JP6771606B2 - Optical wavelength filter - Google Patents

Description

The present invention relates to, for example, an optical wavelength filter that performs combined and demultiplexing of light, which can be used for transmission using a plurality of different wavelengths in one optical fiber.

In recent years, a passive optical network (PON: Passive Opticial Network) has become the mainstream as a subscriber optical access system. In a PON, one station-side device (OLT: Optical Line Thermal) and a plurality of subscriber-side devices (ONU: Optical Network Unit) are connected via an optical fiber and a star coupler, and one OLT is connected to a plurality of units. Shared by ONU. In PON, the optical signal wavelength used for downlink communication and the optical signal wavelength used for uplink communication are set so that the downlink communication from OLT to ONU and the uplink communication from ONU to OLT do not interfere with each other. I'm wrong.

Therefore, a combined demultiplexing element is required to demultiplex and combine optical signals having different wavelengths used for downlink communication and uplink communication. In general, OLTs and ONUs space an optical wavelength filter, a photodiode (PD: Photodiode), and a laser diode (LD: Laser Diode) as a demultiplexing element in order to realize a function of transmitting and receiving optical signals having different wavelengths. It is composed by combining.

In order to spatially couple, an alignment work for aligning the optical axis between the optical wavelength filter, PD, and LD is required. On the other hand, in order to eliminate the work for aligning the optical axes, an optical wavelength filter configured by using a waveguide has been developed. Further, in forming this optical wavelength filter, a silicon (Si) waveguide using a silicon-based material as a waveguide material has attracted attention because of its excellent miniaturization and mass productivity (see, for example, Patent Documents 1 to 5). ).

In the Si waveguide, an optical waveguide core, which is substantially a light transmission path, is formed using Si as a material. Then, a clad made of, for example, silica, which has a lower refractive index than Si, covers the periphery of the optical waveguide core. With such a configuration, the difference in refractive index between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, it is possible to realize a small curved waveguide in which the bending radius is reduced to, for example, about 1 μm. Therefore, it is possible to create an optical circuit having the same size as an electronic circuit, which is advantageous for miniaturization of the entire optical device.

Further, in the Si waveguide, it is possible to divert the manufacturing process of a semiconductor device such as CMOS (Complementary Metal Oxide Semiconductor). Therefore, it is expected to realize photoelectric fusion (silicon photonics) in which an electronic function circuit and an optical function circuit are collectively formed on a chip.

By the way, in a PON using a wavelength division multiplexing (WDM) method, a different reception wavelength is assigned to each ONU. The OLT generates a downlink light signal to each ONU at a transmission wavelength corresponding to the reception wavelength of the destination, and multiplexes and transmits these. Each ONU selectively receives an optical signal having a reception wavelength assigned to itself from the downlink light signals multiplexed at a plurality of wavelengths. In the ONU, an optical wavelength filter is used to selectively receive the downlink light signal of each reception wavelength. Then, a technique of constructing the optical wavelength filter by the above-mentioned Si waveguide has been realized.

Examples of the optical wavelength filter using the Si waveguide include those using a Mach-Zehnder interferometer and those using an array waveguide grating. Further, as an optical wavelength filter using a Si waveguide, a ring resonator (for example, see Patent Documents 6 to 8), a grating type (for example, see Patent Document 9), or a directional coupler type (for example, see Patent Document 10) light. There is a wavelength filter. These optical wavelength filters have the advantages of being easy to use because the output wavelength can be made variable and the element structure is simple.

U.S. Pat. No. 4,860,294 U.S. Pat. No. 5,764,826 U.S. Pat. No. 5,960,135 U.S. Pat. No. 7,072,541 Japanese Unexamined Patent Publication No. 08-163028 Japanese Unexamined Patent Publication No. 2003-215515 Japanese Unexamined Patent Publication No. 2013-03627 Japanese Unexamined Patent Publication No. 2006-278770 Japanese Unexamined Patent Publication No. 2006-330104 JP-A-2002-353556

In optical communication, the polarization state becomes unspecified because the signal light propagates over a long distance in the optical fiber. Therefore, a polarization conversion type or polarization independent type optical waveguide element is required as an optical wavelength filter for wavelength division multiplexing or wavelength separation used in optical communication.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is a grating type optical waveguide element having a simple structure, which is a polarization conversion type or polarization independent type optical wavelength. To provide a filter.

In order to achieve the above-mentioned object, the polarization conversion type optical wavelength filter of the present invention is provided in parallel to the upper surface of the support substrate, the support substrate, the clad formed on the support substrate, and the clad embedded in the clad. A polarization conversion unit is provided which includes a first optical waveguide core and a second optical waveguide core, and the first optical waveguide core and the second optical waveguide core are arranged in parallel at predetermined intervals.

In the polarization conversion unit, the widths of the first optical waveguide core and the second optical waveguide core are different from each other, and the second optical waveguide core is provided with antisymmetric gratings on the left and right, and is different from the second optical waveguide core. , The optical waveguide composed of the cladding around the second optical waveguide core is configured to be asymmetrical in the vertical direction. Of the light input to the input end of the first optical waveguide core, the light of one polarization of TE (Transverse Electrical) polarization and TM (Transverse Magnetic) polarization of a specific wavelength is converted to the other polarization. The light is output from the output end of the second optical waveguide core, and the other light is output from the output end of the first optical waveguide core.

According to a preferred embodiment of the polarization conversion type optical wavelength filter of the present invention, the polarization conversion unit is smaller in thickness than the second optical waveguide core and is integrally formed with the second optical waveguide core. Further provided with a slab waveguide. In the polarization conversion unit, the side surface of the second optical waveguide core may be formed so as to be inclined with respect to the upper surface of the support substrate. Further, in the polarization conversion unit, the upper portion of the clad second optical waveguide core may be air.

Further, according to another preferred embodiment of the polarization conversion type optical wavelength filter of the present invention, in the polarization conversion unit, an electrode for a heater is placed on a cladding in a region where a second optical waveguide core is provided. Is provided.

Here, if the width of the first optical waveguide core is set to be larger than the width of the second optical waveguide core in the polarization conversion unit, the light input to the input end of the first optical waveguide core is specified. The TM-polarized light of wavelength is converted into TE-polarized light and output from the output end of the second optical waveguide core, and the other light is output from the output end of the first optical waveguide core. On the other hand, if the width of the first optical waveguide core is set to be smaller than the width of the second optical waveguide core in the polarization conversion unit, the specific wavelength of the light input to the input end of the first optical waveguide core The TE-polarized light is converted into TM-polarized light and output from the output end of the second optical waveguide core, and the other light is output from the output end of the first optical waveguide core.

Further, the polarization-independent optical wavelength filter of the present invention includes the first element and the second element, which are the polarization conversion type optical wavelength filters . The first polarization conversion unit, which is the polarization conversion unit included in the first element, and the second polarization conversion unit, which is the polarization conversion unit included in the second element, are arranged in series. The output end of the first optical waveguide core that is part of the first polarization converter unit, and an input end of the second optical waveguide core that is part of the second polarization converter unit is connected to the first polarization converter unit the input end of the second optical waveguide core that is part of the output end of the first optical waveguide core that is part of the second polarization conversion unit is connected.

According to another preferred embodiment of the optical wavelength filter of the polarization-independent, and the output end of the first optical waveguide core that is part of the first polarization converter unit, the second polarization converter unit width tapered waveguide between the input end of the second optical waveguide core that is part of is provided. Further, an input end of the second optical waveguide core, the width tapered waveguide between an output terminal of the first optical waveguide core that is part of the second polarization conversion unit provided that is part of the first polarization converter unit Has been done.

According to the present invention, the widths of the first optical waveguide core and the second optical waveguide core arranged in parallel at predetermined intervals are different from each other, and the second optical waveguide core is provided with antisymmetric gratings on the left and right sides. By doing so, a polarization conversion type optical wavelength filter that performs polarization conversion at a specific wavelength can be obtained.

Further, by arranging two of these polarization conversion type optical wavelength filters in series, a polarization-independent optical wavelength filter can be obtained.

It is a schematic diagram for demonstrating the 1st light wavelength filter. It is a schematic diagram (1) for demonstrating another configuration example of the 1st light wavelength filter. It is a schematic diagram (2) for demonstrating another configuration example of the 1st light wavelength filter. It is a figure (1) which shows the simulation result which evaluates the characteristic of the 1st light wavelength filter. It is a figure (2) which shows the simulation result which evaluates the characteristic of the 1st light wavelength filter. It is a schematic diagram for demonstrating the 2nd light wavelength filter.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the shape, size, and arrangement of each component are only schematically shown to the extent that the present invention can be understood. Further, although a preferable configuration example of the present invention will be described below, the material and numerical conditions of each component are merely suitable examples. Therefore, the present invention is not limited to the following embodiments, and many modifications or modifications can be made that can achieve the effects of the present invention without departing from the scope of the constitution of the present invention.

(1st light wavelength filter)
The light wavelength filter (hereinafter, the first light wavelength filter) according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining a first light wavelength filter. FIG. 1A is a schematic plan view showing a first light wavelength filter. Further, FIG. 1 (B) is a schematic end view of the first optical wavelength filter shown in FIG. 1 (A) cut out by the line I-I. Here, in FIG. 1A, the planar shape of the optical waveguide core is shown, and other components are omitted.

In the following description, for each component, the direction along the light propagation direction is defined as the length direction. Further, the direction along the thickness of the support substrate is defined as the thickness direction. Further, the direction orthogonal to the length direction and the thickness direction is defined as the width direction.

The first optical wavelength filter includes a support substrate 10, a clad 20, a first optical waveguide core 30, and a second optical waveguide core 40.

The support substrate 10 is made of, for example, a flat plate made of single crystal Si.

The clad 20 is formed by coating the upper surface 10a of the support substrate 10 on the support substrate 10. The clad 20 is formed of, for example, silicon oxide (SiO ₂ ) as a material.

The first optical waveguide core 30 and the second optical waveguide core 40 are embedded in the clad 20 in parallel with the upper surface 10a of the support substrate 10.

The first optical waveguide core 30 and the second optical waveguide core 40 are formed of, for example, silicon (Si), which has a refractive index (3.5) higher than the refractive index (1.45) of the cladding 20 of SiO _2. Has been done. As a result, the first optical waveguide core 30 and the second optical waveguide core 40 function as light transmission paths, and the light input to the first optical waveguide core 30 and the second optical waveguide core 40 is the first optical waveguide. It propagates in the propagation direction according to the planar shape of the core 30 and the second optical waveguide core 40.

The thickness of the first optical waveguide core 30 and the second optical waveguide core 40 is preferably 200 to 500 nm, which is a value that can achieve the single mode condition in the depth direction. For example, when used in the wavelength band of 1550 nm, the thickness of the first optical waveguide core 30 and the second optical waveguide core 40 can be set to 300 nm.

Here, in order to prevent the light propagating through the first optical waveguide core 30 and the second optical waveguide core 40 from escaping to the support substrate 10, the first optical waveguide core 30 and the second optical waveguide core 40 are supported substrates. It is preferably formed at a distance of at least 1 μm or more from 10.

The first light wavelength filter is provided with a polarization conversion unit 90. In the polarization conversion unit 90, the first optical waveguide core 30 and the second optical waveguide core 40 are arranged in parallel at predetermined intervals, and in this example, they are arranged in parallel and close to each other.

In the polarization conversion unit 90, the widths of the first optical waveguide core 30 and the second optical waveguide core 40 are different from each other. Here, the width of the first optical waveguide core 30 is set wider than the width of the second optical waveguide core 40. The TM polarization in symmetric mode is excited by a wide waveguide, here the first optical waveguide core 30. On the other hand, the TE polarization in the antisymmetric mode is excited by the narrow waveguide, here the second optical waveguide core 40.

The larger the difference in width between the first optical waveguide core 30 and the second optical waveguide core 40, the greater the degree of concentration of light on the excited waveguide.

A grating is formed on the second optical waveguide core 40 of the polarization conversion unit 90. The second optical waveguide core 40 of the polarization conversion unit 90 is configured to integrally include the base portion 42 and the protruding portions 44a and 44b. The base 42 is formed to have a constant width and extend along the light propagation direction, and the protrusions 44a and 44b are periodically formed on both side surfaces 42a and 42b of the base 42 with the same period Λ. A plurality of them are formed to form a so-called grating.

The protrusion 44a formed on one side surface (42a in this example) of the base 42 and the protrusion 44b formed on the other side surface (42b in this example) have a half cycle (that is, Λ/2). They are staggered. That is, at a certain position in the longitudinal direction, when the protrusion 44a is arranged on one side surface 42a, the protrusion 44b is not arranged on the other side surface 42b, and the protrusion 44a is arranged on one side surface 42a. When not, the protrusion 44b is arranged on the other side surface 42b. As a result, the grating is antisymmetric on the left and right.
Further, the base portion 42 and the protruding portions 44a and 44b are formed to have the same thickness.

At the bottom of the grating groove between the protrusions 44a or 44b adjacent to each other in the light propagation direction, a base 42 and Si having a thickness smaller than the protrusions 44a and 44b are formed as the slab waveguide 46. As a result, the optical waveguide having a grating, which is composed of the second optical waveguide core 40 of the polarization conversion unit 90 and the clad 20 around it, is asymmetrical in the vertical direction.

Note that FIG. 1A shows an example in which the slab waveguide 46 is formed in a place other than between the protrusions 44a or 44b, but the present invention is not limited to this. The slab waveguide 46 may be configured to exist only between the protrusions 44a or 44b. Alternatively, the width of the slab waveguide 46 may be increased.

A vertically asymmetric structure of the grating is required for diffraction between TE and TM polarizations. Further, by configuring the grating to be antisymmetric, the TE polarization in the basic mode and the TM polarization in the basic mode are diffracted. By selecting the combination of the TE polarization of the basic mode and the TM polarization of the basic mode, it is possible to suppress the occurrence of diffraction to other modes.

Here, a configuration in which the grating is made vertically asymmetric by providing the slab waveguide 46 has been described, but the present invention is not limited to this. An oblique side wall structure may be formed in which the side surface of the second optical waveguide core 40 is formed so as to be inclined with respect to the upper surface 10a of the support substrate 10. Further, the upper portion of the clad of the second optical waveguide core 40 on which the grating is formed may be air.

The phase matching conditions in the grating are as follows, where the Bragg wavelength is λ0, the grating period is Λ, the equivalent refractive index in the antisymmetric mode of TE polarization is NTE0, and the equivalent refractive index in the symmetry mode of TM polarization is NTM0. It can be represented by 1).

2π (NTE0 + NTM0) / λ0 = 2π / Λ (1)
If the design satisfies the above equation (1), diffraction of TE polarization in antisymmetric mode and TM polarization in symmetry mode occurs at a wavelength of λ0.

The first input waveguide 52 is connected to the input end 30a, which is one end of the first optical waveguide core 30. Further, the first output waveguide 62 is connected to the output end 30b, which is the other end of the first optical waveguide core 30. Similarly, the second input waveguide 54 is connected to the input end 40a, which is one end of the second optical waveguide core 40. Further, the second output waveguide 64 is connected to the output end 40b, which is the other end of the second optical waveguide core 40. The input end 40a of the second optical waveguide core 40 is arranged on the same side as the output end 30b of the first optical waveguide core 30, and the output end 40b of the second optical waveguide core 40 is the input end of the first optical waveguide core 30. It is arranged on the same side as 30a.

In order to smoothly connect the second input waveguide 54 and the second output waveguide 64 to the second optical waveguide core 40, the input end 40a side of the second optical waveguide core 40 and the slab waveguide 46 A width taper waveguide 76 having the same thickness as the slab waveguide 46 is provided on both sides of the output end 40b.

When the TM polarization of the basic mode is input from the first input waveguide 52 of the first optical wavelength filter, the symmetric basic mode in which light mainly exists in the first optical waveguide core 30 is excited. The light propagating through the first optical waveguide core 30 is diffracted and reflected in the antisymmetric basic mode of TE polarization in which light mainly exists in the second optical waveguide core 40 due to the action of the grating. The TE polarized wave propagating through the second optical waveguide core 40 is output from the second output waveguide 64. The light that is not diffracted and reflected propagates through the first optical waveguide core 30 as it is, and is output from the first output waveguide 62.

Another configuration example of the first optical wavelength filter will be described with reference to FIG. FIG. 2 is a schematic plan view showing another configuration example of the first light wavelength filter. In FIG. 2, the planar shape of the optical waveguide core is shown, and other components are omitted.

In the embodiment shown in FIG. 1, the width of the first optical waveguide core 30 is set to be larger than the width of the second optical waveguide core 40, but in the embodiment shown in FIG. 2, the polarization conversion unit 190 The width of the first optical waveguide core 130 is set smaller than the width of the second optical waveguide core 140.

When the TE polarization in the basic mode is input from the first input waveguide 52 of the other configuration example of the first optical wavelength filter, the light mainly exists in the first optical waveguide core 130, and the TE polarization is antisymmetric. The basic mode is excited. The light propagating through the first optical waveguide core 130 is diffracted and reflected by the action of the grating in the symmetrical basic mode of TM polarization in which the light mainly exists in the second optical waveguide core 140. The TM polarization propagating through the second optical waveguide core 140 is output from the second output waveguide 64. The light that is not diffracted and reflected propagates through the first optical waveguide core 130 as it is, and is output from the first output waveguide 62.

In the embodiment shown in FIGS. 1 and 2, an example in which the slab waveguide 46 is provided between the protrusions adjacent to each other in the light propagation direction, that is, the first optical waveguide core is provided on both sides of the second optical waveguide core. Although the configuration is shown not provided on both sides of the slab, as shown in FIG. 3, a slab waveguide may be provided between the first optical waveguide core and the second optical waveguide core. FIG. 3 is a schematic plan view showing another configuration example of the first light wavelength filter.

FIG. 3A shows an example in which the width of the first optical waveguide core 230 is set to be larger than the width of the second optical waveguide core 240 in the polarization conversion unit 290, and FIG. 3B shows an example. An example is shown in which the polarization conversion unit 390 sets the width of the first optical waveguide core 330 to be smaller than the width of the second optical waveguide core 340. In the embodiment shown in FIG. 3, both sides of the first optical waveguide cores 230 and 330 and the second optical waveguide cores 240 and 340, and between the first optical waveguide cores 230 and 330 and the second optical waveguide cores 240 and 340. , A slab waveguide 246 is provided.

The optical wavelength filters of the configuration examples shown in FIGS. 3A and 3B have the same configurations as the optical wavelength filters of the configuration examples shown in FIGS. 1 and 2, respectively, except for the configuration of the slab waveguide. Omit. Further, since the operation is the same, the description thereof will be omitted.

The electrode 70 is formed at a position that covers a part or all of the second optical waveguide core (grating) 40 via the clad 20. When the electrode 70 is formed on the grating, Joule heat is generated by passing an electric current through the electrode 70, and the refractive index of the grating can be changed by the thermo-optical effect. As a result, it is possible to change the wavelength that satisfies the phase matching condition in the grating.

Although FIG. 1B shows an example in which the electrode 70 is provided on the grating, the arrangement location of the electrode 70 is not limited to this. Any position may be used as long as the refractive index of the grating is changed by heat generation, and the grating can be arranged at any suitable location depending on the structure of the optical waveguide composed of the second optical waveguide core 40 and the clad 20.

(Production method)
This optical wavelength filter can be easily manufactured, for example, by using an SOI (Silicon On Insulator) substrate. Hereinafter, an example of a method for manufacturing the first optical wavelength filter of the configuration example shown in FIG. 1 will be described.

First, an SOI substrate is prepared in which a support substrate layer, a SiO ₂ layer, and a Si layer are sequentially laminated. Next, for example, dry etching is performed in two steps, and the Si layer is patterned to form a thick base portion and a protruding portion, and a portion of a grating groove having a small thickness. As a result, the structure in which the SiO ₂ layer is laminated on the support substrate layer as the support substrate 10, and the first optical waveguide core 30, the second optical waveguide core 40, and the slab waveguide 46 are further formed on the SiO ₂ layer. Can be obtained.

Next, for example, by using a CVD (Chemical Vapor Deposition) method, SiO ₂ is formed by covering the first optical waveguide core 30, the second optical waveguide core 40, and the slab waveguide 46 on the SiO ₂ layer. As a result, the clad 20 includes the first optical waveguide core 30, the second optical waveguide core 40, and the slab waveguide 46, and an optical waveguide element used as an optical wavelength filter is obtained.

Although an example of a Si waveguide has been described here, it can also be realized by using a compound semiconductor.

(Characteristic evaluation)
With reference to FIGS. 4 and 5, a simulation for evaluating the characteristics of the first optical wavelength filter performed using a three-dimensional FDTD (Finite Difference Time Domain) will be described.

In FIGS. 4 and 5, the horizontal axis represents the wavelength (nm) and the vertical axis represents the output intensity (dB).

FIG. 4A is a simulation result for the configuration example shown in FIG. In FIG. 4A, when TM polarization is input to the first input waveguide 52, the TM polarization output from the first output waveguide 62 is shown by a curve I, and is output from the second output waveguide 64. The TE polarization is shown by curve II.

Here, the width of the first optical waveguide core 30 is 500 nm, and the width of the second optical waveguide core 40 is 300 nm. The width of the second optical waveguide core 40 is the average width of the structure including the base portion 42 and the protruding portions 44a and 44b. Further, the thickness of the first optical waveguide core 30 and the second optical waveguide core 40 is 220 nm, and the thickness of the slab waveguide 46 is 150 nm.

The period Λ of the grating is 414 nm, and the digging of the grating, that is, the length of the protrusion in the width direction is 150 nm. Further, the distance between the first optical waveguide core 30 and the second optical waveguide core 40 is set to 400 nm. At this time, the distance between the slab waveguide 46 and the first optical waveguide core 30 is 325 nm.

As shown in FIG. 4A, in the configuration example shown in FIG. 1, TE polarization of a specific wavelength can be extracted from the second output waveguide 64.

FIG. 4B is a simulation result for the configuration example shown in FIG. In FIG. 4B, when TE polarization is input to the first input waveguide 52, the TE polarization output from the first output waveguide 62 is shown by a curve I, and is output from the second output waveguide 64. The TM polarization is shown by curve II.

Here, the width of the first optical waveguide core 130 is 300 nm, the width of the second optical waveguide core 140 is 500 nm, and the grating period Λ is 439 nm. Other conditions are the same as those in FIG. 4 (A), so the description thereof will be omitted.

As shown in FIG. 4B, in the configuration example shown in FIG. 2, TM polarization of a specific wavelength can be extracted from the second output waveguide 64. Comparing FIG. 4 (A) and FIG. 4 (B), in FIG. 4 (A) corresponding to the configuration example shown in FIG. 1, the peak height is higher than that in FIG. 4 (B), and the diffraction efficiency is increased. Is excellent. In addition, the occurrence of unnecessary peaks is suppressed.

FIG. 5A is a simulation result for the configuration example shown in FIG. 3A. In FIG. 5A, when TM polarization is input to the first input waveguide 52, the TM polarization output from the first output waveguide 62 is shown by a curve I, and is output from the second output waveguide 64. The TE polarization is shown by curve II.

Here, the width of the first optical waveguide core 230 is 500 nm, and the width of the second optical waveguide core 240 is 300 nm. Further, the thickness of the first optical waveguide core 230 and the second optical waveguide core 240 is 220 nm, and the thickness of the slab waveguide 246 is 150 nm.

The period Λ of the grating is 373 nm, and the digging of the grating, that is, the length of the protruding portion in the width direction is 150 nm. Further, the distance between the first optical waveguide core 230 and the second optical waveguide core 240 is set to 500 nm.

As shown in FIG. 5 (A), in the configuration example shown in FIG. 3 (A), the TE polarized light is extracted from the second output waveguide 64 at around 1600 nm as planned. It should be noted that the desired characteristics have not been obtained as the output from the first output waveguide 62. However, this configuration example of the first light wavelength filter can be used if it is limited to the polarization conversion of a specific wavelength.

FIG. 5B is a simulation result for the configuration example shown in FIG. 3B. In FIG. 5B, when TE polarization is input to the first input waveguide 52, the TE polarization output from the first output waveguide 62 is shown by a curve I, and is output from the second output waveguide 64. The TM polarization is shown by curve II.

Here, the width of the first optical waveguide core 330 is set to 300 nm, the width of the second optical waveguide core 340 is set to 500 nm, and the grating period Λ is set to 375 nm. Since the other conditions are the same as those in FIG. 5 (A), the description thereof will be omitted. The distance between the first optical waveguide core 330 and the second optical waveguide core 340 is set to 500 nm.

As shown in FIG. 5B, in the configuration example shown in FIG. 3B, the TM polarization is extracted from the second output waveguide 64 at around 1600 nm as planned. In addition, unnecessary diffraction peaks of different orders are seen on both sides of the main diffraction peak, but they are suppressed to some extent.

Further, the width of the slab waveguide is set wider in the configurations shown in FIGS. 3 (A) and 3 (B) than in the configurations shown in FIGS. 1 and 2. Therefore, the manufacturing becomes easy, for example, the degree of freedom for alignment when the dry etching is performed in two steps is increased.

As described above, the first light wavelength filter outputs light having a specific wavelength that satisfies the phase matching condition in grating by polarization conversion between TE polarization and TM polarization. Functions as a conversion type optical wavelength filter. Here, if the width of the slab waveguide is narrowed, the diffraction efficiency from TM polarization to TE polarization or from TE polarization to TM polarization can be improved, especially in grating. Further, if the width of the slab waveguide is widened, the manufacturing becomes easy.

(Second light wavelength filter)
A light wavelength filter (hereinafter referred to as a second light wavelength filter) according to a second embodiment of the present invention will be described with reference to FIG. FIG. 6 is a schematic diagram for explaining the second light wavelength filter. FIG. 6 is a schematic plan view showing a second light wavelength filter. Here, in FIG. 6, the planar shape of the optical waveguide core is shown, and other components are omitted.

The second light wavelength filter includes a first element 91 and a second element 92 connected in series. Here, the first element 91 and the second element 92 have the same configuration as each other, and the polarization conversion unit of the first optical wavelength filter described above is used. FIG. 6 shows a case where the polarization conversion unit of the first optical wavelength filter described with reference to FIG. 1 is used.

Here, the widths of the first optical waveguide cores 31 and 32 in the first element 91 and the second element 92 are set to be larger than the widths of the second optical waveguide cores 41 and 42.

The output end 31b of the first optical waveguide core 31 included in the first element 91 and the input end 42a of the second optical waveguide core 42 included in the second element 92 are connected. Further, the input end 41a of the second optical waveguide core 41 included in the first element 91 and the output end 32b of the first optical waveguide core 32 included in the second element 92 are connected to each other.

Between the first optical waveguide core 31 of the first element 91 and the second optical waveguide core 42 of the second element 92, and between the second optical waveguide 41 of the first element 91 and the first optical waveguide core 32 of the second element 92. In order to reduce the loss of light between them, it is preferable to use the width taper waveguide 74. In this case, the input end 42a of the second optical waveguide core 42 of the second element 92 is connected to the output end 31b of the first optical waveguide core 31 of the first element 91 via the width taper waveguide 74. Further, the output end 32b of the first optical waveguide core 32 of the second element 92 is connected to the input end 41a of the second optical waveguide core 41 of the first element 91 via the width taper waveguide 74. In order to prevent light from migrating between the waveguides in the same width of the width taper waveguide 74, a low refractive index region 78 is provided between the width taper waveguides 74, or a curved waveguide is connected. It is necessary to take measures such as widening the waveguide interval.

Further, the first input waveguide 52 is connected to the input end 31a of the first optical waveguide core 31 of the first element 91. Further, the first output waveguide 62 is connected to the output end 42b of the second optical waveguide core 42 of the second element 92. Similarly, the second input waveguide 54 is connected to the input end 32a of the first optical waveguide core 32 of the second element 92. Further, the second output waveguide 64 is connected to the output end 41b of the second optical waveguide core 41 of the first element 91.

In the second optical wavelength filter, when TE polarization and TM polarization in the basic mode are input from the first input waveguide 52, TM polarization of a specific wavelength is diffracted into TE polarization in the first element 91. Will be converted. The converted TE polarized wave propagates through the second waveguide core 41 of the first element 91 and is output from the second output waveguide 64. The TE polarized wave input to the first input waveguide 52 propagates through the first optical waveguide core 31 of the first element 91 as it is, and is input to the second optical waveguide core 42 of the second element 92.

In the second element 92, TE polarized waves having a specific wavelength are diffracted and converted into TM polarized waves. This converted TM polarization propagates through the first optical waveguide core 32 of the second element 92 and is sent to the second optical waveguide core 41 of the first element 91. The TM polarization sent to the first element 91 propagates through the second optical waveguide core 41 of the first element 91 and is output from the second output waveguide 64.

As a result, TE polarized waves and TM polarized waves having a specific wavelength are output from the second output waveguide 64. As described above, the second optical wavelength filter functions as a polarization-independent optical wavelength filter.

10 Support substrate 20 Clad 30, 31, 32, 130, 230, 330 1st optical waveguide core 40, 41, 42, 140, 240, 340 2nd optical waveguide core 42 Base 44a, 44b Protruding parts 46, 246 Slab waveguide 52 1st input waveguide 54 2nd input waveguide 62 1st output waveguide 64 2nd output waveguide 70 Electrodes 74,76 Width tapered waveguide 78 Low refractive index region 90, 190, 290, 390 Polarization converter 91 1st element 92 2nd element

Claims (9)

Support board and
The clad formed on the support substrate and
It includes a first optical waveguide core and a second optical waveguide core that are embedded in the clad and provided parallel to the upper surface of the support substrate.
A polarization conversion unit in which the first optical waveguide core and the second optical waveguide core are arranged in parallel at predetermined intervals is provided.
In the polarization conversion unit
The widths of the first optical waveguide core and the second optical waveguide core are different from each other.
The second optical waveguide core is provided with antisymmetric gratings on the left and right sides.
The optical waveguide composed of the second optical waveguide core and the clad around the second optical waveguide core is configured to be vertically asymmetrical.
Of the light input to the input end of the first optical waveguide core, one polarized light of TE (Transverse Electrical) polarization and TM (Transverse Magnetic) polarization of a specific wavelength becomes the other polarization. A polarization conversion type optical wavelength filter characterized in that it is converted and output from the output end of the second optical waveguide core, and the other light is output from the output end of the first optical waveguide core. In the polarization conversion unit
The polarization conversion type optical wavelength according to claim 1, further comprising a slab waveguide formed integrally with the second optical waveguide core and having a thickness smaller than that of the second optical waveguide core. filter. In the polarization conversion unit
The polarization conversion type optical wavelength filter according to claim 1, wherein the side surface of the second optical waveguide core is formed so as to be inclined with respect to the upper surface of the support substrate. In the polarization conversion unit
The polarization conversion type optical wavelength filter according to claim 1, wherein the upper portion of the second optical waveguide core of the clad is air. In the polarization conversion unit
The polarization conversion type according to any one of claims 1 to 3, wherein an electrode for a heater is provided on the clad in the region where the second optical waveguide core is provided. Optical wavelength filter. In the polarization conversion unit
The width of the first optical waveguide core is larger than the width of the second optical waveguide core.
Of the light input to the input end of the first optical waveguide core, TM-polarized light of a specific wavelength is converted into TE-polarized light and output from the output end of the second optical waveguide core. The polarization conversion type optical wavelength filter according to any one of claims 1 to 5, wherein the other light is output from the output end of the first optical waveguide core. In the polarization conversion unit
The width of the first optical waveguide core is smaller than the width of the second optical waveguide core.
Of the light input to the input end of the first optical waveguide core, TE-polarized light of a specific wavelength is converted into TM-polarized light and output from the output end of the second optical waveguide core. The polarization conversion type optical wavelength filter according to any one of claims 1 to 5, wherein the other light is output from the output end of the first optical waveguide core. The first element and the second element which are the polarization conversion type optical wavelength filters according to any one of claims 1 to 7 are provided.
The first polarization conversion unit, which is the polarization conversion unit included in the first element, and the second polarization conversion unit, which is the polarization conversion unit included in the second element, are arranged in series.
An output terminal of said first optical waveguide core that is part of the first polarization converter unit, the input end of the second optical waveguide core that is part of the second polarization conversion unit is connected,
The input end of the second optical waveguide core that is part of the first polarization converter unit, that the output end of said first optical waveguide core that is part of the second polarization conversion unit is connected A characteristic polarization-independent optical wavelength filter. Width tapered waveguide between an output end of said first optical waveguide core that is part of the first polarization converter unit, an input end of said second optical waveguide core that is part of the second polarization converter unit Is provided,
Width tapered waveguide between an output end of said input end of said second optical waveguide core first Ru contained in the polarization conversion unit, wherein the first optical waveguide core that is part of the second polarization converter unit The polarization-independent optical wavelength filter according to claim 8, wherein the light wavelength filter is provided.

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