of a quartz-based material that has been proven in the past, the method of making the central wavelength independent of temperature, which has been developed so far, can be applied as it is. Become.

(Fourth embodiment)
In this embodiment, a multichannel optical variable attenuator having a thermo-optic light modulator of the present invention having a TiO ₂ / SiO ₂ material as a second core and an arrayed waveguide diffraction grating are integrated on a single substrate. An optical circuit using a TiO ₂ / SiO ₂ material in order to achieve temperature independence of the center wavelength in an arrayed waveguide diffraction grating will be described.

  Therefore, at least a necessary part, that is, a lower part of the heater of the thermo-optic light modulator and an arrayed waveguide part that gives an optical path difference of the arrayed waveguide diffraction grating, preferably a material having a different dn / dT, preferably only at the necessary part. The core is arranged via a spot size converter, and other circuit parts are manufactured using a quartz-based material, thereby reducing the loss of the entire circuit.

  In order to achieve temperature independence in the arrayed waveguide diffraction grating without using a temperature controller such as a Peltier element, the right side of Equation (3) should be as close to 0 as possible. Therefore, in the present embodiment, the arrayed waveguide of the arrayed waveguide diffraction grating includes a core made of a material having a refractive index temperature coefficient having a large absolute value of the refractive index temperature coefficient and having a negative sign. is there.

Since the manufacturing method of the optical circuit of this embodiment is almost the same as the method shown in the first embodiment, the details are omitted here.
FIGS. 9A and 9B are diagrams illustrating a method for manufacturing an optical circuit in which a thermo-optic light modulator and an arrayed waveguide diffraction grating are integrated on the same substrate according to an embodiment of the present invention. is there. In FIGS. 9A and 9B, the heater and the wiring (electrode) are omitted in order to prevent the drawing from being complicated. Further, in FIGS. 9A and 9B, the core is indicated by a thin line in order to make the drawing easy to see.

First, on the substrate 91, the under cladding, the glass 92 of the first core after the deposition of (SiO ₂ with the addition of GeO _2), the _TiO 2 / SiO ₂ with a refractive index temperature coefficient having a negative absolute value greater A mixed film is deposited on the glass 92. Thereafter, a core 93 as a second core is formed in the lower part of the heater to be formed later so that both end portions become tapered portions 94. At the same time, a second core is formed together with the spot size converter in a part of the array waveguide diffraction grating 97 integrated on the substrate 91. That is, the core 95 as the second core is formed so that both end portions are tapered portions 96 (FIG. 9A). In FIG. 9A, taper portions 94 and 96 function as spot size converters.

  Further, the glass 92 is processed to form the first core 98 and the slab waveguide 99, which are then embedded in the clad, and the electric wiring including the heater is formed, so that the variable optical attenuator and the array conductor are formed on the same substrate. An optical circuit integrated with a waveguide diffraction grating can be obtained (FIG. 9B).

  Circuit evaluation was performed on the optical circuit in which the optical variable attenuator and the arrayed waveguide diffraction grating manufactured as described above were integrated. When the temperature dependence on the center wavelength of the arrayed waveguide diffraction grating was examined, only a change of 1 pm / ° C. was observed. This is 1/10 or less of an arrayed waveguide diffraction grating made of ordinary quartz. The power required to completely extinguish the optical variable attenuator was 9 mW. This was about one-sixth of the power required for the same structure fabricated with only a quartz-based waveguide. Regarding the loss, it was confirmed that the excess loss was a slight increase of 0.6 dB compared to the same structure manufactured only with the silica-based waveguide.

  As described above, conventionally, when an optical circuit in which an optical variable attenuator and an arrayed waveguide diffraction grating are integrated on a single substrate using only a single material, material loss is large. It has been difficult to produce a stable coupler by reducing the waveguide core size. For these problems, as produced in this embodiment, a core made of a material having a large negative refractive index temperature coefficient is disposed only in a necessary portion via a spot size converter, and other portions are proven. By using a quartz-based material, an optical circuit that achieves temperature dependence of the arrayed waveguide diffraction grating on the same substrate at the same time as low power consumption of the optical variable attenuator and low loss is achieved. It becomes feasible.

  As described above, according to one embodiment of the present invention, for the purpose of reducing the power consumption of the thermo-optic light modulator, thermo-optic light modulation is performed using a material having a larger absolute value of dn / dT than quartz glass. When the device is manufactured, the connection loss with the fiber increases and the propagation loss due to material loss increases. At least a portion using the thermo-optic effect, specifically, the lower portion of the heater, preferably the A waveguide made of a material having a large absolute value of dn / dT is installed only in the portion using a spot size converter. As a result, an increase in propagation loss due to a connection loss with a fiber and a material loss can be suppressed and power consumption can be reduced.

  Further, in the conventional technique, when only a material having a large absolute value of dn / dT is used and an array waveguide diffraction grating circuit and a circuit using a thermo-optic light modulator are integrated and manufactured at once, an array is obtained. It was difficult to make the waveguide grating circuit temperature independent. However, according to an embodiment of the present invention, a waveguide made of a material having a large absolute value of negative dn / dT can be formed via a spot size converter at least at a necessary place, preferably only at the necessary place. An optical attenuator using a thermo-optic optical modulator capable of low power consumption while maintaining the characteristics of an existing arrayed waveguide grating circuit and a temperature-independent arrayed waveguide grating circuit on the same substrate Accumulation is possible.

It is a bird's-eye view which shows the thermo-optic light modulator which concerns on one Embodiment of this invention. 1A is a cross-sectional view taken along line AA ′ in FIG. 1, FIG. 2B is a cross-sectional view taken along line BB ′ in FIG. 1, and FIG. 1C is a cross-sectional view taken along line CC ′ in FIG. FIG. 1 is a configuration diagram of a 1 × 2 optical switch using a thermo-optic light modulator according to an embodiment of the present invention. FIG. It is explanatory drawing of operation | movement of the spot size converter based on one Embodiment of this invention. (A)-(c) is a figure which shows the mode of arrangement | positioning with the 1st core and 2nd core based on one Embodiment of this invention. According to an embodiment of the present invention, in the mixed film of TiO ₂ and SiO _2, in the case of changing the addition concentration of TiO _2, a diagram illustrating a change in refractive index temperature coefficient. (A) is the schematic of the thermo-optic switch based on one Embodiment of this invention, (b) is the figure which expanded the arm waveguide part shown with the broken line of (a). It is a figure which shows the optical circuit which integrated the thermo-optic light modulator and the arrayed-waveguide diffraction grating on the same board | substrate based on one Embodiment of this invention. (A) And (b) is a figure explaining the manufacturing method of the optical circuit which integrated the thermo-optic light modulator and the arrayed-waveguide diffraction grating on the same board | substrate based on one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2 Cladding 3 1st core 4 2nd core 5 Tapered part 6 Heater 7 Wiring 8 Spot size converter

Claims (7)

A thermo-optic light modulator comprising a buried optical waveguide with a core buried in a cladding layer,
The first core,
The second core is formed of a material having a refractive index different from that of the first core and having a higher temperature coefficient of refractive index than that of the first core. The core,
A spot size converter connecting the first core and the second core;
A heater formed on the cladding layer to apply temperature to the second core,
The first core is made of a material having a smaller propagation loss than the second core,
The core includes the first core and the second core;
Among the first core and the second core, the spot size converter is formed in an optical connection portion between the core having a higher refractive index and the core having a lower refractive index,
A thermo-optic light modulator, wherein at least the second core is formed below a heater formed in the clad layer. A first 3 dB coupler formed on the input side;
A second 3 dB coupler formed on the output side,
2. The thermo-optic light modulator according to claim 1, wherein the first 3 dB coupler and the second 3 dB coupler are connected by the two buried optical waveguides.   3. The thermo-optic light modulator according to claim 1, wherein the first core is made of a quartz material containing B, P, and Ge. Said second core, Si, amorphous _{Si, SiO 2, SiO x (} 0 <x <2), according to claim 1 to 3, characterized in that it consists of a material including any one of TiO 2, SiON The thermo-optic light modulator according to any one of the above.   The thermo-optic light modulator according to any one of claims 1 to 3, wherein the second core is made of a material containing at least 50 mol% of Ti oxide. A thermo-optic light modulator according to claim 1;
An arrayed waveguide diffraction grating having an arrayed waveguide formed on the same substrate as the thermo-optic light modulator and including a core made of a material having a temperature coefficient of refractive index smaller than that of the second core. An optical circuit characterized by that. A thermo-optic light modulator according to claim 1;
An arrayed waveguide diffraction having an arrayed waveguide formed on the same substrate as the thermo-optic light modulator and including a core made of a material having a temperature coefficient of negative refractive index larger than that of the first core. With lattice,
An optical circuit, wherein spot size converters are formed at both ends of the arrayed waveguide.

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