Patent Description:
The disclosure relates to optical coupling, such as among an array of fibers and an array of waveguides within a waveguide circuit, e.g., a planar lightwave circuit (PLC) and/or photonic integrated circuit (PIC) (e.g., silicon photonic circuit). In particular, this disclosure relates to a fiber optic-to-waveguide coupling assembly including an interposer for evanescent coupling to a waveguide circuit and edge coupled to optical fibers of a fiber array unit (FAU).

<FIG> is a perspective view of a fiber-to-waveguide coupling system <NUM> (e.g., planar lightwave circuit (PLC) assembly) including a fiber array unit coupler <NUM> (e.g., FAU coupler) of a fiber array unit <NUM> and a waveguide coupler <NUM> of a waveguide assembly <NUM>. The fiber array unit coupler <NUM> includes a plurality of optical fibers <NUM> with end faces <NUM>. The waveguide coupler <NUM> includes a plurality of waveguides <NUM> with end faces <NUM>. When the fiber array unit coupler <NUM> is engaged with the waveguide coupler <NUM>, the end faces <NUM> of the optical fibers <NUM> are in contact with (or closely proximate to) and aligned with the end faces <NUM> of the waveguides <NUM>. In this way, the optical fibers <NUM> and the waveguides <NUM> are edge coupled permitting optical communication between the fiber array unit <NUM> and a waveguide circuit <NUM> of the waveguide assembly <NUM>.

Edge coupling between the optical fibers <NUM> and the waveguides <NUM> may require an optical quality edge on the waveguide circuit <NUM>, which adds manufacturing cost and process complexity. Such a configuration may also require precise alignment between the optical fibers <NUM> and the waveguides <NUM>, which may be difficult, time consuming and/or expensive. Achieving precise alignment may require complex manufacturing processes and/or components which are not compatible with standard electronic integrated circuit assembly processes, such as high throughput pick and place machines used to place surface mount devices onto a printed circuit board (PCB).

<FIG> is a cross-sectional view of another fiber-to-waveguide coupling system <NUM>' for edge coupling. The fiber-to-waveguide coupling system <NUM>' includes a planar waveguide array <NUM>' of an interposer <NUM>' that is intermediate and positioned between a waveguide circuit <NUM>' and a plurality of optical fibers <NUM>' of a fiber array unit (FAU) coupler <NUM>'. The interposer <NUM>' is positioned to edge couple light to or from optical fibers <NUM>' of the FAU coupler <NUM>'. The fiber-to-waveguide coupling system <NUM>' avoids the need for an optical quality edge for the waveguide circuit <NUM>' and allows for surface mounting of the interposer <NUM>', but still requires actively aligning the FAU coupler <NUM>' to waveguides <NUM>' of the interposer <NUM>' using all six translational and rotational degrees of freedom. Attempts to simplify this complex alignment process and reduce the number of degrees of freedom for highly precise passive alignment may require complicated manufacturing steps. For example, the use of complementary insertion pins and receptacles in a male-female relationship may align the FAU coupler <NUM>' and the waveguide coupler <NUM>', but manufacturing of couplers with such insertion pins requires specific relative sizing and placement (using all six translational and rotational degrees of freedom). This may be further complicated when formation of corresponding alignment features requires precise manufacturing of very different materials and different manufacturing processes between respective couplers.

While passive alignment freedom leads to faster, lower cost integrated photonic packages, what is needed is a simple fabrication and assembly compatible with existing processes.

<CIT> discloses a method and system for passively aligning optical fibers, a first waveguide array, and a second waveguide array using chip-to-chip vertical evanescent optical waveguides, that can be used with fully automated die bonding equipment. The assembled system can achieve high optical coupling and high process throughput for needs of high volume manufacturing of photonics, silicon photonics, and other applications that would benefit from aligning optical fibers to lasers efficiently.

<CIT> discloses an optical waveguide device which can be connected with an optical fiber through a simple positioning adjustment. The optical waveguide device comprises a substrate, a first cladding with a uniform thickness formed on the substrate, a core formed on the first cladding, a recognizable thin layer formed on the first cladding on both sides of the core, and a second cladding formed on the first cladding so as to cover the core. The thin layer is parallel to the top surface of the substrate and defines a reference plane which is substantially in the same plane as the bottom surface of the core. Connection of the optical waveguide device with the optical fiber is achieved by bringing the reference plane into contact with a plane which is in a predetermined relative position to the optical fiber and only making a planar positioning adjustment with the planes held in contact.

<CIT> discloses an optical fiber-to-waveguide coupler which automatically aligns five of the six possible degrees of freedom associated with the alignment process. Silicon v-grooves are used to hold the fibers in place in the silicon substrate, but in contrast to prior art arrangements, the silicon substrate overlaps the top surface of the waveguide substrate. A cover plate disposed over the silicon substrate is cut and polished so that the endface of the cover plate lies in the same plane as the ends of the fibers. When the endface of the cover plate is butted against the endface of the waveguide substrate, and the silicon v-grooves have been etched to the proper predetermined depth, five of the six degrees of freedom are automatically aligned.

In a first aspect, a fiber optic-to-waveguide coupling assembly is provided, comprising: a first coupler comprising: a first substrate comprising a first surface and at least one substate alignment groove in the first surface; the first substrate defining at least one mounting groove and at least one data fiber positioned in the at least one mounting groove; and an interposer comprising: a second surface and at least one waveguide positioned proximate the second surface; wherein at least a first portion of the first surface of the first substrate of the first coupler is positioned proximate at least a second portion of the second surface of the interposer to form a first overlap between the first portion of the first surface and the second portion of the second surface to align the at least one data fiber of the first coupler with the at least one waveguide of the interposer to allow for transmitting a signal between the at least one data fiber and the at least one waveguide; and the interposer further comprising at least one interposer alignment fiducial cutting into the second surface of the interposer, wherein the at least one interposer alignment feature is positioned to facilitate alignment between the at least one data fiber and the at least one waveguide when the at least one interposer alignment feature is aligned with the at least one substrate alignment groove in the first surface of the first substrate.

Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the accompanying drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments, and together with the description serve to explain principles and operation of the various embodiments.

Reference will now be made in detail to the present preferred embodiments, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

Terms such as "left," "right," "top," "bottom," "front," "back," "horizontal," "parallel," "perpendicular," "vertical," "lateral," "coplanar," and similar terms are used for convenience of describing the attached figures and are not intended to limit this description. For example, terms such as "left side" and "right side" are used with specific reference to the drawings as illustrated and the embodiments may be in other orientations in use. Further, as used herein, terms such as "horizontal," "parallel," "perpendicular," "vertical," "lateral," etc., include slight variations that may be present in working examples.

As used herein, the terms "optical communication," "in optical communication," and the like mean, with respect to a group of elements, that the elements are arranged such that optical signals are passively or actively transmittable therebetween via a medium, such as, but not limited to, an optical fiber, one or more ports, free space, index-matching material (e.g., structure or gel), reflective surface, or other light directing or transmitting means.

As used herein, it is intended that terms "fiber optic cables" and/or "optical fibers" include all types of single mode and multi-mode light waveguides, including one or more optical fibers that may be coated, uncoated, colored, buffered, ribbonized and/or have other organizing or protective structure in a cable such as one or more tubes, strength members, jackets or the like.

As used herein, the term "signal" refers to modulated or unmodulated light intended to be transmitted or received at a device.

As used herein, the term "data fiber" refers to any type of optical fiber for propagating a modulated signal.

As used herein, the term "coupler" refer to a device for connecting light from one device to another. A coupler need not be permanently attached and may be removable.

Disclosed herein is a fiber optic-to-waveguide coupling assembly with an overlap for edge coupling. In particular, disclosed is a fiber optic-to-waveguide coupling assembly with an interposer having an intermediate waveguide for evanescent coupling to waveguides (e.g., planar waveguides) within a waveguide circuit (e.g., planar lightwave circuit (PLC) and/or photonic integrated circuit (PIC) (e.g., silicon photonic circuit)) and edge coupling to optical fibers of a fiber array unit. The fiber optic-to-waveguide coupling assembly includes an interposer, a first coupler, and, in some embodiments, a second coupler. The first coupler includes a substrate and at least one data fiber. The interposer includes at least one waveguide. An x axis is perpendicular to the at least one data fiber, the at least one waveguide of the interposer, and a y axis. A first coupler overlap portion of the substrate is positionable proximate a first interposer overlap portion of the interposer to form a first overlap therebetween to align the at least one data fiber of the first coupler with the at least one waveguide of the interposer in a y direction along the y axis intersecting the substrate and the interposer. The substrate and the interposer may each include complementary alignment features (e.g., optical and/or mechanical, etc.) to further align the at least one data fiber and the at least one waveguide in an x direction along the x axis, in a z direction along the z axis, and/or around the y axis (i.e., rotation). These complementary alignment features may be made using similar manufacturing processes used to create the substrate and/or interposer. The fiber optic-to-waveguide coupling assembly provides simple and accurate passive alignment of the at least one data fiber with the at least one waveguide.

<FIG> are views of an exemplary embodiment of a fiber optic-to-waveguide coupling assembly <NUM> (e.g., fiber array unit (FAU)-to-planar waveguide passive alignment assembly) with an overlap for edge coupling. In particular, the fiber optic coupling assembly <NUM> includes a first coupler <NUM> (e.g., fiber array unit coupler, fiber optic coupler, etc.) and an interposer <NUM>. The fiber optic coupling assembly <NUM> provides for a simple and effective way to decrease the number of degrees of freedom for passive alignment between the first coupler <NUM> and the interposer <NUM>.

The first coupler <NUM> includes a first substrate <NUM> with a first end 208A, a second end 208B (opposite the first end 208A), a first side 210A, a second side 210B (opposite the first side 210A), and a first surface <NUM> (e.g., top surface). In certain embodiments, the first surface <NUM> is planar and may define a plurality of mounting grooves <NUM> (e.g., V-grooves) in the first surface <NUM> extending at least partially (e.g., partially or fully) between the first end 208A and the second end 208B. In the embodiment illustrated in <FIG>, for example, the mounting grooves <NUM> extend from the first end 208A to the second end 208B (i.e., along the entire length of the first substrate <NUM>), however, in other embodiments, the fiber V-grooves extend only partially along the first substrate <NUM>. The first substrate <NUM> includes a first coupler overlap portion <NUM> defined near the first end 208A of the first substrate <NUM> for alignment with the interposer <NUM>.

The first coupler <NUM> also includes a fiber array <NUM> including a plurality of data fibers <NUM> positioned proximate the first surface <NUM>. In particular, each of the plurality of data fibers <NUM> is positioned in (e.g., mounted within) one of the plurality of mounting grooves <NUM>. End faces <NUM> (e.g. cleaved fiber ends) are positioned between the first end 208A and the second end 208B along the length of the mounting grooves <NUM> instead of being positioned at the edge of the first substrate <NUM>. The plurality of data fibers <NUM> extend from the second end 208B toward the first end 208A and, in certain embodiments, the data fibers <NUM> do not extend into the first coupler overlap portion <NUM> of the first substrate <NUM> of the first coupler <NUM>. As illustrated in <FIG>, the end faces <NUM> of the data fibers <NUM> may be spaced slightly apart from the first end 226A of the interposer <NUM>. In other embodiments, the end faces <NUM> of the data fibers <NUM> are in contact with or directly adjacent to the first end 226A of the interposer <NUM>.

The interposer <NUM> (may also be referred to as an ion-exchange waveguide interposer, second substrate, etc.) includes a first end 226A, a second end 226B (opposite the first end 226A), a first side 228A, a second side 228B (opposite the first side 228A), a second surface <NUM> (e.g., bottom surface) and a top surface <NUM> (opposite the bottom surface). In certain embodiments, the second surface <NUM> is planar and defines a plurality of waveguide channels <NUM> (e.g., V-grooves) extending at least partially (e.g., partially or fully) between the first end 226A and the second end 226B. In certain embodiments, the plurality of waveguide channels <NUM> may be defined by a photolithographic process, such as ultraviolet exposure and development of photoresist. In this way, the plurality of waveguide channels <NUM> may be formed by a transformation of a portion of the interposer <NUM> rather than removal of material from the interposer <NUM>. In certain embodiments, that transformation can occur through an ion-exchange process. The interposer <NUM> includes a first interposer overlap portion <NUM> defined at the first end 226A of the interposer <NUM> and a second interposer overlap portion <NUM> defined at the second end 226B of the interposer <NUM>.

The interposer <NUM> further includes a waveguide array <NUM> including a plurality of waveguides <NUM> (e.g., planar waveguides, silicon waveguides, polymer waveguides, glass waveguides, ion-exchange glass waveguides, etc.). In some embodiments, the waveguides <NUM> are made of glass (i.e., glass waveguides). Each of the waveguides <NUM> is positioned in (e.g., defined in) one of the plurality of waveguide channels <NUM>. In certain embodiments, the waveguides <NUM> are defined by a photolithographic process and thereby formed, defined, and positioned within the waveguide channels <NUM>. The waveguides <NUM> extend from the first end 226A to the second end 226B such that the waveguides <NUM> extend into the first interposer overlap portion <NUM> of the interposer <NUM> and into a second interposer overlap portion <NUM>. As explained in more detail below, the second interposer overlap portion <NUM> of the interposer <NUM> provides an area for evanescent coupling of the waveguides <NUM> to waveguides of a waveguide circuit (e.g., silicon inverse-taper waveguides) of a second coupler.

In the embodiments illustrated in <FIG>, the x axis is perpendicular to the data fibers <NUM>, the waveguides <NUM>, and a y axis. In other words, the x axis extends through the first side 210A and the second side 210B of the first substrate <NUM> and/or through the first side 228A and the second side 228B of the interposer <NUM>. The y axis is perpendicular the data fibers <NUM> and the waveguides <NUM>. Further, the y axis intersects the first surface <NUM> of the first substrate <NUM> and the second surface <NUM> of the interposer <NUM>. A z axis is aligned with or parallel to the data fibers <NUM> and the waveguides <NUM>, and perpendicular to the x axis and the y axis.

In general, there are six degrees of freedom for aligning two objects in space: translation along the x axis, y axis, and z axis, as well as rotation around the x axis (i.e., pitch, tip, etc.), rotation around the y axis (i.e., yaw, etc.), and rotation around the z axis (i.e., roll, tilt, etc.). The mechanical features and/or visual aids provided by the fiber optic-to-waveguide coupling assembly <NUM> reduce the number of degrees of freedom between the first coupler <NUM> and the interposer <NUM>, thereby making it easier to align the first coupler <NUM> of a fiber array unit to the interposer <NUM>. For example, as explained below in more detail, the first surface <NUM> of the first coupler <NUM> of the fiber array unit constrains the interposer <NUM> in the y-direction, rotation about the x axis, and/or rotation about the z axis. The end faces <NUM> of the data fibers <NUM> constrain the interposer <NUM> in the z-direction and/or rotation about the y axis. It is noted that alignment along and about the x axis and y axis may require greater precision (e.g., within <NUM> microns, preferably within <NUM> micron, etc.) than alignment along and about the z axis (e.g., within <NUM> microns).

The first coupler overlap portion <NUM> of the first substrate <NUM> of the first coupler <NUM> is positionable proximate the first interposer overlap portion <NUM> of the interposer <NUM> to form a first overlap <NUM> therebetween. Further, the y axis intersects the first surface <NUM> of the first substrate <NUM> and the second surface <NUM> of the interposer <NUM> at the first overlap <NUM>. This first overlap <NUM> aligns the data fibers <NUM> of the first coupler <NUM> with the waveguides <NUM> of the interposer <NUM> in one or more directions. In particular, the first overlap <NUM> aligns the data fibers <NUM> and the waveguides <NUM> in a y direction along the y axis (i.e., alignment is within a plane defined by the x and z axes) by the first surface <NUM> of the first substrate <NUM> contacting (directly or indirectly) the second surface <NUM> of the interposer <NUM>. Because the first surface <NUM> of the first substrate <NUM> and the second surface <NUM> of the interposer <NUM> are planar, contacting the first surface <NUM> and the second surface <NUM> would further align the data fiber <NUM> with the waveguides <NUM> around the x-axis (i.e., tip) and/or around a z-axis (i.e., tilt). As shown in <FIG>, the data fibers <NUM> do not extend into the first overlap <NUM> but the waveguides <NUM> do extends into the first overlap <NUM>.

The remaining degrees of freedom to align the data fibers <NUM> with the waveguides <NUM> include alignment by translation in an x direction along the x axis, alignment by rotation about the y axis, and/or alignment by translation in a z direction along the z axis. In certain embodiments, alignment of the first data fibers <NUM> with the waveguides <NUM> by translation in the z direction along the z axis and/or alignment by rotation about the y-axis may be achieved by translating the interposer <NUM> toward the second end 208B of the first substrate <NUM>, until the first end 226A of the interposer <NUM> abuts the end faces <NUM> of the data fibers <NUM>. In other words, the degrees of freedom may be constrained by aligning the end faces <NUM> of the data fibers <NUM> with the first end 226A of the interposer <NUM>. Additionally, or alternatively, in certain embodiments, the first substrate <NUM> and the interposer <NUM> may include one or more complementary alignment features to passively align in an x direction along the x axis and/or rotationally around the y axis.

As shown in <FIG>, in one embodiment, the first substrate <NUM> defines a first substrate alignment groove 244A in the first surface <NUM> (disposed toward the first side 210A and positioned between the first side 210A and the mounting grooves <NUM>), and a second substrate alignment groove 244B (disposed toward the second side 210B and positioned between the second side 210B and the mounting grooves <NUM>). Thus, the first substrate alignment groove 244A and the second substrate alignment groove 244B (referred to generally as substrate alignment grooves <NUM>) are adjacent the mounting grooves <NUM>. In particular, at least a portion of the substrate alignment grooves <NUM> is provided in the first coupler overlap portion <NUM> toward the first end 208A of the first substrate <NUM>. The same manufacturing processes that are used to manufacture the mounting grooves <NUM> could also be used to manufacture the substrate alignment grooves <NUM> for increased precision and reduced complexity. For example, one such manufacturing process includes machining glass with a diamond wheel (e.g., using a CNC machine). In this way, in certain embodiments, spacing between adjacent mounting grooves <NUM> (also called evenly-spaced mounting grooves) is the same (i.e., plus or minus <NUM> micron) as spacing between the substrate alignment grooves <NUM> and the adjacent mounting grooves <NUM> (to simplify manufacturing thereof). In certain embodiments, the substrate alignment grooves <NUM> have a same depth as the mounting grooves <NUM> (to simplify manufacturing thereof), and in other embodiments the substrate alignment grooves <NUM> may have a different depth. For example, in certain embodiments the substrate alignment grooves <NUM> may have a greater depth to accommodate an alignment cylinder, as discussed in more detail below. Thus, the substrate alignment grooves <NUM> are used as the fiducials since they can be precisely located with respect to the mounting grooves <NUM>.

The interposer <NUM> may include a material that is transparent to visible light (e.g., glass). Further, the interposer <NUM> includes interposer alignment fiducials <NUM> on the second surface <NUM> of the interposer <NUM>. In particular, the interposer alignment fiducials <NUM> are provided in the first interposer overlap portion <NUM> toward the first end 226A of the interposer <NUM>. The interposer alignment fiducial <NUM> may be additive (e.g., extending from the second surface <NUM>), in examples not part of the claimed invention, or subtractive (e.g., cutting into the second surface <NUM>), according to the claimed invention. For example, the interposer alignment fiducials <NUM> may be photolithographically defined (i.e., made by photolithography). In certain embodiments, for alignment with the first substrate <NUM>, the second surface <NUM> of the interposer <NUM> has photolithographically-defined fiducials and/or photolithographically-defined etched grooves adjacent to the waveguides <NUM> with the same spacing as the substrate alignment grooves <NUM>.

The interposer alignment fiducial <NUM> may be any of a variety of shapes (e.g., dot, circle, triangle, square, etc.) and sizes. In some embodiments, for example, the interposer alignment fiducials <NUM> need only be a line that, when aligned with the edge or bottom of the V-groove, defines the lateral translation (e.g., along the x-axis) and/or in-plane rotation (e.g., about the y-axis) needed to align the data fibers <NUM> to the waveguides <NUM>. The same manufacturing processes that are used to manufacture the waveguides <NUM> could also be used to manufacture the interposer alignment fiducials <NUM> for increased precision and reduced complexity. Fiducials typically are intended to align with other fiducials (e.g., photolithographically created fiducials may need registration with respect to other photolithographically-defined features), and here the waveguides <NUM> (e.g., ion-exchange waveguides) of the interposer <NUM> are fabricated after a photolithographically-defined photomask defines openings for the silver in a salt bath to exchange with sodium in the glass.

The interposer alignment fiducial <NUM> is configured to cooperate with the substrate alignment groove <NUM> to align the data fiber <NUM> of the first coupler <NUM> with the waveguides <NUM> of the interposer <NUM> in an x direction along the x axis and/or rotationally around the y axis. For example, the interposer alignment fiducials <NUM> are positioned on the second surface <NUM> of the interposer <NUM> to align with the substrate alignment grooves <NUM>. In particular, the interposer alignment fiducial <NUM> is a square shape and the width of the square is generally the same width as that of the substrate alignment groove <NUM>. By looking at the interposer alignment fiducial <NUM> and the substrate alignment grooves <NUM> through the transparent interposer <NUM>, the square shape of the interposer alignment fiducial <NUM> and the substrate alignment grooves <NUM> could be used to orient the interposer <NUM> relative to the first substrate <NUM> in an x direction along the x axis and/or rotationally around the y axis. In this way, the mechanical substrate alignment grooves <NUM> and optical interposer alignment fiducials <NUM> are used for visual, passive alignment of the vertically-placed interposer <NUM>.

It is noted that in certain embodiments, only a point of the interposer alignment fiducial <NUM> is configured to align with only a point of the substrate alignment grooves <NUM> to orient the interposer <NUM> relative to the first substrate <NUM> in an x direction along the x axis. Further, although only one of the interposer alignment fiducials <NUM> and/or only one of the substrate alignment grooves <NUM> is needed for alignment, multiples can be provided to further facilitate alignment and alignment accuracy.

Once assembled, the mounting grooves <NUM> underneath the waveguides <NUM> may be either left open or filled with adhesive. If exposure to air is a concern, epoxy can be placed along the edges of the first coupler overlap portion <NUM> (e.g., proximate the first side 228A and/or the second side 228B) but not underneath the entire length of the waveguides <NUM>. The positioning of the data fibers <NUM> and waveguides <NUM> is designed to align for maximum coupling of light therebetween. In certain embodiments, index matching material may be applied between the end faces <NUM> of the data fibers <NUM> and the waveguides <NUM> of the interposer <NUM>.

Advantages may include cost savings as there may be no additional alignment parts (at least by using fiducials) and may be a reduction in assembly steps (e.g., FAU polishing) and assembly time by elimination alignment steps. Further, other advantages may include more mechanically robust configurations since an interposer <NUM> may be positioned on top of the first coupler <NUM> rather than in front of it (e.g., larger bond area). Larger, unobstructed bond areas may make it easier to use laser bonding, which can provide higher processing temperature and operating temperature performance and less movement of parts when bonded. In certain embodiments, higher processing temperature means that the assembly <NUM> can survive solder reflow temperature cycling in the attachment of electronic integrated circuits (ICs) via a surface-mounted/ball-grid-array process.

<FIG> is a perspective view another exemplary embodiment of a first substrate <NUM> of the first coupler <NUM> of <FIG> with substrate alignment fiducials 302A, 302B, <NUM> and mounting grooves <NUM> terminated within the first substrate <NUM>. In other words, the mounting grooves <NUM> (e.g., fiber V-grooves) terminate part way through the first substrate <NUM> and/or do not extend into the first coupler overlap portion <NUM>. In particular, the first substrate <NUM> defines a first substrate longitudinal alignment fiducial 302A and a second substrate longitudinal alignment fiducial 302B (may be referred to generally as substrate longitudinal alignment fiducials <NUM>) defined (e.g., cut) in the first surface <NUM> within the first coupler overlap portion <NUM>. The mounting grooves <NUM> include a termination end <NUM> offset from the first end 208A of the first substrate <NUM>, formed by a trench <NUM> formed in the first surface <NUM> of the first substrate <NUM> proximate the first coupler overlap portion <NUM> and extending between the first side 210A and the second side 210B. In other words, the mounting grooves <NUM> terminate at the trench <NUM> that is transverse (e.g., perpendicular) to the mounting grooves <NUM>. In this way, the mounting grooves <NUM> do not extend into the first coupler overlap portion <NUM> and there is no gap or open space beneath the waveguides <NUM> of the interposer <NUM>, which can avoid adhesive from causing additional propagation loss due to proximity of the waveguides <NUM>. The trench <NUM> can be parallel to the x-axis and/or perpendicular to the substrate longitudinal alignment fiducials <NUM>.

In use, the first end 226A of the interposer <NUM> (see <FIG>) is positioned proximate a termination end <NUM> of the mounting grooves <NUM> so that the end faces <NUM> of the data fibers <NUM> (see <FIG>) can be proximate to the waveguides <NUM> (see <FIG>) for efficient optical coupling. The substrate alignment fiducials <NUM> may be created by the same cutting tool used for forming the mounting grooves <NUM> and may be less deep. Further, the part or cutting tool may be rotated to create substrate orthogonal alignment fiducials <NUM> to align the first coupler <NUM> and the interposer <NUM> (see <FIG>) in the z direction along the y axis and/or to create the trench <NUM>.

In certain embodiments, the trench <NUM> in the termination region defines a <NUM> degree edge <NUM> opposite the mounting grooves <NUM>, which can be used as a mechanical stop for the data fibers <NUM> (see <FIG>). The fiducials <NUM>, <NUM> may be shallower than the mounting grooves <NUM>. Since the fiducials <NUM>, <NUM> are cut using the same tool and preferably without moving the part, their location with respect to the mounting grooves <NUM> may be highly precise.

<FIG> are perspective views of an example fiber optic-to-waveguide coupling assembly <NUM>, not according to the claimed invention, including alignment cylinders <NUM> and interposer alignment grooves 404A and 404B. In particular, the first coupler <NUM> includes a first alignment cylinder 402A positioned in the first substrate alignment groove 244A and a second alignment cylinder 402B (may also be referred to generally as alignment cylinders <NUM>) positioned in the second substrate alignment groove 244B. In certain embodiments, the alignment cylinder <NUM> includes a non-data optical fiber that has a same diameter as the at least one data fiber. The alignment cylinder <NUM> extends toward the interposer <NUM> beyond the data fiber <NUM> into the first coupler overlap portion <NUM>.

In certain embodiments, the alignment cylinders <NUM> (may also be referred to as alignment pins) are made from non-active fibers (may also be referred to as dummy fibers) of the same fiber ribbon as the data fibers <NUM> (may also be referred to as signal fibers) and have the same diameter as the data fibers <NUM>, but may be cleaved to extend beyond the ends of the data fibers <NUM> into the first coupler overlap portion <NUM> (and/or not extending beyond the first end 208A of the first substrate <NUM>). This can be achieved by cleaving the two outermost fibers of the ribbon to a longer length than the signal fibers.

The interposer <NUM> defines a first interposer alignment groove 404A and a second interposer alignment groove 404B (may be referred to generally as interposer alignment grooves <NUM>). The interposer alignment grooves <NUM> are configured to receive at least a portion of the alignment cylinder <NUM> to align the data fibers <NUM> of the first coupler <NUM> with the waveguides <NUM> of the interposer <NUM> in an x direction along the x axis and/or rotationally around the y axis. A depth D of the interposer alignment grooves <NUM> (along the y axis) may be larger than a width W along the x axis of the interposer alignment grooves <NUM> because the interposer alignment grooves <NUM> are used for alignment along the x axis and/or rotationally around the y axis, not for alignment along the y axis. As a result, the width W is more precisely defined than the depth D. Instead of alignment fiducials within the photomask of the interposer <NUM>, the photomask defines an opening for subsequent chemical or physical etching of the interposer alignment grooves <NUM>. Given the isotropic nature of etching glass, the interposer alignment grooves <NUM> may be rectangular at the top and have slightly rounded bottoms. In certain embodiments, alignment cylinder <NUM> is partially positioned in both the substrate alignment groove <NUM> and the interposer alignment grooves <NUM>. In other words, the top part of the alignment cylinder <NUM> fits within the interposer alignment grooves <NUM> while the bottom part of the alignment cylinder <NUM> fits within the substrate alignment groove <NUM> of the first coupler <NUM>.

The interposer alignment grooves <NUM> can be at the same or different depth, but in certain embodiments may be deeper if the alignment cylinder <NUM> has a larger diameter so that the alignment cylinder <NUM> does not interfere with alignment in the y direction along the y axis between the data fibers <NUM> and the waveguides <NUM>. Accordingly, the interposer alignment grooves <NUM> are configured to receive at least a portion of the alignment cylinder <NUM> to align the at least one data fiber <NUM> of the first coupler <NUM> with the at least one waveguide <NUM> of the interposer <NUM> in an x direction along the x axis and/or rotationally around the y axis.

The alignment cylinder <NUM> may reduce the assembly complexity as there is no need for vision-based alignment. Once the data fibers <NUM> and alignment cylinder <NUM> are bonded to the first substrate <NUM>, the interposer <NUM> can be placed onto the first surface <NUM> of the first substrate <NUM> and can slide against the data fibers <NUM> before bonding, thereby simplifying assembly. Alternatively, the interposer <NUM> can be bonded first and the data fibers <NUM> slide against the interposer <NUM>.

In certain embodiments, the substrate alignment grooves <NUM> have the same diameter as the mounting grooves <NUM>. In other embodiments, the substrate alignment grooves <NUM> have a depth greater than the depth of the mounting grooves <NUM>. In such embodiments, the substrate alignment grooves <NUM> are greater (i.e., have a greater depth) than the mounting grooves <NUM> and are configured such that the top surface of the data fibers <NUM> in the mounting grooves <NUM> is in the same plane as the alignment cylinders <NUM> positioned in the substrate alignment grooves <NUM>. In such a configuration, the interposer alignment grooves <NUM> do not need to be made deeper to accommodate the larger diameter of the alignment cylinders <NUM>, which simplifies the process and the amount of etching that may be required to form the interposer alignment grooves <NUM>.

<FIG> is a perspective view of the fiber optic-to-waveguide coupling assembly <NUM> of <FIG> in which the fiber array unit (FAU) cover <NUM> has a planar bottom surface <NUM>. In particular, the fiber array unit cover <NUM> is positioned over the first substrate <NUM> (outside of the first overlap <NUM>). In other words, the fiber array unit cover <NUM> is at least partially positioned over the data fibers <NUM>. The cover <NUM> (may also be referred to as a lid) includes the planar bottom surface <NUM> to push or compress the data fibers <NUM> (see <FIG>) and/or alignment cylinder <NUM> into the first substrate <NUM>, where the first substrate <NUM> includes mounting grooves <NUM>. Attachment of a cover <NUM> or interposer <NUM> can be with organic or inorganic adhesive, such as a UV or thermally curable epoxy, sol gel or liquid glass, or by a laser bonding process.

<FIG> is a perspective view of the fiber optic-to-waveguide coupling assembly <NUM> of <FIG> with a fiber array unit cover <NUM> with a grooved bottom surface <NUM>. In particular, the fiber array unit cover <NUM> includes v-grooves <NUM>, with the data fiber <NUM> (see <FIG>) at least partially positioned within the one of the v-grooves <NUM>. The bottom surface of the fiber array unit cover <NUM> pushes or compresses the data fibers <NUM> (see <FIG>) and/or alignment cylinder <NUM> into the first substrate <NUM>.

<FIG> is perspective view the fiber optic-to-waveguide coupling assembly <NUM> of <FIG> including pitch spacing fibers <NUM>. In this embodiment, the first coupler <NUM> further comprises a plurality of pitch spacing fibers <NUM> alternatingly interposed between the plurality of data fibers <NUM>. In certain embodiments, the first surface <NUM> of the first substrate <NUM> and the bottom surface <NUM> of the fiber array unit cover <NUM> are both flat. Instead of grooves, pitch spacing fibers <NUM> of a certain diameter are compactly alternatingly interposed between the plurality of data fibers <NUM> between the alignment cylinders <NUM> to achieve the desired precise core pitch between the data fibers <NUM>.

<FIG> are views of an exemplary fiber optic-to-waveguide coupling system <NUM> including the fiber optic-to-waveguide coupling assembly of <FIG>. It is noted that the orientation of the fiber optic-to-waveguide coupling system <NUM> is for illustrative purposes only and that assembly may occur in any orientation. Referring to <FIG> and <FIG>, the fiber optic-to-waveguide coupling system <NUM> includes a first coupler <NUM>, the interposer <NUM>, and a second coupler <NUM> in a disengaged position. In particular, a first interposer overlap portion <NUM> of the first waveguides <NUM> of the interposer <NUM> is configured for edge coupling with data fibers <NUM> of the first coupler <NUM> and a second interposer overlap portion <NUM> of the first waveguides <NUM> are configured for evanescent coupling with second waveguides of the second coupler <NUM> (see <FIG>).

The first coupler <NUM> includes a first substrate <NUM> and a cover <NUM> with data fibers <NUM> positioned therebetween, and a jacket <NUM> surrounding a portion of the data fibers <NUM>. The second coupler <NUM> includes a waveguide circuit <NUM> (e.g., planar lightwave circuit (PLC) and/or photonic integrated circuit (PIC) (e.g., silicon photonic circuit)) in communication with the interposer <NUM>, and a second substrate <NUM> (may also be referred to as a carrier substrate) attached to the waveguide circuit <NUM> (via solder bumps <NUM>) and in electrical communication with the waveguide circuit <NUM>. In certain embodiments, the second substrate <NUM> includes a printed circuit board (PCB). The waveguide circuit <NUM> includes electrical circuitry mounted to the second substrate <NUM> and/or optical components (e.g., wavelength multiplexers, couplers, and/or taps, etc.). In particular, the waveguide circuit <NUM> includes a third substrate <NUM>, a buried oxide layer <NUM>, and a silicon waveguide <NUM> (with the buried oxide layer <NUM> positioned between the third substrate <NUM> and the silicon waveguide <NUM> to separate the layers). The silicon waveguide <NUM> may include a silicon photonic integrated circuit (e.g., including modulators, detectors, etc.) and is evanescently coupled with the waveguides <NUM> of the interposer <NUM>. In other words, the silicon waveguides <NUM> are placed proximate the waveguides <NUM> of the interposer <NUM> so that their optical fields overlap. Adiabatic or evanescent coupling reduces or eliminates edge quality issues since the light coupling is from the top surface of the planar waveguides, and relaxes the alignment tolerance in the direction of propagation. Further, end faces of the waveguides <NUM> do not need to be polished or finished.

Referring to <FIG>, the fiber optic-to-waveguide coupling system <NUM> includes the first coupler <NUM> and the second coupler <NUM> in an engaged position with the interposer <NUM> for communication between the first coupler <NUM> and the second coupler <NUM>. In particular, the interposer <NUM> (may also be referred to as a third coupler) is edge coupled to the first coupler <NUM> and evanescently coupled to the second coupler <NUM>. In other words, at least a portion of the first surface <NUM> of the first substrate <NUM> of the first coupler <NUM> is positionable proximate at least a portion of the second surface <NUM> of the interposer <NUM> to form a first overlap <NUM> therebetween to align at least one data fiber <NUM> of the first coupler <NUM> with at least one of the plurality of waveguides <NUM> of the second coupler in a y direction along the y axis intersecting the first surface <NUM> of the first substrate <NUM> and the second surface <NUM> of the interposer <NUM>. At least a portion (e.g., a second coupler overlap portion <NUM>) of the second coupler <NUM> is positionable proximate at least a portion (e.g., a second interposer overlap portion <NUM>) of the second surface <NUM> of the interposer <NUM> to form a second overlap <NUM> therebetween to evanescently couple the interposer <NUM> and the second coupler <NUM>.

Once aligned, the interposer <NUM> is fixedly attached to the first substrate <NUM> (e.g., by adhesive, etc.). The waveguides <NUM> act as intermediate waveguides (e.g., intermediate glass waveguides, intermediate ion-exchange glass waveguides, polymer waveguides, intermediate silicon waveguides, etc.) in optical communication between the data fibers <NUM> and the silicon waveguide <NUM>. Circuitry in the silicon waveguide <NUM> converts the optical signal to an electrical signal and transmits the electrical signal to electronic components on the second substrate <NUM> through the solder bumps <NUM>. As discussed above, when engaged, the data fibers <NUM> of the first coupler <NUM> are aligned with the waveguides <NUM> of the interposer <NUM> of the second coupler <NUM> for optical communication therebetween.

<FIG> is a flowchart of steps <NUM> for manufacturing the fiber optic-to-waveguide coupling assembly of <FIG>. In step <NUM>, at least one data fiber is positioned proximate a first surface of a first substrate of a first coupler. In step <NUM>, at least one waveguide is positioned proximate a second surface of an interposer. In step <NUM>, the at least one data fiber of the first coupler is aligned with the at least one waveguide of the interposer in a y direction along a y axis intersecting the first surface of the first substrate and the second surface of the interposer by positioning at least a portion of the first surface of the first substrate of the first coupler proximate at least a portion of the second surface of the interposer to form a first overlap therebetween. In step <NUM>, the at least one data fiber of the first coupler is aligned with the at least one waveguide of the second coupler in a z direction along a z axis parallel to the at least one data fiber and the at least one waveguide, and perpendicular to the y axis (e.g., by moving the first substrate in the z direction with respect to the interposer). In step <NUM>, the at least one data fiber of the first coupler is aligned with the at least one waveguide of the second coupler in an x direction along an x axis perpendicular to the at least one data fiber, the at least one waveguide, and the y axis. In some embodiments, the interposer is then aligned with a second coupler that includes a plurality of silicon waveguides. The waveguides of the interposer are aligned with the silicon waveguides of a second coupler by placing at least a portion of the second surface of the interposer in contact with the second coupler.

In certain embodiments, the interposer is attached to the first coupler before the interposer is attached to the second coupler. In certain embodiments, the interposer is attached to the second coupler before the interposer is attached to the first coupler.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the invention as defined by the claims.

Claim 1:
A fiber optic-to-waveguide coupling assembly (<NUM>), comprising:
a first coupler (<NUM>) comprising:
a first substrate (<NUM>) comprising a first surface (<NUM>) and at least one substate alignment groove (<NUM>) in the first surface (<NUM>); the first substrate defining at least one mounting groove (<NUM>) and at least one data fiber (<NUM>) positioned in the at least one mounting groove (<NUM>); and
an interposer (<NUM>) comprising:
a second surface (<NUM>) and at least one waveguide (<NUM>) positioned proximate the second surface (<NUM>);
wherein at least a first portion of the first surface (<NUM>) of the first substrate (<NUM>) of the first coupler (<NUM>) is positioned proximate at least a second portion of the second surface (<NUM>) of the interposer (<NUM>) to form a first overlap (<NUM>) between the first portion of the first surface (<NUM>) and the second portion of the second surface (<NUM>) to align the at least one data fiber (<NUM>) of the first coupler (<NUM>) with the at least one waveguide (<NUM>) of the interposer (<NUM>) to allow for transmitting a signal between the at least one data fiber (<NUM>) and the at least one waveguide (<NUM>); and
the interposer further comprising at least one interposer alignment fiducial (<NUM>) cutting into the second surface (<NUM>) of the interposer (<NUM>), wherein the at least one interposer alignment feature (<NUM>) is positioned to facilitate alignment between the at least one data fiber (<NUM>) and the at least one waveguide (<NUM>) when the at least one interposer alignment feature (<NUM>) is aligned with the at least one substrate alignment groove (<NUM>) in the first surface (<NUM>) of the first substrate (<NUM>).