JP2019219484A - Polarization cleaner and optical module - Google Patents

Abstract

To provide a polarization cleaner and an optical module which have a sufficient polarization extinction ratio and can realize downsizing of a receiver.SOLUTION: A polarization cleaner 10 includes: a core 13 for propagating light; and a clad near the core 13, the clad having a lower index of refraction than that of the core 13 and at least a part of the waveguide made of the core 13 and the clad being a bending waveguide formed by bending and extending the core 13. In at least the bending waveguide, the aspect ratio w/h is at least 5 when the thickness of the core 13 is the thickness h of the waveguide and the width of the core 13 is the width w of the waveguide. The polarization cleaner further includes a tapered structure 13A as a tapered core extending from the core 23 of another waveguide connected to the bending waveguide to the core 13 of the bending waveguide.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a polarization cleaner and an optical module.

  In recent years, digital coherent communication has been actively studied in ultra-high-speed communication exceeding 100 Gbit / s. As one type of digital coherent communication, a polarization multiplexing quaternary phase modulation (DP-QPSK: Dual Polarization-Quadrature Phase Shift Keying) method is known. In the DP-QPSK method, a function of performing polarization separation in a receiver and a 90 ° optical hybrid function for extracting phase information from the separated optical signal are required. A receiver (ICR: Integrated Coherent Receiver) having such a function is currently realized by a planar lightwave circuit (PLC) using an optical waveguide technology based on quartz. In the future, miniaturization of transceivers such as CFP2-ACO (c form-factor pluggable 2-Analog Coherent Optics) and CFP4-ACO is desired. Therefore, miniaturization of the receiver (ICR) is strongly desired. Also, there is a strong demand for downsizing of the 90 ° hybrid mixer itself constituted by a PLC used for a receiver (ICR).

In a technology for realizing miniaturization of a 90 ° hybrid mixer composed of PLC, a semiconductor material having a higher refractive index, particularly Si photonics using Si, is promising. In Si photonics, by using Si for the core of the optical waveguide and using SiO ₂ for the cladding of the optical waveguide, the refractive index difference between the core and the cladding can be made extremely high. Thereby, light can be strongly confined in the optical waveguide. Further, the bending radius of the optical waveguide can be made smaller than that of the quartz waveguide. The channel type optical waveguide has a structure in which a Si waveguide (core) having a rectangular cross section is surrounded by SiO ₂ (cladding). In the channel type optical waveguide, a sharp bending radius on the order of several μm can be realized. Therefore, by using the channel type optical waveguide, the entire circuit can be downsized. Although the channel-type optical waveguide has a large propagation loss per unit length, the loss of the entire chip using the channel-type optical waveguide can be equivalent to the loss of the PLC.

  Patent Literature 1 describes a technique for suppressing polarization conversion in a rib-type optical waveguide having a curved waveguide. Specifically, the core of the rib-type optical waveguide of Patent Document 1 has a slab portion and a rib portion protruding from the slab portion, and the thickness of the slab outside the curve in the curved waveguide is set to the inside slab. It is thinner than the thickness.

  For miniaturization of a receiver (ICR), miniaturization of a polarization beam splitter (PBS: Polarization Beam Splitter) used in the receiver is strongly expected. The polarization separation element (PBS) is an element that separates a TE mode (Transverse Electro mode) and a TM mode (Transverse Magnetic mode) of signal light, and is an important component that determines the transmission performance of the receiver (ICR). The polarization extinction ratio of the Mach-Zehnder interferometer-type polarization splitter (PBS) and the directional coupler-type polarization splitter (PBS) is sufficient when these polarization splitters (PBS) are mounted on the transceiver. Not really. Therefore, there is a case where the polarization extinction ratio is improved by serially connecting the same polarization separation element (PBS). However, when a polarization separation element (PBS) is serially connected, there is a problem that the propagation loss of the waveguide becomes excessive. In addition, there is also a problem that the receiver becomes large when the polarization separation elements (PBS) are connected in multiple stages.

JP 2017-215526 A

  Patent Document 1 suppresses polarization conversion in a bent waveguide, and thus suppresses polarization extinction in the bent waveguide. Therefore, the polarization extinction ratio required for the receiver cannot be obtained.

  An object of the present disclosure is to provide a polarization cleaner and an optical module which have a sufficient polarization extinction ratio and can realize downsizing of a receiver.

  A polarization cleaner according to a first aspect of the present invention includes a core that propagates light, and a clad formed around the core and having a lower refractive index than the core, and a waveguide including the core and the clad. Is a bent waveguide in which the core is curved and extended, and in at least the bent waveguide, a waveguide thickness h that is the thickness of the core and a waveguide that is the width of the core An aspect ratio w / h, which is a ratio with respect to the width w, is 5 or more, and a tapered shape extending from the core of the other waveguide connected to the bent waveguide to the core of the bent waveguide. And a tapered structure as the core.

  An optical module according to a second aspect of the present invention includes a polarization cleaner, wherein the polarization cleaner includes a core that propagates light and a clad formed around the core and having a lower refractive index than the core. At least a part of the waveguide comprising the core and the clad is a bent waveguide in which the core is curved and extended, and at least the bent waveguide has a waveguide thickness that is the thickness of the core. h and an aspect ratio w / h, which is a ratio of the waveguide width w that is the width of the core, is 5 or more, and the bent waveguide is connected to the bent waveguide from the core of another waveguide connected to the bent waveguide. And a tapered structure which is the tapered core extending to the core.

  A polarization cleaner and an optical module having a sufficient polarization extinction ratio and realizing miniaturization of a receiver can be provided.

FIG. 2 is a top view illustrating an example of the polarization cleaner according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view illustrating an example of the polarization cleaner according to Embodiment 1 of the present invention. FIG. 5 is a cross-sectional view illustrating an example of another waveguide according to Embodiment 1 of the present invention. FIG. 3 is a sectional view illustrating an aspect ratio of the polarization cleaner according to the first embodiment of the present invention. 5 is a graph illustrating a radius of curvature of a curved waveguide of the polarization cleaner according to the first embodiment of the present invention. FIG. 9 is a top view illustrating an example of a polarization cleaner according to Embodiment 2 of the present invention. FIG. 7 is a cross-sectional view illustrating an example of a polarization cleaner according to Embodiment 2 of the present invention. FIG. 9 is a cross-sectional view illustrating an example of a polarization cleaner according to Embodiment 2 of the present invention. FIG. 9 is a cross-sectional view illustrating an example of a polarization cleaner according to Embodiment 2 of the present invention. 4 is a graph showing a propagation loss of light in a TE mode and a propagation loss of light in a TM mode, which are propagated by the bent waveguide of the polarization cleaner according to the first embodiment of the present invention. FIG. 13 is a block diagram illustrating an example of an optical hybrid mixer according to Embodiment 3 of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Embodiment 1
Hereinafter, the polarization cleaner 10 according to Embodiment 1 of the present invention will be described. FIG. 1 is a top view illustrating an example of the polarization cleaner 10 according to the first embodiment. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. FIG. 3 is a cross-sectional view taken along the line III-III of FIG. As shown in FIGS. 1 and 2, the polarization cleaner 10 includes a substrate 11, a lower clad 12, cores 13 and 13A, and an upper clad 14. The polarization cleaner 10 shown in FIGS. 1 and 2 is a channel-type waveguide in which the cores 13 and 13A are surrounded by claddings 12 and 14. Further, as shown in FIG. 1, the polarization cleaner 10 includes a bent waveguide in which the core 13 is curved and extended. Further, the polarization cleaner 10 includes a tapered core (hereinafter, also referred to as a tapered structure) 13A that connects the core 23 of the other waveguide 20 and the core 13 of the bent waveguide.

The substrate 11 is, for example, a silicon substrate, and a lower cladding 12 is provided on an upper layer of the silicon substrate. The lower cladding 12 is, for example, an SiO ₂ film, and is formed of, for example, a buried oxide film (BOX: Burried Oxide). The cores 13 and 13A are provided on the upper layer side of the lower clad 12. The core 13 is made of, for example, a Si film.

An upper clad 14 is provided on the upper layers of the cores 13 and 13A so as to cover the cores 13 and 13A. The upper cladding 14 is, for example, a SiO ₂ film. Instead of using the SiO ₂ film as the upper clad 14, an air layer may be used as the upper clad. The refractive indices of the cores 13 and 13A are higher than those of the lower cladding 12 and the upper cladding 14, and light incident on the cores 13 and 13A is confined in the cores 13 and 13A. As shown in FIGS. 1 to 3, the width of the core 13 of the bent waveguide of the polarization cleaner 10 is larger than the width of the core 23 of the other waveguide 20. The bent waveguide core 13 is curved in an S-shape. The curved shape of the bent waveguide core 13 is not limited to the S-shape. The curved shape of the core 13 of the curved waveguide may be a curved shape, for example, a curved shape having a bending angle of 45 degrees or 90 degrees. Here, the bending angle is the angle (inner angle) formed by the linear waveguides before and after the bent waveguide. When the curved shape of the bent waveguide core 13 is S-shaped, the propagation directions of the straight waveguides before and after the bent waveguide can be made substantially the same.

  The other waveguide 20 includes a substrate 11, a lower clad 12, a core 23, and an upper clad 14, as shown in FIG. The substrate 11, the lower cladding 12, and the upper cladding 14 are the same as the substrate 11, the lower cladding 12, and the upper cladding 14 of the polarization cleaner 10, and thus the description thereof is omitted.

  The core 23 is provided on the upper layer side of the lower clad 12 similarly to the cores 13 and 13A. The core 23 is made of, for example, a Si film. The core 23 is formed in a straight line. In the present invention, the term “straight” includes, for example, a case where the core 23 is displaced by about 10 μm in a direction perpendicular to the direction when the core 23 is extended by 1 mm in a predetermined direction. As shown in FIGS. 1 to 3, the width of the core 23 of the other waveguide 20 is smaller than the width of the core 13 of the polarization cleaner 10.

  The width of the core 13A having the tapered structure gradually increases from the width of the core 23 of the other waveguide 20 to the width of the core 13 of the bent waveguide. Thereby, the tapered structure 13A can couple the TM mode light of the light propagated by the core 23 of the other waveguide 20 to the leak mode. Note that the leak mode is one of the propagation modes of light propagated by the waveguide. That is, the TM mode light of the light propagated by the core 23 of the other waveguide 20 is coupled to the propagation mode radiated in the bent waveguide core 13 by the tapered structure 13A.

  Further, as shown in FIG. 4, in the present embodiment, the waveguide thickness which is the thickness of the core 13 is h, the waveguide width which is the width of the core 13 is w, the waveguide thickness h and the waveguide width Assuming that the aspect ratio w / h is the aspect ratio w / h, the aspect ratio w / h of the bent waveguide of the polarization cleaner 10 is 5 or more. By setting the aspect ratio to 5 or more, the confinement of the TM mode light by the core 13 is weakened, so that the propagation loss of the TM mode light in the core 13 can be significantly increased. Thereby, the TE mode light can be propagated while emitting the TM mode light in the curved waveguide of the polarization cleaner 10, and the polarization extinction ratio can be improved.

  Further, as shown in FIG. 5, as the radius of curvature of the core of the bent waveguide 13 decreases, the propagation loss of light propagated by the core 13 increases. More specifically, as shown in FIG. 5, the radius of curvature of the core 13 of the curved waveguide when the propagation loss of the TM mode light is large is equal to the radius of curvature of the curved waveguide when the propagation loss of the TE mode light is large. Is larger than the radius of curvature of the core 13. More specifically, as shown in FIG. 5, when the radius of curvature of the core 13 is C, the propagation loss of the light in the TM mode is very large. In contrast, when the radius of curvature of the core 13 is C, propagation loss of TE mode light hardly occurs.

  Therefore, by setting the radius of curvature of the core 13 of the curved waveguide to be a radius at which the propagation loss of the TM mode light is larger than the propagation loss of the TE mode light, the TM mode in the curved waveguide of the polarization cleaner 10 is obtained. , While propagating the TE mode light, thereby improving the polarization extinction ratio. Specifically, by setting the radius of curvature of the curved waveguide core 13 to the value of C shown in FIG. 5, the TE mode light can be propagated while the TM mode light is emitted. The extinction ratio can be significantly improved.

  According to the polarization cleaner 10 according to the first embodiment described above, since the aspect ratio w / h of the core 13 is 5 or more in the bent waveguide, light in the TM mode is emitted in the bent waveguide. While transmitting the TE mode light, the polarization extinction ratio can be improved. Further, since the taper structure 13A which is a tapered core extending from the core 23 of the other waveguide 20 to the core 13 of the bent waveguide is provided, the light propagating by the core 23 of the other waveguide 20 is provided. TM mode light can be coupled into the leaky mode. Thereby, the polarization extinction ratio of the curved waveguide of the polarization cleaner 10 can be further improved. Further, since the polarization extinction ratio of the polarization cleaner 10 is improved, it is not necessary to serially connect the polarization splitter (PBS), and the receiver (ICR) can be downsized. Therefore, it is possible to provide the polarization cleaner 10 having a sufficient polarization extinction ratio and realizing the miniaturization of the receiver.

  The radius of curvature of the core 13 of the curved waveguide is a radius at which the propagation loss of the TM mode light is larger than the propagation loss of the TE mode light. Therefore, in the curved waveguide of the polarization cleaner 10, the TE mode light can be propagated while emitting the TM mode light, and the polarization extinction ratio can be improved.

Embodiment 2
Next, a polarization cleaner 30 according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 6 is a top view illustrating an example of the polarization cleaner 30 according to the second embodiment. FIG. 7 is a sectional view taken along the line VII-VII in FIG. FIG. 8 is a cross-sectional view taken along line VIII-VIII of FIG. FIG. 9 is a cross-sectional view taken along the line IX-IX in FIG.

  As shown in FIGS. 6, 7, and 9, in the polarization cleaner 30 according to the second embodiment, a core 33A having a tapered structure is different from the core 13A of the polarization cleaner 10 according to the first embodiment. Further, in another waveguide 40 of the second embodiment, the core 43 is different from the other waveguide 20 of the first embodiment. Therefore, other configurations of the polarization cleaner 30 according to the second embodiment and other configurations of the other waveguides 40 of the second embodiment are the same as those of the first embodiment, and thus are denoted by the same reference numerals. The description is omitted.

  As shown in FIGS. 6 to 8, the width of the core 43 of the other waveguide 40 is substantially the same as the width of the core 13 of the curved waveguide of the polarization cleaner 30. On the other hand, as shown in FIGS. 7 to 9, the thickness of the core 43 of the other waveguide 40 is larger than the thickness of the core 13 of the curved waveguide of the polarization cleaner 30.

  Then, as shown in FIG. 9, the thickness of the core 33A having the tapered structure gradually decreases from the thickness of the core 43 of the other waveguide 40 to the thickness of the core 13 of the bent waveguide. Thus, the tapered structure 33A can couple the TM mode light of the light propagated by the core 43 of the other waveguide 40 to the leak mode.

  According to the polarization cleaner 30 according to the second embodiment described above, the same effects as those of the polarization cleaner 10 according to the first embodiment can be obtained. Specifically, since the thickness of the core 33A having the tapered structure gradually decreases from the thickness of the core 43 of the other waveguide 40 to the thickness of the core 13 of the bent waveguide, the thickness of the other waveguide is reduced. The TM mode light of the light propagated by the core 43 of the waveguide 40 can be coupled to the leaky mode. Therefore, the polarization extinction ratio of the curved waveguide of the polarization cleaner 30 can be improved. Further, since the polarization extinction ratio of the polarization cleaner 30 is improved, it is not necessary to serially connect the polarization splitter (PBS), and the receiver (ICR) can be downsized. Therefore, it is possible to provide the polarization cleaner 30 having a sufficient polarization extinction ratio and realizing the miniaturization of the receiver.

Example 1
The polarization extinction ratio of the polarization cleaner according to the first embodiment of the present invention will be described with reference to FIG. FIG. 10 is a graph showing the propagation loss of the light in the TE mode and the propagation loss of the light in the TM mode propagated by the bent waveguide of the polarization cleaner according to the first embodiment of the present invention. In the graph shown in FIG. 10, the horizontal axis represents wavelength (nm) and the vertical axis represents propagation loss (dB).

  The polarization cleaner according to the first embodiment of the present invention has a structure in which the tapered structure 13A is omitted from the polarization cleaner 10 according to the first embodiment. That is, the polarization cleaner according to the first embodiment has a curved waveguide that is curved and extended in an S-shape. The width of the core of the curved waveguide of the polarization cleaner according to the first embodiment is about 1 μm, and the thickness of the core is about 50 nm. The aspect ratio of the core is about 20.

  In the polarization cleaner according to the first embodiment, a linear waveguide as another waveguide is connected to the entrance side and the exit side of the polarization cleaner. The linear waveguide has a substantially linearly extending core. The width of the core of the straight waveguide is about 1 μm, and the thickness of the core is about 50 nm. The aspect ratio of the core is about 20.

  As shown in FIG. 10, the polarization cleaner according to the first embodiment of the present invention can secure a polarization extinction ratio of 20 dB or more over the entire C-band (Conventional-band). That is, the polarization cleaner according to the first embodiment of the present invention can achieve a sufficient polarization extinction ratio to be mounted on a receiver.

Embodiment 3
Next, an optical hybrid mixer 100 as an optical module according to Embodiment 3 of the present invention will be described. FIG. 11 is a block diagram illustrating a configuration of an optical hybrid mixer 100 according to Embodiment 3.
As shown in FIG. 11, an optical hybrid mixer 100 includes an OPR (Optical Power Ratio) 101, a variable optical attenuator (Variable Optical Attenuator) 102, a polarization beam splitting element (Polarization Beam Splitter) 103, and a polarization rotator (Polarization Rotator) 104. , A polarization cleaner 105, an optical coupler 106, and a 90-degree optical hybrid 107.

  The input signal light is input to the OPR 101, and is input from the OPR 101 to the variable optical attenuator 102. The variable optical attenuator 102 adjusts the intensity of the input signal light for each wavelength, for example. The variable optical attenuator 102 inputs the intensity-adjusted signal light to the polarization splitter 103.

  The polarization separation element 103 separates the TE component and the TM component of the signal light input from the variable optical attenuator 102. Then, the polarization separation element 103 inputs the polarization-separated signal light (TM mode light) to the polarization rotator 104 and inputs the polarization-separated signal light (TE mode light) to the polarization cleaner 105.

  The polarization rotator 104 rotates the polarization plane of the signal light (TM mode light) input from the polarization separation element 103. The polarization rotator 104 inputs the converted signal light (TE mode light) to the polarization cleaner 105.

  The polarization cleaner 105 is the polarization cleaner 10 according to the first embodiment or the polarization cleaner 30 according to the second embodiment. The polarization cleaner 105 to which the signal light has been input from the polarization separation element 103 extinguishes the TM mode light included in the signal light. The polarization cleaner 105 to which the signal light has been input from the polarization rotator 104 extinguishes the TM mode light included in the signal light. Thus, a sufficient polarization extinction ratio can be obtained without connecting a plurality of polarization splitters 103 serially. The polarization cleaner 105 inputs the polarization-quenched signal light (TE mode light) to the 90-degree optical hybrid 107.

  On the other hand, the optical coupler 106 is an optical coupler having a branching ratio of 1: 1 and branches the input local oscillation light into two. Then, the optical coupler 106 causes the two branched local oscillation lights to enter the respective 90-degree optical hybrids 107.

  The 90-degree optical hybrid outputs interference light generated by causing the signal light (TE mode light) input from the polarization cleaner 105 to interfere with the local oscillation light input from the optical coupler 106.

  According to the optical hybrid mixer 100 according to the third embodiment described above, since the polarization extinction ratio is improved by the polarization cleaner 105, it is not necessary to serially connect a plurality of polarization splitters 103. Therefore, it is possible to provide the optical hybrid mixer 100 having a sufficient polarization extinction ratio and realizing the miniaturization of the receiver.

  The present invention is not limited to the above embodiment, and can be appropriately changed without departing from the gist. For example, the present invention is not limited to a silicon waveguide, but may be applied to a semiconductor waveguide formed of an InP-based compound or the like.

10, 30, 105 Polarization cleaner 11 Substrate 12 Lower cladding 13 Core (bent waveguide)
13A, 33A core (tapered structure)
14 Upper cladding 20, 40 Other waveguides 23, 43 Core 100 Optical hybrid mixer 101 OPR (Optical Power Ratio)
102 Variable Optical Attenuator
103 Polarization Beam Splitter
104 Polarization Rotator
105 Polarization Cleaner
106 Optical Coupler
107 90 ° Optical Hybrid

Claims (8)

A core that propagates light, comprising a clad formed around the core and having a lower refractive index than the core,
At least a part of the waveguide composed of the core and the clad is a bent waveguide in which the core is curved and extended,
In at least the bent waveguide, an aspect ratio w / h, which is a ratio of a waveguide thickness h, which is the thickness of the core, to a waveguide width w, which is the width of the core, is 5 or more;
The polarization cleaner further comprising a tapered structure that is the tapered core extending from the core of the other waveguide connected to the bent waveguide to the core of the bent waveguide.   The polarization cleaner according to claim 1, wherein the width of the core of the tapered structure gradually increases from the width of the core of the other waveguide to the width of the core of the bent waveguide.   The polarization cleaner according to claim 1, wherein the thickness of the core of the tapered structure gradually decreases from the thickness of the core of the other waveguide to the thickness of the core of the curved waveguide. .   4. The polarization cleaner according to claim 1, wherein a radius of curvature of the core of the curved waveguide is a radius at which a propagation loss of TM mode light is larger than a propagation loss of TE mode light. 5. . An optical module including a polarization cleaner,
The polarization cleaner,
A core that propagates light, comprising a clad formed around the core and having a lower refractive index than the core,
At least a part of the waveguide composed of the core and the clad is a bent waveguide in which the core is curved and extended,
In at least the bent waveguide, an aspect ratio w / h, which is a ratio of a waveguide thickness h, which is the thickness of the core, to a waveguide width w, which is the width of the core, is 5 or more;
The optical module further comprising a tapered structure that is the tapered core extending from the core of another waveguide connected to the bent waveguide to the core of the bent waveguide.   The optical module according to claim 5, wherein the width of the core of the tapered structure gradually increases from the width of the core of the other waveguide to the width of the core of the bent waveguide.   The optical module according to claim 5, wherein the thickness of the core of the tapered structure is gradually reduced from the thickness of the core of the other waveguide to the thickness of the core of the bent waveguide.   The optical module according to claim 5, wherein a radius of curvature of the core of the curved waveguide is a radius at which a propagation loss of light in the TM mode is larger than a propagation loss of light in the TE mode.

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