Patent Description:
Certain examples are described in the following detailed description and in reference to the drawings, in which:.

Optical fibers may be used to carry or transmit light or optical signals to and from a PIC. Therefore, efficient coupling of light or optical signals to and from the one or more optical fibers is an important aspect of PIC connector design. In particular, single mode optical fibers present unique challenges due to the two orthogonal polarization modes supported by such fibers which should be considered when designing optical connectors between such fibers and the PIC.

While single-mode polarization-maintaining fibers are available, they are generally more costly relative to single mode optical fibers. Additionally, such single mode polarization maintaining fibers may need to be "keyed" to align the transmitted polarization modes at a receiver component. Polarization splitting grating couplers (PSGCs) are also presently available which may be used to couple optical signals from single mode optical fibers to the PIC. However, the coupling efficiency of PSGCs to single mode optical fibers may be poor having peak insertion losses generally greater than about <NUM> dB. Polarization independent grating couplers may also offer a potential solution. However, design and fabrication of such polarization independent grating couplers remain challenging with respect to achieving wide optical bandwidth and low peak insertion loss.

Typically, single polarization grating couplers are relatively more simple to design and fabricate with wide optical bandwidth and low peak insertion loss (e.g., generally less than about <NUM> dB). Therefore, there remains a need for improvements in efficiently coupling a single mode optical fiber to single polarization grating couplers. A polarization diversity optical interface assembly including a walk-off crystal may be configured to separate the polarization modes of a single mode optical fiber to be coupled to single polarization grating couplers in an efficient manner as described herein. Implementations of the present disclosure provide improved polarization diversity optical interface assemblies and methods thereof to couple light (e.g., optical signals) between a single mode optical fiber and single polarization first and second grating couplers to achieve efficient coupling, low peak insertion loss, or wide optical bandwidth.

<FIG> illustrate a polarization diversity optical interface assembly <NUM> and components thereof. The polarization diversity optical interface assembly <NUM> includes one or more single mode optical fiber(s) <NUM>, grating couplers <NUM> (e.g., identified individually as a first grating coupler 104a and a second grating coupler 104b) disposed on a substrate <NUM>, and an optical connector <NUM> configured to couple light (e.g., optical signals) between the single mode optical fiber <NUM> and the first and second grating couplers 104a and 104b.

Each of the first and second grating couplers 104a and 104b may be coupled to a waveguide <NUM> (e.g., identified individually as a first waveguide 108a and a second waveguide 108b, respectively). The grating couplers <NUM> may each be single polarization grating couplers. The waveguides <NUM> may each be polarization dependent or sensitive waveguides. The waveguides <NUM> may transmit light to or from the optical fiber <NUM>. For example, the waveguides <NUM> may transmit optical signals from the optical fiber <NUM> across the substrate <NUM> to an optoelectronic device <NUM> (e.g., a light emitter, detector, modulator, or optical transceiver) disposed on the substrate <NUM> to receive, detect, or process such signals. In other implementations, the waveguides <NUM> may transmit optical signals from the optoelectronic device <NUM> off or away from the substrate <NUM> onto the optical fiber <NUM> (e.g., via the grating couplers <NUM> and optical connector <NUM>) to be received, detected, or processed by an optoelectronic device disposed off the substrate <NUM>. In some implementations, waveguides 108a and 108b are integrated or monolithically formed with respective grating couplers 104a and 104b (e.g., a waveguide grating coupler). In other implementations, the waveguides <NUM> and grating couplers <NUM> are separately formed.

The single mode optical fiber <NUM> may also be coupled to an optoelectronic device <NUM> (e.g., a light emitter, detector, modulator, or optical transceiver) disposed off the substrate <NUM>. In this manner, the optical fiber <NUM> may transmit light or optical signals from the optoelectronic device <NUM> onto the substrate <NUM> (e.g., into the waveguides <NUM> via the optical connector <NUM> and grating couplers <NUM>) to be received, detected, or processed by the optoelectronic device <NUM> disposed on the substrate <NUM>. In other implementations, optical signals may be carried or transmitted off the substrate <NUM> by the optical fiber <NUM> from the optoelectronic <NUM> device disposed on the substrate <NUM> as described above. The single mode optical fiber <NUM> may be a single optical fiber or an array of optical fibers.

The optical connector <NUM> couples light between the single mode optical fiber <NUM> and each of the first and second grating couplers 104a and 104b. The optical connector <NUM> includes a ferrule <NUM> coupled to at least a portion of the single mode optical fiber <NUM>. The optical connector <NUM> further includes a walk-off crystal <NUM>. The walk-off crystal <NUM> spatially separates the light into first and second orthogonal polarization modes (e.g., identified individually as Ex and Ey) prior to passing the polarization modes through the respective first and second grating couplers 104a and 104b, combine the first and second polarization modes of the light Ex and Ey (e.g., from the respective first and second grating couplers 104a and 104b) into a single optical output signal prior to passing the light into the single mode optical fiber <NUM>, or both. The optical connector <NUM> may be a single mode expanded beam optical connector. For example, the optical connector <NUM> may include a lens <NUM> coupled to or disposed at an end of the single mode optical fiber <NUM> to collimate the light exiting or emerging from the single mode optical fiber <NUM> as described in more detail below (see <FIG>).

Referring to <FIG>, the first and second grating couplers 104a and 104b may extend co-planar with each other. For example, the grating couplers <NUM> may extend along a same horizontal plane or surface of substrate <NUM> (e.g., a top or front side or bottom or rear side of substrate <NUM>). In some implementations, the first and second grating couplers 104a and 104b may also extend or be oriented at an angle (e.g., orthogonal or non-oblique) relative to each other to ensure the first and second orthogonal polarization modes propagate in the proper orientations or directions across the grating couplers <NUM> (e.g., along optical beam axes of the grating couplers). For example, the first grating coupler 104a may extend orthogonally relative to the second grating coupler 104b such that a direction of the light or beam path of the first polarization mode Ex propagates across the first grating coupler 104a in a direction (e.g., east-west direction) orthogonal to a direction (e.g., north-south direction) that the light or beam path of the second polarization mode Ey propagates across the second grating coupler 104b. In other implementations, the optical connector <NUM> may include one or more lenses or tilt elements <NUM> (see <FIG>), such as a prism to ensure proper propagation or orientation of the light or beam path from the first and second polarization modes along respective optical beam axes of respective grating couplers 104a and 104b, as described in more detail below.

The light from the first and second polarization modes Ex and Ey may have the same polarization or orientation once they are in their respective waveguides <NUM> (e.g., waveguides <NUM> may have one or more turns or bends <NUM> such that the polarizations or beam path directions are the same or parallel). In some implementations, ends of the grating couplers <NUM> may be spaced apart as illustrated.

Referring to <FIG> and as described above, the walk-off crystal <NUM> spatially separates or combines the first and second polarization modes of the light Ex and Ey. The walk-off crystal <NUM> may provide angular walk-off (<FIG>) or position walk-off (<FIG>) to separate (or combine) the polarization modes. The walk-off crystal <NUM> may be constructed from a birefringent crystal such as TiO<NUM> or YVO<NUM>. The degree or amount of angular or position walk-off <NUM> (e.g., between <NUM> and <NUM> degrees or between <NUM> to <NUM> radians) may be achieved by cutting or shaping the walk-off crystal <NUM> at a desired angle, orienting the walk-off crystal <NUM> relative to incoming or outgoing light, or orienting incoming or outgoing light with respect to a crystal optic axis of the walk-off crystal <NUM> to have a desired angle of incidence. In some implementations, the walk-off crystal <NUM> may include only a single crystal. In other implementations, the walk-off crystal <NUM> may include two or more crystals.

In some implementations, a thickness T of the walk-off crystal <NUM> is selected such that a walk-off distance between the first and second polarization modes of the light Ex and Ey matches a spacing X (e.g., <FIG>) between optical axes of grating couplers <NUM>. In other implementations, a thickness of walk-off crystal <NUM> is selected such that a walk-off distance between the first and second polarization modes of the light Ex and Ey matches a spacing between axes of lenses (e.g., lenses <NUM>) configured to tilt or focus the light onto the grating couplers <NUM>. In some implementations, the walk-off crystal <NUM> has a thickness between <NUM> to <NUM> which may provide or correspond to a walk-off distance between <NUM> microns to <NUM> microns between the first and second polarization modes of the light Ex and Ey.

Referring back to <FIG>, the substrate <NUM> may serve as a foundation or common carrier for various electronic and optical components of a PIC including the grating couplers 104a and 104b in which light is coupled from the single mode optical fiber <NUM> by the optical connector <NUM>. In some implementations, the substrate <NUM> is constructed out of silicon, thus providing a silicon photonics platform. In some implementations, the substrate <NUM> includes a silicon photonic interposer on which the various electronic and optical components may be disposed. The substrate <NUM> may include multiple layers (e.g., semiconductor, dielectric, or insulating layers). For example, the substrate <NUM> may include an insulating layer sandwiched between semiconductor layers. The substrate <NUM> may be a silicon-on-insulator (SOI), a silicon-on-glass, silicon on sapphire, or silicon on nothing substrate. In some implementations, the substrate <NUM> includes an insulating layer such as a buried oxide (BOX) layer composed of silicon dioxide or other insulating oxide material. For example, the grating couplers 104a and 104b may be disposed on the BOX layer with the BOX layer disposed over or on a base silicon substrate layer of the substrate <NUM>. In other implementations, the insulating layer may be composed of another insulating material such as sapphire, diamond, or glass.

With reference to <FIG>, various polarization diversity optical interface assemblies 200a-200e are illustrated with a walk-off crystal positioned at various locations along the optical path or train relative the other components. The polarization diversity optical interface assemblies 200a-200e may include one or more of any of the features described herein with respect to the polarization diversity optical interface assembly <NUM> or with respect to one another, in whole or in part. For example, the polarization diversity optical interface assemblies 200a-200e may include the single mode optical fiber <NUM>, grating couplers <NUM> and waveguides <NUM> disposed on a substrate <NUM>, optoelectronic devices <NUM> or <NUM> disposed on or off the substrate <NUM>, or optical connector <NUM> with ferrule <NUM> as described herein.

In an embodiment illustrated in <FIG> but not covered by the claimed invention, the polarization diversity optical interface assembly 200a includes the lens <NUM> to collimate light exiting the single mode optical fiber <NUM> (or focus light entering the single mode optical fiber <NUM>). An optical turning element <NUM> (e.g., internal reflection mirror, parabolic turning mirror) may be used to turn the light to a proper orientation relative to the crystal optic axis of the walk-off crystal <NUM> such that a desired walk-off between the polarization modes of the light is achieved. In this manner, the walk-off crystal <NUM> is disposed after the optical turning element <NUM> along the optical path. In other implementations, as described in more detail below, the walk-off crystal <NUM> may be positioned or disposed at other various locations along the optical path or train between the single mode optical fiber <NUM> and the grating couplers <NUM> (e.g., before the optical turning element <NUM>, within the ferrule <NUM>, outside the ferrule <NUM>, disposed on the substrate <NUM>, between the one or more lenses <NUM> and the grating couplers <NUM>).

After exiting the walk-off crystal <NUM>, the first and second polarization modes of the light Ex and Ey may be tilted or oriented to ensure the modes propagate along optical axes of the respective grating couplers 104a and 104b. For example, the polarization diversity optical interface assembly 200a may include one or more light tilting lenses or elements <NUM> identified individually as 232a and 232b (e.g., beam tilting prisms). In other implementations, the polarization diversity optical interface assembly 200a may include one or more lenses <NUM> identified individual as 236a and 236b (e.g., disposed, mounted, or established on the substrate <NUM>) instead of or in addition to the tilting elements <NUM>. The one or more lenses <NUM> may be spherical or aspherical lenses.

The one or more lenses <NUM> may focus, tilt, or focus and tilt the first and second polarization modes of the light Ex and Ey onto the grating couplers 104a and 104b such that light propagates along the optical axes of the respective grating couplers. As described above, the light may then enter respective waveguides 108a and 108b to be carried to optoelectronic device <NUM> disposed on the substrate <NUM> to be further processed. For example, the light may be carried to a data modulation system, polarization diversity circuit, or other photonic integrated circuit.

With reference to <FIG>, as illustrated and according to the invention, the walk-off crystal <NUM> of polarization diversity optical interface assembly 200b is positioned within the ferrule <NUM>. In this implementation, the walk-off crystal <NUM> is positioned or disposed after the optical turning element <NUM> similar to polarization diversity optical interface assembly 200a. After the light is spatially separated into the first and second polarization modes of the light Ex and Ey, first and second tilting elements <NUM> disposed on or coupled to the ferrule <NUM> may tilt or orient the respective light paths of each mode such that they propagate in proper directions (e.g., east-west and north-south when the grating couplers are orthogonal to each other) along optical axes of grating couplers <NUM> as described above. In some implementations, as illustrated, the first polarization mode is tilted about an x-axis while the second polarization mode is tilted about a y-axis prior to entering respective grating couplers 104a and 104b (e.g., at the optimum angle of incidence). In some implementations, the tilted first and second polarization modes of the light Ex and Ey may then pass through respective lenses 236a and 236b to be focused onto the grating couplers 104a and 104b (e.g., to ensure mode matching to the grating coupler and they reach the grating couplers at the optimum angle of incidence). The lenses 236a and 236b may be mounted on the substrate <NUM>.

With reference to <FIG>, the walk-off crystal <NUM> of polarization diversity optical interface assembly 200c is positioned within the ferrule <NUM> before the optical turning element <NUM> such that the first and second polarization modes of the light Ex and Ey are spatially separated along the optical path prior to being turned by the optical turning element <NUM>.

In some non-claimed implementations, the walk-off crystal <NUM> may be positioned or disposed outside or off of the ferrule <NUM>. For example, as illustrated in <FIG> but not covered by the claimed invention, the walk-off crystal <NUM> of polarization diversity optical interface assembly 200d may be positioned between one or more lenses <NUM> and grating couplers 104a and 104b. The walk-off crystal <NUM> may be established or disposed on the substrate <NUM>. The one or more lenses <NUM> may be disposed on the substrate <NUM> or coupled to the ferrule <NUM>. The polarization diversity optical interface assembly 200d may further includes one or more tilting elements <NUM> as described herein between the walk-off crystal <NUM> and the grating couplers 104a and 104b.

With reference to <FIG>, in some implementations, the polarization diversity optical interface assembly 200e includes a single lens <NUM> disposed after the walk-off crystal <NUM> to focus or orient the first and second polarization modes of the light Ex and Ey onto and along optical axes of the grating couplers 104a and 104b. In some implementations, the polarization diversity optical interface assembly 200e may further include tilting elements <NUM> to further ensure that the first and second polarization modes of the light Ex and Ey propagate along the optical axes of grating couplers 104a and 104b. The walk-off crystal <NUM> is disposed within the ferrule <NUM>.

With reference to <FIG>, any of the polarization diversity optical interface assemblies described herein may include an optical connector <NUM> with a ferrule <NUM> configured to carry or couple to an array <NUM> of single mode optical fibers 102a-102d. As illustrated, the walk-off crystal <NUM> may extend across the entire array <NUM> to spatially separate the light from each fiber <NUM> into first and second polarization modes prior to passing through pairs of corresponding grating couplers <NUM>. In other implementations, multiple spaced apart walk-off crystals <NUM> may be located between each fiber 102a-102d and grating couplers.

<FIG> is a flowchart illustrating a method of coupling light between a single mode optical fiber and single polarization first and second grating couplers with a polarization diversity optical interface assembly (200b, 200c, 200e, <NUM>). The method <NUM> begins at block <NUM>, wherein a single mode optical fiber is coupled to a ferrule. At bock <NUM>, light is emitted from an optoelectronic device through the single mode optical fiber. The light is then passed through a walk-off crystal such that the light is spatially separated into first and second orthogonal polarization modes prior to entering respective single polarization first and second grating couplers disposed on a substrate in block <NUM>. At block <NUM>, the first and second polarization modes are passed through the respective single polarization first and second grating couplers.

In some implementations, the method <NUM> further includes passing the first and second polarization modes through first and second polarization dependent waveguides coupled to the respective single polarization first and second grating couplers after passing the first and second polarization modes through the respective single polarization first and second grating couplers. The light is passed through the walk-off crystal such that the light is spatially split into the first and second polarization components prior to exiting the ferrule. In some implementations, the method <NUM> further includes passing the first and second polarization modes through respective first and second light tilting elements to orient the first and second polarization modes to propagate along respective optical axes of the respective single polarization first and second grating couplers. In yet other implementations, the method <NUM> further includes passing the first and second polarization modes through one or more lenses mounted on the substrate to focus the first and second polarization modes onto the respective single polarization first and second grating couplers.

Optoelectronic devices <NUM> and <NUM> such as light emitters, modulators, or photodetectors described herein may include, but are not limited to, vertical cavity surface emitting lasers, distributed feedback lasers, mach-zehnder or ring modulators, or p-i-n or avalanche photodiodes.

In the foregoing description, numerous details are set forth to provide an understanding of the subject disclosed herein. However, implementations may be practiced without some or all of these details, or may include additions, modifications, or variations from the details discussed above, insofar as the implementations are covered by the appended claims. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive.

It will be recognized that the terms "comprising," "including," and "having," as used herein, are specifically intended to be read as open-ended terms of art. The term "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, mechanical, logical, optical, electrical, or a combination thereof.

Claim 1:
A polarization diversity optical interface assembly (<NUM>) comprising:
a single mode optical fiber (<NUM>);
first and second grating couplers (104a, 104b), wherein the first and second grating couplers (104a, 104b) are disposed on a substrate (<NUM>); and
an optical connector (<NUM>; <NUM>) to couple light between the single mode optical fiber and each of the first and second grating couplers (104a, 104b), the optical connector (<NUM>; <NUM>) comprising a walk-off crystal (<NUM>; <NUM>) to spatially separate the light into first and second orthogonal polarization modes prior to passing the first and second orthogonal polarization modes through the respective first and second grating couplers (104a, 104b), wherein
the optical connector (<NUM>; <NUM>) comprises a ferrule (<NUM>; <NUM>) coupled to a portion of the single mode optical fiber (<NUM>), wherein
the walk-off crystal (<NUM>; <NUM>) is disposed within the ferrule (<NUM>; <NUM>) such that the light is spatially separated into the first and second polarization modes prior to exiting the ferrule (<NUM>; <NUM>) and passing through the respective first and second grating couplers (104a, 104b).