Optical fiber mounted photonic integrated circuit device for single mode optical fibers

The invention relates to an optical fiber mounted photonic integrated circuit device where the tolerance in the positioning of the coupling between a single mode optical fiber and an optical waveguide provided in the photonic integrated circuit device is increased. A second optical waveguide of which the cross-section of the core is in the form of a slab having a width that is greater than the mode diameter of the single mode optical fiber, and which is tapered in such a manner that the thickness of the core is reduced as the location is closer to the connection portion with the single mode optical fiber, is provided on the input/output end side of the first optical waveguide through which light propagates in such a manner that the inclined connection end surface of the single mode optical fiber is coupled to the upper surface of the second optical waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-061829, filed on Mar. 25, 2016, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to an optical fiber mounted optical integrated circuit device, and to an optical connection structure in a silicon photonic integrated circuit (PIC) device where optical fibers and optical parts are mounted in high density, for example.

BACKGROUND

In order to implement an optical transceiver made of one chip that covers the capacity of a class of terabits per second, it is necessary to use single mode optical fibers as the optical fibers used for light transmission in the case of wavelength division multiplexing or long distance transmission.

Si photonic wire waveguides used for silicon PIC's have a size as small as one μm or less, and therefore are appropriate for high integration. However, the size is greatly different from that of single mode optical fibers, and therefore, it is a theme to enhance the efficiency in the coupling of the Siphotonic wire waveguides with optical fibers.

According to a conventional method, a spot size converter of which an end is tapered is used to make the spot size compatible with an optical fiber in order to enhance the efficiency in the coupling with an optical fiber.FIGS. 15A and 15Bare diagrams illustrating a conventional optical fiber mounted photonic integrated circuit device.FIG. 15Ais a cross-sectional diagram along the optical axis, andFIG. 15Bis a plan diagram where the main portions can be clearly seen from the top.

An SOI wafer is used in such a manner that a Si fine wire core63having a thickness of 0.25 μm is provided on top of a silicon substrate61with a BOX layer62that also works as a lower clad layer in between. A spot size converter64of which an end is tapered is provided to an end of the Si fine wire core63. A SiO2film is provided on the upper surface of the Si fine wire core63as an upper clad layer65, and a wide SiON core66is provided so as to cover the sides of the spot size converter64. A single mode optical fiber67is arranged so that the core68faces the SiON core66. Here,69and70in the figures are a clad and a ferrule, respectively.

As illustrated in the figures, the propagating light beam having a spot diameter of approximately 0.25 μm is expanded through the spot size converter64so that the spot diameter is approximately 9 μm in the SiON core66when the light beam enters into the core68of the single mode optical fiber67.

PRIOR ART LITERATURE

Patent Literature

SUMMARY

In the case of the mounting structure inFIGS. 15A and 15B, the spot size of the single mode optical fiber is approximately 9 μm, and therefore, only a shift in the position of several μm is allowed when positioning, and thus, fiber positioning with high precision is required. Accordingly, active alignment is required where light is made to pass through the Si fine wire core, and positioning can be done so that the coupled optical power becomes maximum. Thus, a problem arises that the mounting cost is high as compared to systems where a multimode fiber having a larger diameter is used.

An optical fiber mounted photonic integrated circuit device, including:

a photonic integrated circuit device configured to be provided with an optical waveguide on a substrate; and

a single mode optical fiber having an inclined connection end surface configured to be optically coupled to the optical waveguide and incline relative to the direction in which light propagates, wherein

the optical waveguide is formed of a first optical waveguide through which light propagates and a second optical waveguide that is coupled to the first optical waveguide on the input/output end side of the first optical waveguide,

the cross-section of the core of the second optical waveguide is in the form of a slab having a width that is greater than the mode diameter of the single mode optical fiber, and the second optical waveguide is tapered in such a manner that the thickness of the core is reduced as the location is closer to the connection portion with the single mode optical fiber, and

the inclined connection end surface of the single mode optical fiber and the upper surface of the core of the second optical waveguide face each other for coupling.

According to one aspect of the invention, it becomes possible to increase the tolerance in the positioning of the coupling between a single mode optical fiber and an optical waveguide provided in a photonic integrated circuit device.

DESCRIPTION OF EMBODIMENTS

In reference toFIGS. 1 through 8, the optical fiber mounted photonic integrated circuit device according to an embodiment of the present invention is described below.FIGS. 1A and 1Bare diagrams illustrating the optical fiber mounted photonic integrated circuit device according to an embodiment of the present invention.FIG. 1Ais a cross-sectional diagram along the optical axis, andFIG. 1Bis a plan diagram where the main portions can be clearly seen from the top. As illustrated in the figures, the core13of the first optical waveguide is provided on a substrate11with a lower clad layer12in between, and the core16of the second optical waveguide is provided on the input/output end side of the core13of the first optical waveguide. Here, it is desirable to provide a spot size converter14at the input/output end of the core13of the first optical waveguide. Here, the shape of the spot size converter14may be tapered in the direction of the width or in the direction of the thickness.

The core16of the second optical waveguide is in a tapered shape where the cross-section of the core is in the form of a slab with a width greater than the mode diameter of the single mode optical fiber17, and the thickness of the core decreases as the location is closer to the connection portion with the single mode optical fiber17. It is also desirable to provide such a structure that the width of the core of the second optical waveguide gradually widens as the location is closer to the coupling portion with the single mode optical fiber17, where it is desirable for the width of the core to be two times or more greater than the mode diameter of the single mode optical fiber17. In addition, it is desirable for the refractive index of the core16of the second optical waveguide to be smaller than the refractive index of the core13of the first optical waveguide.

The single mode optical fiber17is provided with an inclined connection end surface so that this inclined connection end surface and the upper surface of the core16of the second optical waveguide face each other for coupling. It is desirable for a transparent member21such as a resin or a liquid that is transparent for the propagating light, for example, a transparent oil, to intervene between the inclined connection end surface of the single mode optical fiber17and the upper surface of the core16of the second optical waveguide. The transparent member21may be a material of which the refractive index is close to that of the clad19of the single mode optical fiber. In the case where the transparent member21is a liquid, it is preferable for it to be difficult to volatilize. In the case where a resin is used, it may be a transparent resin that is used for conventional optical devices. Here, the angle of the inclined connection end surface can be 80° or greater so that the coupling loss can be lowered, and the closer to 90° this angle is, the better.

A silicon photonic integrated circuit device can be cited as an example of the photonic integrated circuit device. In this case, an SOI wafer is used in such a manner that the BOX layer is used for the lower clad layer12, and the core13of the first optical waveguide is a silicon fine wire core. In addition, it is desirable to use SiON of which the refractive index is smaller than that of Si or Ge-doped SiO2for the core16of the second optical waveguide. Furthermore, SiO2may be used for the upper clad layer15. In the case where an SOI wafer is not used, the fine wire core may be made of SiON having a high N concentration instead of the Si fine wire core.

The single mode optical fiber17is fitted to a ferrule20to be fixed to the photonic integrated circuit device side. The ferrule is provided with a first engagement mechanism, and at the same time, the substrate11of the photonic integrated circuit device is provided with a second engagement mechanism which engages with the first engagement mechanism. At this time, it is desirable to further provide a weight applying mechanism that applies weight on the photonic integrated circuit side at the time of engagement.

When light in the core13of the first optical waveguide is guided into the core16of the second optical waveguide that is in the form of a flat slab, the spot expands to a range that is wider than the diameter of the core18of the single mode optical fiber17. When the core16of the second optical waveguide in the form of a slab is tapered in the direction of the thickness, the light spot in the core16of the second optical waveguide in the form of a slab expands in the vertical direction. At the same time, the light spot is coupled to the core18of the single mode optical fiber17through the inclined connection end surface of the single mode optical fiber17.

FIGS. 2A through 2Care diagrams illustrating how light propagates from the first optical waveguide to the second optical waveguide and exhibiting the results of calculation in accordance with a beam propagation method (BPM). As light propagates from the core13of the first optical waveguide to the core16of the second optical waveguide in the order ofFIG. 2A,FIG. 2BandFIG. 2C, the cross-section of the light beam becomes flatter.

FIGS. 3A through 3Care diagrams illustrating how light propagates from the second optical waveguide to the optical fiber in the case where the optical fiber is located at the center and exhibiting the results of calculation in accordance with the BPM as well. In the case where the optical axis of the single mode optical fiber17and the optical axis of the core13of the first optical waveguide are aligned, light moves from the core16of the second optical waveguide to the core18of the single mode optical fiber17without much loss in the order ofFIG. 3A,FIG. 3BandFIG. 3C.

FIGS. 4A through 4Care diagrams illustrating how light propagates from the second optical waveguide to the optical fiber in the case where the optical fiber is shifted to the left and exhibiting the results of calculation in accordance with the BPM as well. In the case where the optical axis of the single mode optical fiber17and the optical axis of the core13of the first optical waveguide are shifted away from each other by 15 μm, light again moves from the core16of the second optical waveguide to the core18of the single mode optical fiber17without much loss in the order ofFIG. 4A,FIG. 4BandFIG. 4C. Accordingly, it is confirmed that the optical power is coupled to the single mode optical fiber17irrelevant of the location of the single mode optical fiber17.

FIGS. 5A and 5Bare diagrams illustrating the spatial distribution of the optical power in the embodiment of the present invention.FIG. 5Aillustrates the spatial distribution of the optical power as viewed from the top, andFIG. 5Billustrates the spatial distribution of the optical power in the cross-section along the optical axis. It can be seen from the figures how the light in the core13of the first optical waveguide spreads in the core16of the second optical waveguide in the form of a slab, and after that, how the optical power is coupled to the core18of the single mode optical fiber17having the inclined connection end surface. Here,FIG. 5Aillustrates the spatial distribution of the optical power in proximity to the portion lower than the inclined connection end surface of the single mode optical fiber17, and therefore, the optical power is lower as the light propagates through the single mode optical fiber17.

FIGS. 6A and 6Bare a diagram and a graph illustrating the tolerance in the lateral direction in the embodiment of the present invention.FIG. 6Ais a plan diagram as viewed from the top illustrating how the optical waveguide and the single mode optical fiber are arranged, andFIG. 6Bexhibits the results of calculation of the power coupling efficiency between the Si fine wire core and the single mode optical fiber in accordance with the BPM. In the case where the single mode optical fiber17is shifted in the lateral direction (in the upward and downward directions in the figure) as illustrated inFIG. 6A, a coupling efficiency of −2 dB or greater is gained in the bandwidth of 35 μm as illustrated inFIG. 6B. This exhibits that a shift that is approximately the same as the width of the core16of the second optical waveguide in the form of a slab (40 μm) is tolerable. In addition, an improvement of one digit or more can be seen as compared to the fact that the tolerance (loss <2 dB) in the lateral direction in the conventional optical fiber mounted photonic integrated circuit device inFIG. 15is approximately 3 μm.

FIGS. 7A and 7Bare a diagram and a graph illustrating the tolerance in the direction of the optical axis in the embodiment of the present invention.FIG. 7Ais a plan diagram as viewed from the top illustrating how the optical waveguide and the single mode optical fiber are arranged.FIG. 7Billustrates the results of calculation of the power coupling efficiency between the Si fine wire core and the single mode optical fiber in accordance with the BPM. In the case where the single mode optical fiber17is shifted in the direction of the optical axis (in the left and right directions in the figure) as illustrated inFIG. 7A, there is almost no change in the coupling efficiency over a wide range of 200 μm as illustrated inFIG. 7B. The results indicate an improvement of two digits or more as compared to the fact that the tolerance (loss <2 dB) in the direction of the optical axis of the conventional optical fiber mounted photonic integrated circuit device inFIG. 15is approximately 3 μm.

FIGS. 8A and 8Bare a diagram and a graph illustrating the tolerance in the vertical direction in the embodiment of the present invention.FIG. 8Ais a diagram as viewed from the top illustrating how the optical waveguide and the single mode optical fiber are arranged, andFIG. 8Bexhibits the results of calculation of the power coupling efficiency between the Si fine wire core and the single mode optical fiber in accordance with the BPM. In the case where the single mode optical fiber17is shifted in the vertical direction (in the upward and downward directions in the figure) as illustrated inFIG. 8A, it can be seen fromFIG. 8Bthat the shift in the vertical direction, that is to say, the gap between the core16of the second optical waveguide in the form of a slab and the inclined connection end surface of the single mode optical fiber17, may be as great as approximately 5 μm. Here, this gap occurs due to the limit of precision in tapering the core16of the second optical waveguide in the form of a slab, the limit of precision in the process of the inclined connection end surface of the single mode optical fiber17, or a foreign substance that may interfere between the core16and the inclined connection surface.

According to the embodiment of the present invention, the core16of the second optical waveguide is tapered so that the thickness of the core is reduced and is coupled to the inclined connection end surface of the single mode optical fiber17, and therefore, the tolerance in the positioning when coupled to the single mode optical fiber can be increased.

Next, the optical fiber mounted photonic integrated circuit device according to Example 1 of the present invention is described in reference toFIGS. 9A through 12.FIGS. 9A and 9Bare diagrams illustrating the optical fiber mounted photonic integrated circuit device according to Example 1 of the present invention.FIG. 9Ais a cross-sectional diagram along the optical axis, andFIG. 9Bis a plan diagram where the main portions can be clearly seen from the top. As illustrated in the figures, an SOI wafer is used in such a manner that an Si fine wire core33having a thickness of 0.25 μm and a width of 0.5 μm is provided on a silicon substrate31with a BOX layer22, which also works as a lower clad layer, in between. A spot size converter34is provided on the input/output end side of the Si fine wire core33. A SiON core36is provided so as to cover the spot size converter34.

The SiON core36is in the form of a slab of which the core cross-section has a width greater than the mode diameter (9 μm) of the single mode optical fiber37. The SiON core36is tapered so that the thickness is reduced from 0.5 μm to 0 μm. Here, the form of the SiON core36of which the width is reduced is not limited to that in the figure. The SiON core36may be tapered, but it is desirable for the width of the form to be gradually reduced in order to avoid undesired reflection from a portion where the form changes.

An inclined connection end surface is provided to the single mode optical fiber37, which is arranged in such a manner that this inclined connection end surface and the upper surface of the SiON core36face each other and are coupled with a transparent oil41in between. 18061 (product number of the oil made by Cargille Labs Inc.) having a refractive index of 1.44 is used for the transparent oil41. In addition, the coupling loss can be lowered by making the angle of the inclined connection end surface 80° or greater. The closer to 90° this angle is, the better, but here it is 88°.

FIGS. 10A and 10Bare diagrams illustrating the coupling portion in the optical fiber mounted photonic integrated circuit device according to Example 1 of the present invention.FIG. 10Ais a cross-sectional diagram along the optical axis, andFIG. 10Bis a plan diagram where the main portions can be clearly seen from the top. As illustrated inFIG. 10A, the thickness of the BOX layer32is 3 μm excluding the end portion, and the thickness of the end portion is 10 μm. As illustrated inFIG. 10B, the width of the end portion on the spot size converter34side in the SiON core36is 10 μm, the width of the end portion on the opposite side is 40 μm, and the length of the flat portion is 500 μm.

FIGS. 11A through 11Care diagrams illustrating the process for forming the SiON core in the optical fiber mounted photonic integrated circuit device according to Example 1 of the present invention. First, as illustrated inFIG. 11A, the single crystal Si layer on the BOX layer32is processed so as to form an Si fine wire core33and a spot size converter34, on top of which an SiO2film is provided as an upper clad layer35. Next, part of the upper clad layer35is removed, and then, a SiON film42is provided so as to cover the spot size converter34so that the thickness thereof is 0.25 μm above the spot size converter34. Next, a metal film is provided, and then, a patterned resist44is provided in order to etch the metal film, and thus, a metal mask43is formed.

Next, as illustrated inFIG. 11B, etching is carried out using an etchant for the metal mask43and the SiON film42. At this time, an etchant of which the etching rate is higher for the metal mask43is selected to etch the sides of the metal mask43so that the surface of the SiON film43that is exposed as the sides of the metal mask43are etched is sequentially etched so as to be in an inclined form.

When the etching is further progressed as illustrated inFIG. 11C, a tapered SiON core36of which the core thickness changes from 0.5 μm to 0 μm is gained. After that, the SiON core36is etched so as to be the form in a plane as illustrated inFIG. 9B. Here, the SiON film42may be etched in advance at the stage inFIG. 11Ato the form in a plane as illustrated inFIG. 9B.

FIG. 12is a diagram illustrating the mounting structure of the optical fiber mounted photonic integrated circuit device according to Example 1 of the present invention. The Si photonic integrated circuit device is mounted on a mounting substrate45, and positioning pins46that are fixed to the ferrule40are inserted into the holes provided in the silicon substrate31for positioning. The shape of the positioning pins46may be columnar, prism-shaped or tapered. In addition, the material of the positioning pins46is generally a metal but may be a material other than metal.

When engagement members47provided to the mounting substrate45and engagement members49provided to the lid48for pressing the ferrule40are engaged with each other, the ferrule40is pressed toward the Si photonic integrated circuit device side by applying a load so that the state where the Si photonic integrated circuit device and the inclined connection end surface of the single mode optical fiber37are pressed against each other can be maintained.

The weight applying mechanism is not limited to the engagement mechanism illustrated in the figure, and any measure may be taken as long as the weight that is required to maintain the state where the Si photonic integrated circuit device and the inclined connection end surface of the single mode optical fiber37are pressed against each other can be applied. For example, the lid48placed on the ferrule40may be fixed with bolts or the structure may be provided with a spring through which weight is applied, but excessive weight can be prevented from being applied.

In Example 1 of the present invention, the SiON core36is tapered so that the thickness of the core is reduced and is connected to the inclined connection end surface of the single mode optical fiber37, and therefore, the tolerance in the positioning for the coupling to the single mode optical fiber can be increased. Typically, the tolerance in the positioning for the coupling between the single mode optical fiber37and the Si fine wire core33can be improved by one digit in the lateral direction and by two digits in the direction of the optical axis. As a result, a simple positioning mechanism makes optical fiber connection possible, and thus, it becomes possible to achieve high coupling efficiency in a device mounted with an inexpensive mechanism.

Next, the optical fiber mounted photonic integrated circuit device according to Example 2 of the present invention is described in reference toFIGS. 13A and 13B.FIGS. 13A and 13Bare diagrams illustrating the coupling portion in the optical fiber mounted photonic integrated circuit device according to Example 2 of the present invention.FIG. 13Ais a cross-sectional diagram along the optical axis, andFIG. 13Bis a plan diagram where the main portions can be clearly seen from the top. The basic structure is the same as in Example 1. In Example 2 of the present invention, however, a transparent resin50is used for the transparent member instead of a transparent oil. GA700H (product number of resin made by NTT Advanced Technology (NTT-AT) Corporation) is used for the transparent resin.

In Example 2 of the present invention, the transparent resin50is used for the transparent member, and therefore, a weight applying mechanism is not essential in the case where the adhesiveness of the transparent resin50is sufficiently strong to maintain the state where the Si photonic integrated circuit device and the inclined connection end surface of the single mode optical fiber37are pressed against each other.

Next, the optical fiber mounted photonic integrated circuit device according to Example 3 of the present invention is described in reference toFIG. 14.FIG. 14is a diagram illustrating the mounting structure of the optical fiber mounted photonic integrated circuit device according to Example 3 of the present invention. The basic structure is the same as in Example 1 illustrated inFIG. 12. In Example 3 of the present invention, however, positioning pins51are provided on the Si photonic integrated circuit device side in the structure so as to be inserted into the holes provided in the ferrule40.

Here, the precision in positioning is more relaxed to the amount of several tens of μm due to the above-described improvement in the tolerance in the lateral direction, and therefore, the mechanism for positioning to this degree is not limited to the mechanisms illustrated inFIG. 12 or 14. For example, the step into which the external periphery of the ferrule40is engaged may be created in the Si photonic integrated circuit device for positioning. Alternatively, a step into which the external periphery of the Si photonic integrated circuit device is engaged may be created in the ferrule40.