JP2001272561A - Polarization independent waveguide type optical circuit - Google Patents

Abstract

(57) [Problem] To provide a polarization independent waveguide type optical circuit which completely eliminates polarization dependence and reduces reflected return light. SOLUTION: An intermediate portion between two connecting waveguides 9 and 10 has two curved waveguides 9B, 9E, 10B and 1 respectively.
0E or two curved waveguides and an S-shaped waveguide of the same shape consisting of straight waveguides 9C, 9F, 10C, and 10F connecting the curved waveguides.
One or two grooves are provided in the groove 11 dug so as to cross the waveguide, and the perpendicular to the light incidence surface of the polarization rotator 12 and the S-shaped waveguide are larger than 0 degree. an angle Θ _1.

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 used for construction of an optical communication system, an optical information processing apparatus, etc., and more particularly, to a polarization independent waveguide type optical circuit having no polarization dependency. It is about technology that is effective to apply.

[0002]

2. Description of the Related Art Conventionally, various optical components have been researched and developed with the progress of optical communication technology. Among them, a waveguide type optical component based on an optical waveguide on a flat substrate is the most important component. Has become. This is because the waveguide type optical component has a feature that it can be reproducibly mass-produced with a precision equal to or less than the light wavelength by photolithography technology and fine processing technology.

For example, a waveguide type Mach-Zehnder interferometer optical circuit is composed of two optical couplers on a substrate and two connecting waveguides connecting the two optical couplers. Various functions can be realized by controlling the optical path length difference between the optical paths and the phase state at the time of interference by the optical coupler, so that it is widely used and practically used.

FIG. 9 shows an example of a conventional waveguide type Mach-Zehnder interferometer optical circuit. FIG. 9A is a plan view, and FIG.
FIG. 10 is a cross-sectional view taken along line BB ′ of FIG.

[0005] The Mach-Zehnder interferometer optical circuit is shown in FIG.
As shown in FIG. 3, a first input waveguide 23 and a second input waveguide 24 formed on a clad 22 on a silicon substrate 21 are formed.
A first directional coupler 25 in which the first input waveguide 23 and the second input waveguide 24 are brought close to each other, a first output waveguide 26 and a second output waveguide 27, and a first output waveguide 26. A second directional coupler 28 in which the second output waveguide 27 is brought close to the first output waveguide 27;
It comprises a first connecting waveguide 39 and a second connecting waveguide 40 connecting the directional coupler 25 and the second directional coupler 28, and a thermo-optic phase shifter (thin film heater) 41. As a material of the optical waveguide, quartz glass produced by a flame deposition method is used. As shown in FIG. 9B, a cross section of a core having a cross section of 7 μm × 7 μm, for example, a first connection conductor is formed at the center of a 50 μm thick clad 22 deposited on a silicon substrate 21 as shown in FIG. This is a structure in which the waveguide 39 and the second connection waveguide 40 are buried. The refractive index difference between clad and core is 0.7
5%.

As the first directional coupler 25 and the second directional coupler 28, a 3 dB coupler having a branching ratio of 1: 1 is generally used. The light input from the input waveguide is split into two equal parts by the first directional coupler 25, and the first connection waveguide 39
And the second coupling waveguide 40. First connecting waveguide 39
The light that has propagated through the second coupling waveguide 40 joins and interferes in the second directional coupler 28. The amount of light output from the first output waveguide 26 and the light output from the second output waveguide 27 varies depending on the phase state of the light combined by the second directional coupler 28. For example, when the light having the wavelength λ incident from the first input waveguide 23 is bisected by the first directional coupler 25 and merged by the second directional coupler 28, the second directional coupler 28
If the phase difference between the two lights that are merged in step (1) is 0 or an integer multiple of the wavelength λ, the merged light is output from the second output waveguide 27.
On the other hand, if the phase difference between the two lights to be merged is an odd multiple of half a wavelength (λ / 2), the merged light is output from the first output waveguide 26. In addition, the phase difference of the converging light is in an intermediate state, that is, the phase difference is neither 0 nor an integral multiple of the wavelength,
If the state is not an odd multiple of half a wavelength, light is output from both the first output waveguide 26 and the second output waveguide 27 at a ratio according to the state.

The first connecting waveguide 39 and the second connecting waveguide 40
In the Mach-Zehnder interferometer optical circuit in which the optical path length difference ΔL is 0 or about half the wavelength of the propagating light, as shown in FIG.
By providing a thin-film heater that operates as the thermo-optic phase shifter 41 on the optical fiber 9, the device can be operated as an optical attenuator or an optical switch.

FIG. 10 shows an optical output characteristic when the thermo-optic phase shifter of the Mach-Zehnder interferometer circuit having the optical path length difference ΔL = 0 is operated. When the thermo-optic phase shifter 41 is not operated, the light incident from the first input waveguide 23 is transmitted to the second output waveguide 2
7 to output. Therefore, a thermo-optic phase shifter (thin film heater)
When the first input waveguide 2 is heated by operating the first input waveguide 39 and effectively increasing the optical path length by increasing the refractive index of the first connection waveguide 39 by the thermo-optic effect,
A part of the light incident from 3 is output to the first connection waveguide 39. When the temperature of the first coupling waveguide 39 is adjusted and the optical path length difference ΔL becomes a half wavelength, the first input waveguide 23
From the first output waveguide 26. As described above, by using the thermo-optic phase shifter 41 to variably adjust the optical path length difference ΔL between the two coupled waveguides from 0 to half a wavelength, the optical waveguide can be operated as an optical attenuator.
If the optical path length difference ΔL between the two coupling waveguides is used only at two points of 0 and a half wavelength, it operates as a spatial optical switch. The power required for switching by the optical switch is about 0.5 W when the length of the thin film heater is 5 mm and the width is 50 μm.
At this time, the temperature rise of the thin film heater is about 30 ° C.

On the other hand, a Mach-Zehnder interferometer optical circuit in which the optical path difference ΔL between the two coupling waveguides is several μm or more operates as a wavelength filter. FIG. 11 is a plan view showing a schematic configuration of an asymmetric Mach-Zehnder interferometer optical circuit. In this asymmetric Mach-Zehnder interferometer optical circuit, a first input waveguide 23 and a second input waveguide 24 formed on a clad 22 on a silicon substrate 21 are placed close to each other. The first directional coupler 25 and the first
An output waveguide 26 and a second output waveguide 27; a second directional coupler 28 in which the first output waveguide 26 and the second output waveguide 27 are brought close to each other; a first directional coupler 25; It comprises a first connecting waveguide 39 and a second connecting waveguide 40 for connecting the sex coupler 28. As the waveguide material, quartz glass produced by a flame deposition method is used. Its cross section is a 50 μm thick clad 2 deposited on a silicon substrate 21.
In the approximate center of 2, a core having a dimension of 7 μm × 7 μm, for example, the first
This is a structure in which the connection waveguide 39 and the second connection waveguide 40 are buried. 0.75% relative refractive index difference between clad and core
It is. For example, of the light incident from the first input waveguide 23, the optical path length difference ΔL between the coupling waveguides is exactly 2 wavelengths.
Light having a wavelength of N times (N is an integer) is output from the second output waveguide 27, and is output from the first output waveguide 26 in the case of 2 (N-1) times. For example, the optical path length difference ΔL is about 1.4
In the case of 8 mm, it operates as a wavelength filter having a period of 200 GHz (approximately 1.6 nm in wavelength). FIG. 12 is a conceptual diagram of wavelength characteristics when used as a wavelength filter.

In the Mach-Zehnder interferometer optical circuit, as shown in FIGS. 10 and 12, the optical path length difference ΔL between the two coupling waveguides is zero or half, depending on the polarization state of light incident from the input waveguide. In the case of an optical attenuator or an optical switch of about the wavelength, the power and switching power of the thermo-optical phase shifter 41 that can obtain the same attenuation are different. In the case of a wavelength filter having a large ΔL, the period of the wavelength is also the peak position (wavelength).
Is also different. This is for the following reason.

That is, in the case of an optical attenuator or an optical switch having an optical path length difference ΔL of about 0 or a half wavelength, the optical effect of the thermal stress generated by the thermo-optical phase shifter differs depending on each polarization. is there. Specifically, it is generated by the following mechanism. When the thermo-optic phase shifter operates, the generated heat diffuses to the surroundings, but tends to propagate through a material having particularly high heat conductivity. The thermal conductivity in air is 2.61 × 10 −4 W / (c)
m · deg), and the thermal conductivity of the glass waveguide constituting the Mach-Zehnder interferometer optical circuit is 0.014 W / (cm).
Deg), the thermal conductivity of the silicon substrate is 1.70 W /
(cm · deg), the heat generated by the thermo-optic phase shifter 41 diffuses mainly through the glass waveguide and is transmitted to the silicon substrate 21. However, since the thermal conductivity of the silicon substrate 21 is extremely high, It flows almost perpendicular to 21 and does not diffuse much to the surroundings. Therefore, the connection waveguide immediately below the thermo-optic phase shifter 41 is efficiently heated, and the periphery thereof expands locally.

The glass waveguide is set at 1000 ° C. when the waveguide is manufactured.
After being heated as described above, during the process of cooling to room temperature, a strong compressive stress is applied to the silicon substrate in the horizontal direction due to a difference in thermal expansion coefficient from the silicon substrate. Therefore, the periphery of the locally expanded coupling waveguide receives a new compressive stress from the silicon substrate in the horizontal direction with respect to the substrate, so that the refractive index changes due to the compressive stress in addition to the refractive index change due to the temperature rise. A phenomenon in which the refractive index changes with a change in compressive stress is called a photoelastic effect, and is expressed by the following equation (1). [Equation 1] ΔnTE = (Δnx) = C1Δσxx + C2 (Δσyy +
Δσzz) ΔnTM = (Δny) = C1Δσyy + C2 (Δσxx +
Δσzz) In Equation 1, x is a direction horizontal to the silicon substrate, y is a direction perpendicular to the silicon substrate, z is a light guiding direction, and Δ
σxx, Δσyy, and Δσzz are the amounts of stress change in the x, y, and z directions, respectively, and the tensile widening force is a positive value. ΔnTE is the refractive index felt by light having an electric field component in the x direction horizontal to the silicon substrate (hereinafter referred to as TE mode light). ΔnTM is the refractive index felt by light having a magnetic field component in the x direction horizontal to the silicon substrate (hereinafter referred to as TM mode light). C1 and C2 are photoelastic coefficients of quartz glass, and C1 = −0.74 × 10−5 mm ² · k
g, C2 = −4.1 × 10 −5 mm ² · kg.

As can be seen from Equation 1, when a compressive stress is applied in a direction parallel to the silicon substrate, the stress change Δσxx
Occurs, and the refractive index of the glass waveguide increases. The amount of change in the refractive index at this time differs between the TE mode light and the TM mode light because the photoelastic coefficients C1 and C2 are different. The refractive index change ΔnTM of the TM mode light is larger than the refractive index change ΔnTE of the TE mode light. large. That is, when the thermo-optic phase shifter 41 is driven, the change in the refractive index of the TM mode light becomes larger than that of the TE mode light due to the change in the refractive index due to the local thermal stress in addition to the change in the refractive index due to the thermo-optic effect. Thus, the optical change of the TM mode light progresses faster than the TE mode light. The drive power of the thermo-optic phase shifter 41 having the same light output is smaller by about 4% in the TM mode light. Therefore,
When a Mach-Zehnder interferometer having an optical path length difference ΔL of two coupling waveguides of 0 is used as an optical variable attenuator, for example, when TE mode light is attenuated by 10 dB, TM mode light is attenuated by about 11.5 dB. As a result, the amount of attenuation differs depending on the change in the polarization plane of the light incident on the optical attenuator.

On the other hand, in a Mach-Zehnder interferometer having a large optical path length difference ΔL of the coupling waveguide, the effective optical path length difference between the coupling waveguides is expressed by the following equation: ΔL = Δl · n In Equation 2, Δl is the physical optical path length difference of the coupling waveguide, and n is the refractive index of the coupling waveguide. Due to the strong compressive stress in the x direction received from the silicon substrate 21, the refractive index of the waveguide is expressed as follows: nTE = n0 + ΔnTE nTM = n0 + ΔnTM In Equation 3, n0 is the refractive index of the waveguide when no stress is applied, ΔnTE, and ΔnTM are respectively
It is a refractive index change amount of the TE mode light and the TM mode light due to the stress obtained from Expression 1.

The refractive index change ΔnTE of TE mode light and T
Since the refractive index change ΔnTM of the M mode light is larger than the refractive index change ΔnTM of the TM mode light, the optical path length difference between the coupling waveguides is effectively larger in the TM mode light than in the TE mode light. When this optical circuit is used as a wavelength filter, for example, the wavelength λ at which light input from the first input waveguide 23 is output to the second output waveguide 27 satisfies the following expression: ΔL = 2Nλ. In Equation 4, N is an integer.

The refractive index change ΔnTE of TE mode light and T
Since the value of the refractive index change amount ΔnTM of the M mode light is different,
The output wavelength differs depending on the polarization state of the incident light.
Further, the period of the output wavelength, that is, the difference Δλ between the output wavelength and the cutoff wavelength is represented by the following equation (5): Δλ = λ ² / 2nΔl. For example, the physical optical path length difference Δ of the coupling waveguide
If l is 20.4 mm and the wavelength λ of the incident light is 1.55 μm, the wavelength period Δλ is 0.04 nm from Equation 5.
However, also here, since the refractive index differs depending on the polarization state, the period differs depending on the polarization state. For these reasons, for example, there is a problem that even if light of a certain wavelength can be separated in the TE mode, it cannot be separated by the TM mode light.

As a method for solving these problems, the center of two connecting waveguides of the Mach-Zehnder interferometer optical circuit is
There is a method of inserting a half (1/2) wavelength plate. As a result, the light incident in the TE mode is T
Since the light is converted into the M mode and the light incident in the TM mode is converted into the TE mode, the coupling waveguides have effectively the same length for both incident polarized lights. Therefore, the polarization dependence is eliminated.

FIG. 13 shows an example of a polarization-independent waveguide type Mach-Zehnder interferometer optical circuit in which a half-wave plate is inserted to eliminate polarization dependence. The Mach-Zehnder interferometer optical circuit shown in FIG. 13 includes a first input waveguide 23 and a second input waveguide 24 formed on a clad 22 on a silicon substrate 21, a first output waveguide 26 and a second output waveguide. 27, a first directional coupler 25 and a second directional coupler 28, first connecting waveguides 39A and 39B, and second connecting waveguides 40A and 40B.
From the thermo-optical phase shifters (thin film heaters) 41A and 41B and the thin film type 1/2 wave plate 32 inserted into the half wave plate insertion structure 31 formed at the center of the optical path of each coupling waveguide. It is configured. The first directional coupler 25 and the second directional coupler 28 use a 3 dB coupler.

The fabrication of the half-wavelength plate insertion structure 31 uses mechanical processing such as reactive ion etching and a dicing saw. After the thin-film half-wave plate 32 is inserted into the half-wave plate insertion structure 31, it is fixed with an optical adhesive or the like.
The half-wave plate may be a crystal such as calcite, but if a glass substrate holding the crystal is included, the loss is large because the thickness is about 100 μm. For this reason, it is common to use a thin film half-wave plate 32 which is a thin film obtained by stretching a polyimide film and having birefringence. As a result, although there is a slight increase in loss, the optical characteristics of the Mach-Zehnder interferometer optical circuit show the average value of the TE mode light and the TM mode light as shown in FIGS. Disappears.

Although the Mach-Zehnder interferometer optical circuit has been described above, similar effects can be obtained in other circuits, for example, an arrayed waveguide grating optical circuit. FIG.
FIG. 1 shows an example of a polarization independent array waveguide grating optical circuit in which the polarization dependence is eliminated. The input waveguide bundle 34, the output waveguide bundle 36, the first slab waveguide 35, the second slab waveguide 37, the half-wave plate insertion structure 31, the thin-film half-wave plate 3
2, a first array waveguide 42A and a second array waveguide 42B arranged between the first slab waveguide 35 and the second slab waveguide 37;

The first array waveguide 42A and the second array waveguide 42B have a constant optical path length difference ΔL between adjacent waveguides.
Is given. The light of a certain wavelength incident from the input waveguide bundle 34 is diffracted at the entrance of the first slab waveguide 35 and
The first array waveguide 42A extends through the slab waveguide 35 and
Is output to Light that has propagated through the first array waveguide 42A and the second array waveguide 42B reaches the second slab waveguide 37, and each array waveguide is given a constant optical path length difference ΔL between adjacent waveguides. Therefore, a phase difference corresponding to the optical path length difference is reached. The light that has entered the second slab waveguide 37 is diffracted and spreads, but the light output from each array waveguide interferes with each other, diffracts in the direction in which the wavefronts are aligned (diffraction angle) as a whole, and is connected at the connection with the output waveguide. Collect light. If an output waveguide is arranged at this position, light of that wavelength can be split. Since the speed of light is different depending on the wavelength, the phase difference given by the array waveguide is different, and the focusing position is different depending on the wavelength. That is, if the output waveguide bundle 36 in which the output waveguides are arranged at the condensing position of the light of each wavelength is connected to the second slab waveguide 37, light of a different wavelength is output from each output waveguide. be able to.

The effective optical path length difference ΔL of the array waveguide differs between TE mode light and TM mode light due to the compressive stress from the silicon substrate. Therefore, the wavelength output to a certain output waveguide differs depending on the polarization state. Therefore, by inserting a thin-film half-wave plate 32 in the middle of the array waveguide, the optical path length difference of the array waveguide is the same for both polarized lights, and the wavelength of the output light is polarization independent. Can be eliminated.

The method of eliminating the polarization dependence by inserting a half-wave plate is a method that can be applied to other optical circuits, such as a ring resonator and a directional coupler.

[0024]

However, in the prior art, in order to remove the polarization dependence by the thin-film half-wave plate 32, strictly speaking, the thin-film half-wave plate 32 is placed at the axially symmetric center of the circuit. It is necessary to insert the wave plate 32, and the optical circuit is generally laid out while maintaining axial symmetry. Therefore, the half-wave plate insertion structure 31 is formed perpendicular to each coupling waveguide. When the thin film half-wave plate 32 is inserted into the half-wave plate insertion structure 31 and fixed with an optical adhesive, the glass waveguide, the optical adhesive, and the optical adhesive
Due to the difference in the refractive index from the half-wave plate, a part of the light propagating in each coupling waveguide is reflected and returns to the input waveguide side. Hereinafter, the light returning to the input waveguide side will be referred to as reflected return light. For example, FIG. 17 shows the spectrum of the reflected return light in the arrayed waveguide grating optical circuit shown in FIG. From FIG. 17, it can be seen that a maximum of -35 dB light is reflected to the incident port. There is a problem that the reflected return light may adversely affect a system using the device. For example, when the reflected return light returns to the semiconductor laser, the output intensity of the laser fluctuates and the system becomes unstable.

On the other hand, the groove 31 for inserting the λ / 2 plate is
As shown in FIG. 16 and FIG. 16, the reflection return light can be reduced by forming it obliquely with respect to the axis of symmetry. However, in that case, there is a problem that the polarization dependence of the optical circuit cannot be completely eliminated because the axial symmetry is lost.

The present invention has been made in view of such a problem, and it is an object of the present invention to completely eliminate polarization dependence and reduce reflected return light. An object of the present invention is to provide a dependent waveguide type optical circuit.

[0027]

According to the present invention, in order to achieve the above object, the invention described in claim 1 comprises one or more input waveguides formed on a substrate, and A first optical coupler connected to an input waveguide, one or more output waveguides, a second optical coupler connected to the output waveguide, the first optical coupler and a second optical coupler In the center of the optical path of the coupling waveguide of the optical circuit composed of a plurality of coupling waveguides connecting the horizontal polarization light is converted to vertically polarized light, and the vertically polarized light is converted to horizontally polarized light. In a polarization independent waveguide type optical circuit provided with a polarization rotator that converts light into light, an intermediate portion of the plurality of coupling waveguides is two curved waveguides or two curved waveguides, respectively. The polarization rotator is composed of S-shaped waveguides of the same shape, which are linear waveguides connecting curved waveguides. Wherein is 1 or 2 provided dug groove so as to traverse the S-shaped optical waveguide,
A perpendicular line to the light incident surface of the polarization rotator and the S-shaped waveguide form an angle larger than 0 degree.

According to a second aspect of the present invention, there is provided a first directional coupler and a second directional coupler, wherein two optical waveguides formed on a substrate are close to each other, and In the Mach-Zehnder interferometer type optical waveguide circuit comprising a coupler and two coupling waveguides coupling the second directional coupler, the horizontally polarized light is vertically polarized light at the center of the optical path of the coupling waveguide. In a polarization independent waveguide type optical circuit provided with a polarization rotator for converting vertically polarized light into horizontally polarized light, an intermediate portion between the two coupling waveguides is Or two curved waveguides or an S-shaped waveguide having the same shape composed of two curved waveguides and a linear waveguide connecting the curved waveguides, and the polarization rotator is configured to control the S-shaped optical waveguide. One or two are provided in a trench dug across the wave path,
A perpendicular line to the light incident surface of the polarization rotator and the S-shaped waveguide form an angle larger than 0 degree.

According to a third aspect of the present invention, there is provided a first multi-mode interference coupler and a second multi-mode interference coupler each having two optical waveguides formed on a substrate approaching each other;
In the Mach-Zehnder interferometer type optical waveguide circuit comprising two coupling waveguides for coupling the first multi-mode interference coupler and the second multi-mode interference coupler, the horizontally polarized light is perpendicular to the center of the optical path of the coupling waveguide. In a polarization independent waveguide type optical circuit provided with a polarization rotator for converting polarized light into vertically polarized light and converting horizontally polarized light into horizontally polarized light, an intermediate portion between the two coupling waveguides Are each composed of two curved waveguides, or S-shaped waveguides of the same shape consisting of two curved waveguides and a straight waveguide connecting the curved waveguides, and the polarization rotator is One or two grooves are provided so as to traverse the S-shaped optical waveguide, and the perpendicular to the light incident surface of the polarization rotator and the S-shaped waveguide are less than 0 degree. It is characterized by forming a large angle.

The invention described in claim 4 is the same as the invention described in claim 2.
Or a thermo-optical phase shifter is provided on at least one of the coupling waveguides, and the thermo-optical phase shifter is separated into an input side and an output side of the polarization rotator. It is characterized by being provided.

According to a fifth aspect of the present invention, there is provided one or more input waveguides, a first slab waveguide through which light propagating through the input waveguide freely propagates, and a first slab waveguide. An array waveguide in which a plurality of waveguides are connected to a waveguide, and a constant optical path length difference is provided between adjacent waveguides, and the array waveguide is connected and propagated through the array waveguide A second slab waveguide through which light freely propagates;
At the center of the optical path of the array waveguide of the array waveguide grating circuit composed of one or more output waveguides connected to the second slab waveguide, horizontally polarized light is converted to vertically polarized light. In a polarization-independent waveguide type optical circuit provided with a polarization rotator for converting vertically polarized light into horizontally polarized light, the intermediate portion of the array waveguide has two curved waveguides. Or two S-shaped waveguides of the same shape consisting of two curved waveguides and a straight waveguide connecting the curved waveguides, wherein the polarization rotator traverses the S-shaped waveguide. One or more are provided in the trench dug in,
A perpendicular line to the light incident surface of the polarization rotator and the S-shaped waveguide form an angle larger than 0 degree.

The invention according to claim 6 is the first invention.
6. The invention according to any one of items 5 to 5, wherein an angle between a perpendicular to the light incident surface of the polarization rotator and the S-shaped waveguide is 3 to 10 degrees. .

The invention according to claim 7 is the first invention.
7. The invention according to any one of Items 6 to 6, wherein the linear waveguide connecting the curved waveguides has a tapered shape whose width changes in the longitudinal direction, and the width of the linear waveguide is maximum at the portion of the polarization rotator. It is characterized by having become.

The invention described in claim 8 is the first invention.
8. The invention according to any one of Items 7 to 7, wherein the polarization rotator is a half-wave plate, and an optical main axis of the half-wave plate makes an angle of 45 degrees with the waveguide substrate surface. It is characterized by being installed in.

The invention according to claim 9 is the first invention.
9. The invention according to any one of items 8 to 8, wherein the polarization rotator is a thin-film half-wave plate.

According to a tenth aspect of the present invention, in any one of the first to ninth aspects, the optical waveguide is a glass optical waveguide.

[0037]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIG. 1 is a plan view showing a schematic configuration of a polarization independent waveguide type optical circuit according to Embodiment 1 of the present invention.
FIG. 2 is a sectional view taken along line AA ′ of FIG.

1 and 2, reference numeral 1 denotes a silicon substrate, 2 denotes a clad, 3 denotes a first input waveguide, 4 denotes a second input waveguide, and 5 denotes a first direction functioning as a first optical coupler. 6 is a first output waveguide, 7 is a second output waveguide, 8 is a second directional coupler functioning as a second optical coupler, 9 is a first connection waveguide, 10 is a second coupling waveguide. Coupling waveguide, 1
1 is a half-wave plate insertion structure, 12 is a thin film half-wave plate functioning as a polarization rotator, 9A is a first developed waveguide, 9B
Is the first curved waveguide, 9C is the first straight waveguide, 9D is the second
Expanded waveguide, 9E is second curved waveguide, 9F is second linear waveguide, 9G and 9H are thermo-optic phase shifters (thin film heaters), 10
A is a third expanded waveguide, 10B is a third curved waveguide, 10C
FIG. 2 shows a third straight waveguide, 10D a fourth development waveguide, 10E a fourth curved waveguide, and 10F a fourth straight waveguide.
In the above, 13 is an optical main axis.

The polarization-independent waveguide type optical circuit of the first embodiment is a Mach-Zehnder interferometer optical circuit, and is shown in FIGS.
As shown in FIG. 1, a first input waveguide 3 and a second input waveguide 4 formed on a clad 2 on a silicon substrate 1 and a first input waveguide 3 and a second input waveguide 4 in which the first input waveguide 3 and the second input waveguide 4 are close to each other. A directional coupler 5, a first output waveguide 6 and a second output waveguide 7, a second directional coupler 8 in which the first output waveguide 6 and the second output waveguide 7 are brought close to each other, Directional coupler 5 and second
A first connecting waveguide 9 and a second connecting waveguide 10 for connecting the directional coupler 8; a first connecting waveguide 9 and a second connecting waveguide l;
Thin film type 1 inserted in half-wave plate insertion structure 11 penetrating 0
And a half-wave plate 12. A 3 dB coupler is used for the first directional coupler 5 and the second directional coupler 8.

The first connecting waveguide 9 includes a first expanded waveguide 9A connected to the first directional coupler 5, a first curved waveguide 9B connected to the first expanded waveguide 9A, and a first curved waveguide 9B. A first linear waveguide 9C located between the curved waveguide 9B and the half-wave plate insertion structure 11, a second expanded waveguide 9D connected to the second directional coupler 8, and a second expanded waveguide 9D And a second linear waveguide 9F located between the second curved waveguide 9E and the half-wave plate insertion structure 11.

Similarly, the second connecting waveguide 10 includes a third expanded waveguide 10A connected to the first directional coupler 5, a third curved waveguide 10B connected to the third expanded waveguide 10A, A third linear waveguide 10C located between the three-curve waveguide I0B and the half-wave plate insertion structure 11, a fourth deployed waveguide 10D connected to the second directional coupler 8, and a fourth deployed waveguide 10
A fourth curved waveguide 10E connected to D and a fourth straight waveguide 10F located between the fourth curved waveguide 10E and the 波長 wavelength plate insertion structure 11.

In the Mach-Zehnder interferometer optical circuit of the first embodiment, the conventional Mach-Zehnder interferometer optical circuit shown in FIG. 9 is divided into two by a half-wavelength plate insertion structure 11, and has the same radius and the same angle Θ1. First curved waveguide 9B and third curved waveguide 1
0B, the same radius and the same angle Θ1, but the direction of the center of rotation is the first curved waveguide 9B and the third curved waveguide 10B and 1B.
The second curved waveguide 9E and the fourth curved waveguide 10 differing by 80 degrees
E, and the first straight waveguide 9C, the third straight waveguide 10C, the second straight waveguide 9F, and the fourth straight waveguide 10
It can be said that the structure is connected by F.

The first curved waveguide 9B, the first straight waveguide 9C, the second curved waveguide 9E in the middle part of the first connecting waveguide 9,
An S-shaped waveguide constituted by the second straight waveguide 9F;
The S-shaped waveguide composed of the third curved waveguide 10B, the third straight waveguide 10C, the fourth curved waveguide 10E, and the fourth straight waveguide 10F in the middle portion of the coupling waveguide 10 has the same shape. , The optical path length of each is equal. Therefore, the optical path length difference ΔL between the first connection waveguide 9 and the second connection waveguide 10 is equal to the first expansion waveguide 9A and the third expansion waveguide 10
A, given by the deployment distance of the second deployment waveguide 9D and the fourth deployment waveguide 10D.

Each waveguide is a silica-based optical waveguide produced by a flame deposition method and reactive ion etching. As shown in FIG. 2, the cross section is approximately 7 μm at substantially the center of the 50 μm thick clad 2 deposited on the silicon substrate 1.
It has a structure in which a core having a cross section of × 7 μm, for example, a second linear waveguide 9F or a fourth linear waveguide 10F is buried. The relative refractive index difference between the cladding 2 and the core is 0.75%.

As shown in FIG. 1, the half-wave plate insertion structure 11 includes a first connection waveguide 9 connecting the first directional coupler 5 and the second directional coupler 8 and a second connection waveguide. At the center of the optical path of the waveguide 10, the perpendiculars are the first straight waveguide 9 C, the second straight waveguide 9 F, the third straight waveguide 10 C, and the fourth straight waveguide 1.
0F and an angle Θ1, respectively.
The width is about 30 μm and the depth is about 100 μm. The half-wave plate insertion structure 11 is manufactured by machining such as reactive ion etching and a dicing saw. The thin-film half-wave plate 12 inserted into the half-wave plate insertion structure 11 has, for example, a thickness of about 20 in which a polyimide film is stretched in one direction.
A film having a thickness of about μm and having in-plane birefringence because the refractive index is high in the stretching direction, and the optical main axis 13 is operated as a half-wave plate, as shown in FIG.
It is installed so as to have an angle of 45 degrees with the silicon substrate 1.

The thin-film half-wave plate 12 inserted into the half-wave plate insertion structure 11 is fixed with an ultraviolet-curing or thermosetting optical adhesive. The thin-film type half-wave plate 12 has a first
After being divided into two equal parts by the directional coupler 5, the first expansion waveguide 9A, the first curved waveguide 9B, the first straight waveguide 9C of the first connection waveguide 9, and the second connection waveguide 10 3 deployment waveguide 10
A, the polarized liquid surface is changed so that the TE mode light having a wavelength of 1.55 μm transmitted through the third curved waveguide 10B and the third linear waveguide 10C becomes TM mode light, and the TM mode light becomes TE mode light. Rotate 90 degrees. Since the thin-film half-wave plate 12 is arranged at the center of each optical path of the first connecting waveguide 9 and the second connecting waveguide 10, the same optical path length is obtained for either mode light. The polarization dependence of the Mach-Zehnder interferometer optical circuit is eliminated.

Since the refractive indices of the coupling waveguide, the thin film half-wave plate 12 and the optical adhesive fixing the thin film half-wave plate 12 are generally different, the coupling waveguide Light that has propagated through the wave path is reflected at each interface.

FIG. 3 is a diagram for explaining the operation of the polarization-independent waveguide type optical circuit according to the first embodiment. The waveguide forms a perpendicular to the light incidence surface of the half-wave plate. The calculation result of the amount of reflected return light according to the angle Θ1 is shown.

The calculation results shown in FIG. 3 show that the waveguide has a core cross-sectional dimension of 7 μm × 7 μm, a relative refractive index difference between the cladding and the core of 0.75%, and the half-wave plate insertion structure 11 is filled. In this case, the substance was assumed to be air (refractive index: 1.0). Usually, an adhesive is filled at the boundary with the waveguide, but since the refractive index of the adhesive is close to the refractive index of the waveguide, the amount of reflected return light in this calculation is the value that is the largest. Is less than this calculation.

When the angle Θ1 between the thin-film half-wave plate 12 and the linear waveguide portion of each coupling waveguide is, for example, the reflection end is air and the angle is 0 degrees, there is 14.7 dB of reflected return light.
The angle Θ1 becomes smaller as the angle Θ1 becomes larger. Therefore, considering only from the viewpoint of reducing the reflected return light, it is desirable that the angle Θ1 is as large as possible. However, if Θ1 is too large, the polarization conversion efficiency of the thin-film half-wave plate 12 deteriorates. FIG. 4 shows the relationship between Θ1 and the polarization conversion efficiency. According to FIG. 4, when Θ1 is 10 degrees or more, the polarization conversion efficiency becomes 99.9% or less, and the polarization-independent characteristic rapidly deteriorates. For this reason, 3 is preferably 10 degrees. Further, by adopting such a structure, the thin film type half-wave plate insertion groove 11 becomes one, and can be formed by machining such as a dicing saw. Compared with a manufacturing method such as etching, a groove can be formed easily and in a short time, which is advantageous for mass production.

In this embodiment, the design is performed with Θ1 = 5 degrees. At this time, the reflected return light to the incident port was -39 dB with respect to the incident light. Compared to the case where the conventional Θ1 = 0 degree, the reflected return light was reduced by about 17 dB as compared with that of −22 dB.

[Embodiment 1-1] FIG. 5 is a plan view showing a schematic configuration of a polarization independent waveguide type optical circuit according to Embodiment 1-1 of the present invention. This embodiment is a polarization-independent asymmetric Mach-Zehnder interferometer optical circuit as a wavelength filter in which the optical path length difference ΔL between two coupling waveguides is several μm or more. Thin film type 1
The reduction of reflection by the half-wave plate is performed in the same manner as in the first embodiment.
The first curved waveguides 9B and 10B, the first linear waveguides 9C and 10C, and the second linear waveguides 9F and 10C are provided at the central portions of the connecting waveguide 9 and the second connecting waveguide 10, respectively.
F, is realized by providing the second curved waveguides 9E and 10E. The sum of the lengths of the first curved waveguide 9B, the first straight waveguide 9C, the second straight waveguide 9F, and the second curved waveguide 9E, the first curved waveguide 10B, and the first straight waveguide 1
OC, second straight waveguide 10F, second curved waveguide 10
Since the sum of the lengths of E is equal, the optical path length difference ΔL is given by the first deployed waveguides 9A and 10A and the second deployed waveguides 9D and 10D.

Further, the first straight waveguides 9C and 10C
C, by forming the second linear waveguides 9F and 10F into a tapered shape (T) shown in FIG.
, The field of the guided mode when the light is incident on the waveguide can be expanded, and as a result, the efficiency with which the reflected light generated by the thin-film half-wave plate 12 recombine with the waveguide can be reduced. That is, reflected light can be reduced by using the tapered waveguide shown in FIG. This tapered waveguide is a thin film type 1/2.
There is also an effect of reducing excess loss due to the wave plate insertion groove 11. In this embodiment, the reflected return light is reduced by -41 d by increasing the core width from 7 μm to 12 μm at the widest part.
B was able to be reduced. Approximately when the core width is not changed
2dB reflection suppression was realized.

As described above, according to the first embodiment, in the Mach-Zehnder interferometer optical circuit, two curves are provided at an intermediate portion of the connecting waveguide connecting the first directional coupler and the second directional coupler. A waveguide and its curved waveguide are connected. An S-shaped waveguide of the same shape consisting of a straight waveguide is provided, and the half-wave plate is inserted into the S-shaped waveguide to eliminate polarization dependence. A polarization independent Mach-Zehnder interferometer optical circuit with reduced reflected return light can be obtained. In the above-described embodiment, the directional couplers 5 and 8 are used as the optical couplers constituting the Mach-Zehnder interferometer optical circuit, but a multi-mode interference coupler may be used instead.

[Embodiment 2] FIG. 7 is a plan view showing a schematic configuration of a polarization independent waveguide type optical circuit according to Embodiment 2 of the present invention, wherein 14 is an input waveguide bundle, and 15 is a first slab. Waveguide, 16 an output waveguide bundle, 17 a second slab waveguide, 1
8 is an array waveguide, 18A is a first array waveguide, 18B
Is a first curved waveguide bundle, 18C is a first straight waveguide bundle, 18
D is the second array waveguide, 18E is the second curved waveguide bundle, l
8F is a second straight waveguide bundle.

The polarization-independent waveguide type optical circuit according to the second embodiment is an arrayed waveguide grating optical circuit. As shown in FIG. 7, the input waveguide bundle 14 is connected to the input waveguide bundle 14. The first slab waveguide 15, the output waveguide bundle 16, the second slab waveguide 17 connected to the output waveguide bundle 16, and the first slab waveguide 15 and the second slab waveguide 17 are connected. It comprises an array waveguide 18 and a thin-film half-wave plate 12 inserted into a half-wave plate insertion structure 11 penetrating the array waveguide 18.

The array waveguide 18 is the first slab waveguide 1
5, a first arrayed waveguide 18A connected to the first array waveguide 5, a first curved waveguide bundle 18B connected to the first arrayed waveguide 18A,
A first linear waveguide bundle 18C located between the first curved waveguide bundle 18B and the half-wave plate insertion structure 11, a second array waveguide 18D connected to the second slab waveguide 17, 2
Second curved waveguide bundle 18 connected to array waveguide 18D
E, the second curved waveguide bundle 18E and the half-wave plate insertion structure 1
The second linear waveguide bundle 18F is located between the first and second linear waveguide bundles 18F.

The first arrayed waveguide 18A and the first curved waveguide bundle 18B, the connection points of the first curved waveguide bundle 18B and the first straight waveguide bundle 18C, and the second arrayed waveguide 18
D, second curved waveguide bundle 18E, second curved waveguide bundle 18E
The connection points of the second straight waveguide bundle 18F and the second straight waveguide bundle 18F are shown in FIG.
As shown in FIG.
They are on the same straight line parallel to the wave plate 12. The half-wave plate insertion structure 11 is manufactured by machining such as etching or dicing saw.

The arrayed waveguide grating optical circuit of the second embodiment has a structure for reducing the reflected return light from the thin-film half-wave plate 12 in the same manner as the Mach-Zehnder interferometer optical circuit of the first embodiment. That is, the arrayed waveguide grating optical circuit shown in FIG. 9 is separated by the half-wave plate insertion structure 11, and the first curved waveguide bundle 18B, the second curved waveguide bundle 18E, and the first
It can be said that the structure is connected by an S-shaped waveguide composed of the linear waveguide bundle 18C and the second linear waveguide bundle 18F.

[0060] The first curved waveguide bundle 18B and curved waveguide constituting the second curved waveguide bundle 18E has all the same radius of curvature and the angle theta _2, the curve of the first curved waveguide bundle 18B second
The direction of the center of rotation with respect to the curve of the curved waveguide bundle 18E is 180.
Different degrees. Further, the straight waveguides forming the first straight waveguide bundle 18C and the second straight waveguide bundle 18F all have the same length, and the perpendiculars thereof are the half-wave plate insertion structure 11 and the thin-film half-wavelength. make the plate 12 and the same angle Θ _2.
That is, the first array waveguide 18A and the second array waveguide 1
Each of the waveguides sandwiched between 8Ds has the same S-shaped shape, and has the same optical path length. Therefore, the array waveguide 1 sandwiched between the first slab waveguide 15 and the second slab waveguide 17
Each of the waveguides 8 has an optical path length difference ΔL with an adjacent waveguide, and has the same characteristics as the arrayed waveguide grating optical circuit shown in FIG.

The angle Θ2 between the perpendicular of the thin-film half-wave plate 12 and each of the straight waveguides constituting the first straight waveguide bundle 18C and the second straight waveguide bundle 18F is 3 to 1 as in the first embodiment.
0 degrees is preferred. In the present embodiment, the design is performed so that Θ2 = 5 degrees. FIG. 8 shows the reflection spectrum of the arrayed waveguide grating optical circuit in this case. The reflection is about -50 dB at the maximum, and it can be seen that the reflection suppression of 15 dB can be realized as compared with -35 dB (see FIG. 16) of the related art. Further, by adopting such a structure, the thin film type half-wave plate insertion groove 11 becomes one, and can be formed by machining such as a dicing saw. Therefore, as in the first embodiment, a groove can be formed easily and in a short time as compared with a manufacturing method such as etching, which is advantageous for mass production.

As described above, according to the second embodiment, in the arrayed waveguide grating optical circuit, two intermediate waveguides are provided at the intermediate portion of the arrayed waveguide between the first slab waveguide and the second slab waveguide. Equipped with an S-shaped waveguide of the same shape consisting of a curved waveguide and a straight waveguide connecting the curved waveguides, and inserting a half-wave plate into the S-shaped waveguide to eliminate polarization dependence In addition, a polarization-independent arrayed waveguide grating optical circuit with reduced reflected return light can be obtained.

Although the present invention has been described in detail with reference to the embodiments, the present invention is not limited to the above-described embodiments, and may be variously modified without departing from the gist thereof. Of course. For example, in the first and second embodiments, the ／ wavelength plate insertion structure 11 is formed so as to cross each straight waveguide of the S-shaped waveguide.
The S-shaped waveguide is connected to a curved waveguide by omitting the straight waveguide, and the half-wave plate insertion structure 1 is connected to the connection position of the curved waveguide.
1 may be formed. In the embodiment, the thin-film half-wave plate 12 is inserted at a physical intermediate point of the optical circuit because the optical circuit has a symmetrical structure in the direction in which light propagates. If the reference numeral 12 is within the range of the S-shaped waveguide 18, the optical characteristic does not change even if it is moved in the horizontal direction in FIG. For this reason, it is not an essential condition of the present invention that the half-wave plate insertion groove 11 and the thin-film half-wave plate 12 are located at a physical intermediate point of the optical circuit.

[0064]

As described above, according to the present invention, in a waveguide type optical circuit, it is possible to completely eliminate polarization dependence and reduce reflected return light.

[Brief description of the drawings]

FIG. 1 is a plan view showing a schematic configuration of a polarization independent waveguide type optical circuit according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along line A-A 'of FIG.

FIG. 3 is a diagram for explaining the operation of the polarization independent waveguide type optical circuit according to the first embodiment.

FIG. 4 is a diagram illustrating the dependence of the polarization conversion efficiency of a thin-film half-wave plate on the incident angle Θ1.

FIG. 5 is a plan view showing a schematic configuration of a polarization independent waveguide type optical circuit of Example 1-1.

FIG. 6 is a plan view showing a schematic configuration of a polarization independent waveguide type optical circuit using a tapered waveguide of Example 1-1.

FIG. 7 is a plan view illustrating a schematic configuration of a polarization independent waveguide type optical circuit according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a reflection spectrum in Example 2.

9A and 9B are diagrams showing a conventional Mach-Zehnder interferometer optical circuit, where FIG. 9A is a plan view and FIG. 9B is a cross-sectional view.

FIG. 10 is a diagram illustrating characteristics of a conventional Mach-Zehnder interferometer optical circuit.

FIG. 11 is a plan view of an asymmetric Mach-Zehnder interferometer optical circuit.

FIG. 12 is a diagram illustrating characteristics of a conventional Mach-Zehnder interferometer optical circuit.

FIG. 13 is a plan view of a conventional polarization-independent Mach-Zehnder interferometer optical circuit.

FIG. 14 is a plan view of a Mach-Zehnder interferometer optical circuit in which reflected return light is suppressed.

FIG. 15 is a plan view of a conventional polarization-independent arrayed waveguide grating optical circuit.

FIG. 16 is a plan view of an arrayed waveguide grating optical circuit in which reflected return light is suppressed.

17 is a diagram showing a reflection spectrum of the conventional arrayed waveguide grating optical circuit of FIG.

[Explanation of symbols]

 1,21 silicon substrate 2,22 cladding 3,23 first input waveguide 4,24 second input waveguide 5,25 first directional coupler 6,26 first output waveguide 7,27 second output waveguide 8, 28 Second directional coupler 9 First connecting waveguide 9A First developing waveguide 9B First curved waveguide 9C First straight waveguide 9D Second developing waveguide 9E Second curved waveguide 9F Second straight waveguide Waveguide 9G Thermo-optic phase shifter (thin film heater) 9H Thermo-optic phase shifter (thin film heater) 10 Second connection waveguide 10A Third development waveguide 10B Third curve waveguide 10C Third linear waveguide 10D Fourth development guide Waveguide 10E Fourth curved waveguide 10F Fourth straight waveguide 11, 31 1/2 wavelength plate insertion structure 12, 32 Thin film type 1/2 wavelength plate 13 Optical main axis 14, 34 Input waveguide bundle 15, 35 First slab Waveguide 16, 36 outputs Waveguide bundles 17, 37 Second slab waveguide 18 Array waveguide 18A First array waveguide 18B First curved waveguide bundle 18C First straight waveguide bundle 18D Second array waveguide 18E Second curved waveguide bundle 18F Second Linear waveguide bundles 19, 19A, 19B First connection waveguide 20, 20A, 20B Second connection waveguide 21, 21A, 21B Thermo-optic phase shifter 22A First array waveguide 22B Second array waveguide 39, 39A, 39B First connecting waveguide 40, 40A, 40B Second connecting waveguide 41, 41A, 41B Thermo-optic phase shifter 42A First array waveguide 42B Second array waveguide

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akira Himeno 2-3-1 Otemachi, Chiyoda-ku, Tokyo Within Nippon Telegraph and Telephone Corporation (72) Inventor Yoshinori Hibino 2-3-1, Otemachi, Chiyoda-ku, Tokyo No. 1 Nippon Telegraph and Telephone Corporation

Claims (10)

[Claims] 1. An input waveguide formed on a substrate, one or more input waveguides, a first optical coupler connected to the input waveguide, one or more output waveguides, and one or more output waveguides. An optical circuit comprising a second optical coupler connected to the optical path and a plurality of connecting waveguides connecting the first optical coupler and the second optical coupler has a horizontal polarization center at the center of the optical path of the connecting waveguide. In the polarization independent waveguide type optical circuit provided with a polarization rotator for converting wave light into vertically polarized light and converting vertically polarized light into horizontally polarized light, The middle part of the waveguide is composed of two curved waveguides, or an S-shaped waveguide of the same shape composed of two curved waveguides and a straight waveguide connecting the curved waveguides, One or two devices are provided in a groove dug across the S-shaped optical waveguide, Serial and normal to the incident surface of the light polarization rotator the S-shaped waveguide and the polarization independent waveguide type optical circuit, characterized in that it forms an angle greater than 0 degrees. 2. A first directional coupler and a second directional coupler formed by approaching two optical waveguides formed on a substrate, and the first directional coupler and the second directional coupler. In the Mach-Zehnder interferometer type optical waveguide circuit composed of two coupling waveguides connecting the optical waveguides, the horizontally polarized light is converted into vertically polarized light, and the vertically polarized light is Is a polarization independent waveguide type optical circuit provided with a polarization rotator for converting to horizontally polarized light, wherein the intermediate portion between the two connecting waveguides is two curved waveguides or two Of the same shape consisting of a curved waveguide and a straight waveguide connecting the curved waveguides, and the polarization rotator is dug so as to cross the S-shaped optical waveguide. One or two grooves are provided in the groove, and the perpendicular to the light incident surface of the polarization rotator and the S-shaped A polarization independent waveguide type optical circuit, wherein the waveguide and the waveguide form an angle larger than 0 degree. 3. A first multi-mode interference coupler and a second multi-mode interference coupler in which two optical waveguides formed on a substrate are close to each other, and the first multi-mode interference coupler and the second multi-mode interference coupler. At the center of the optical path of the coupling waveguide of the Mach-Zehnder interferometer type optical waveguide circuit composed of two coupling waveguides, the horizontally polarized light is converted into vertically polarized light, and the vertically polarized light is In a polarization independent waveguide type optical circuit provided with a polarization rotator for converting to horizontally polarized light, an intermediate portion between the two coupling waveguides may be two curved waveguides or two curved waveguides, respectively. A groove formed by an S-shaped waveguide having the same shape and composed of a curved waveguide and a linear waveguide connecting the curved waveguides, wherein the polarization rotator is dug so as to cross the S-shaped optical waveguide; One or two are provided in the polarization rotator. Polarization-independent waveguide type optical circuit normal to the incident surface and the S-shaped waveguide, characterized in that it forms an angle greater than 0 degrees. 4. A thermo-optical phase shifter is provided on at least one of the coupling waveguides, and the thermo-optical phase shifter is provided separately on an input side and an output side of the polarization rotator. 4. The polarization-independent waveguide type optical circuit according to claim 2, wherein: 5. One or more input waveguides, a first slab waveguide through which light propagating through the input waveguide freely propagates, and a plurality of waveguides connected to the first slab waveguide. An array waveguide which is a waveguide and has a constant optical path length difference between adjacent waveguides, and a second slab waveguide to which the array waveguide is connected and light propagated through the array waveguide freely propagates. In the center of the optical path of the array waveguide of the array waveguide grating circuit composed of one or more output waveguides connected to the second slab waveguide, the horizontally polarized light is vertically polarized. In the polarization-independent waveguide type optical circuit provided with a polarization rotator that converts light into light and converts vertically polarized light into horizontally polarized light, the intermediate portion of the array waveguide has two Connecting a curved waveguide or two curved waveguides with one another It is constituted by an S-shaped waveguide of the same shape composed of a linear waveguide, and the polarization rotator is provided in one or a plurality of grooves dug so as to cross the S-shaped waveguide, A polarization independent waveguide type optical circuit, wherein a perpendicular to the light incident surface of the polarization rotator and the S-shaped waveguide form an angle larger than 0 degrees. 6. The optical system according to claim 1, wherein an angle between a perpendicular to the light incidence surface of the polarization rotator and the S-shaped waveguide is 3 to 10 degrees. A polarization independent waveguide type optical circuit as described in the above. 7. A linear waveguide connecting the curved waveguides has a tapered shape whose width changes in a longitudinal direction,
7. The polarization-independent waveguide type optical circuit according to claim 1, wherein the width of the polarization rotator is maximized. 8. The polarization rotator is a half-wave plate, and the optical main axis of the half-wave plate is in contact with the waveguide substrate surface.
The polarization-independent waveguide type optical circuit according to any one of claims 1 to 7, wherein the optical circuit is installed at an angle of degrees. 9. The polarization-independent waveguide type optical circuit according to claim 1, wherein the polarization rotator is a thin-film half-wave plate. 10. The polarization-independent waveguide type optical circuit according to claim 1, wherein the optical waveguide is a glass optical waveguide.