FIBER TO CHIP OPTICAL COUPLER

[0001] The present disclosure relates generally to coupling an optical fiber to a substrate, and more particularly to coupling the optical fiber to an optoelectronic integrated circuit (IC).

[0002] Communications systems and datacenters are required to handle massive data at ever increasing speeds and ever decreasing costs. To meet these demands, optical fibers and optical ICs (such as, a photonic integrated circuit (PIC) or integrated optical circuit) are used together with high speed electronic ICs. A PIC is a device that integrates multiple photonic functions (similar to an electronic IC or RF IC). PICs are typically fabricated using indium phosphide or silicon oxide (SiO₂), which allows for the integration of various optically active and passive functions on the same circuit.

[0003] The coupling of PICs to optical fibers is not as well advanced as the integration and/or coupling of electronic ICs. Specifically, the challenges facing optical connections are different and much more complex than connecting electronic ICs to, for example, a printed circuit board (PCB). Some difficulties are inherent in wavelength, signal losses, assembly tolerance, and polarization characteristics of optical packaging.

[0004] Existing solutions utilize various techniques for connecting optical fibers to PICs. One technique suggests using various types of butt connections to the edge and surface fiber connections a PIC. The butt of a fiber can be connected to a planar waveguide at the edge of a PIC. This technique is efficient only if the cross sectional of the propagating mode of the fiber and the waveguide areas of the fiber core and the waveguide are of similar size. In most cases, this technique suffers from poor assembly tolerance.

[0005] An improved technique suggests laying a section of fiber on top of the surface of the PIC where the end of the fiber has been cut at an angle to form an angled tip. The angled tip has a flat surface which reflects a light beam down to a waveguide grating coupler disposed on the integrated circuit. The light beam is reflected off the reflective surface of the angled tip by total internal reflection. The waveguide grating coupler is designed to accept the slightly diverging light beam from the reflective surface of the angled tip of the fiber. The light beam can also propagate through the fiber to a chip coupler in the opposite direction, up from the substrate through the waveguide grating and into an optical fiber after bouncing off the reflective surface of the angled tip. This technique further requires coating on the exterior of the reflective surface with epoxy.

[0006] Among others, all of the above-noted techniques require precise alignment and active positioning of the optical fiber to the PIC. As such, current techniques suffer from poor and very tight alignment tolerance to gain an efficient connectivity. For example, a misalignment between an optical fiber and a PIC of 1-2 microns (μm) would result in a signal loss of about 3 db. Furthermore, the alignment is now performed with expensive equipment or labor intensive assembly solutions. As a result, a mass production of PICs and/or optical couplers is not feasible.

[0007] It would therefore be advantageous to provide a fiber-to-chip optical coupling solution that would overcome the deficiencies of the existing solutions.

[0008] A summary of several example embodiments of the disclosure follows. This summary is provided for the convenience of the reader to provide a basic understanding of such embodiments and does not wholly define the breadth of the disclosure. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later. For convenience, the term some embodiments may be used herein to refer to a single embodiment or multiple embodiments of the disclosure.

[0009] The disclosure relates in various embodiments to an optical coupler for coupling an optical fiber to a photonic integrated circuit (PIC). The optical coupler comprises a first curved mirror included in a first substrate layer of the PIC and at a first predefined lateral distance from an optical transceiver associated with the PIC; a second curved mirror included in a second substrate layer and placed at a second predefined lateral distance from the optical fiber; and a spacer located between the first substrate layer and the second substrate layer.

[0010] The disclosure also relates in various embodiments to a photonic plug comprising a plurality of optical couplers enabling optical connectivity between a plurality of optical fibers and a photonic integrated circuit (PIC) dispose on a first substrate layer, wherein each of the plurality of optical couplers include a second substrate layer, at least one optical focusing element, a tilted flat mirror, a fiber trench etched in the second substrate layer, and a spacer located between the first substrate layer and the second substrate layer.

[0011] The disclosure also relates in various embodiments to a method for manufacturing an optical coupler for coupling an optical fiber to a photonic integrated circuit (PIC). The method comprises: fabricating, in a first substrate layer, a first curved mirror, wherein the first substrate layer is part of the PIC; fabricating, in a second substrate layer, a second curved mirror; and disposing a spacer between the first substrate layer and the second substrate layer.

[0012] The disclosure also relates in various embodiments to a photonic integrated circuit (PIC) package comprising a first substrate layer including at least a first curved mirror; a second substrate layer including at least a second curved mirror; and a spacing layer coupling between the first substrate layer and the second substrate layer.

[0019] It is important to note that the embodiments disclosed herein are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed embodiments. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views.

[0020] By way of example to the various disclosed embodiments, an adaptive optical coupling solution that provides efficient and scalable fiber-to-chip and chip-to-fiber optical connections is presented. The chip includes, but is not limited to, a photonic integrated circuit (PIC). The fiber in the “fiber-to-chip and chip-to-fiber optical” connections can be an optical fiber, a laser, or any type of light source and/or light drain. The scalability of the disclosed optical coupler or (a photonic plug) is achieved due to its optical arrangement that provides high tolerance alignment and a passive positioning of the optical coupler, thus the optical fiber with respect to the PIC. Therefore, the disclosed optical coupler can be mass-produced. In certain embodiments, the disclosed optical coupler allows for compact and secured packaging of PICs. In a further embodiment, the disclosed optical coupler solution provides integrality with flip-chip arrangement. The various disclosed embodiments are discussed in detail below.

[0021] FIG. 1 is a side view of a fiber-to-chip optical coupler 100 constructed according to one embodiment. The optical coupler 100 provides an optical connection between a PIC 110 and an optical fiber 120. In an embodiment, the optical coupler 100 includes a spacer 130 connected between the PIC 110 and the optical fiber 120, a first curved mirror 140, a second curved mirror 150, and a tilted flat mirror 160. The optical coupler 100 may also include a fiber trench (not shown in FIG. 1).

[0022] The optical fiber 120 and the coupler 100 are stacked under a substrate layer 170. Specifically, as will be discussed below, the second curved mirror 150 and the tilted flat mirror 160 are fabricated in the substrate 170. The substrate 170 may be the same or a different type as of the substrate of the PIC 110. In an exemplary embodiment, the substrate layer 170 may be made of silicon oxide (SiO₂), plastic, and the like. In another embodiment, the second curved mirror 150 and the tilted flat mirror 160 are fabricated (and included) in the spacer 130 and not the substrate 170.

[0023] According to one embodiment, the material of the spacer 130 may be any transparent and non-conductive material, such as glass, polydimethylsiloxane, air, or any other index matching materials. The height of the spacer 130 determines, in part, the efficiency of the light beam (optical signal) that propagates through the spacer 130. Specifically, the higher the spacer 130 is, the more the coupler 100 is error-prone to rotation and leveling errors between the PIC 110 and the coupler 100. In an exemplary and non-limiting embodiment, the height of the spacer 130 is set to 300 μm.

[0024] The tilted flat mirror 160 is utilized to direct a light beam from the optical fiber 120 to the first curved mirror 140 and/or from the first curved mirror 140 to the optical fiber 120. This allows for placement of the optical fiber 120 parallel to the PIC 110. The tilted flat mirror 160 is formed by means of anisotropic grayscale etching and tilted at a predefined angle. The angle is determined respective of the optical path between the optical fiber 120 and the first curved mirror 140. In certain implementations, the tilted flat mirror 160 is optional. As a non-limiting example, when the optical fiber 110-is replaced with a laser, then the light can be easily directed to the second curved mirror 150, thus the flat mirror 160 is not required in such an arrangement.

[0025] As illustrated, the first and second curved mirrors 140 and 150 are collimated mirrors placed at opposite directions to each other. Specifically, the first curved mirror 140 is placed at the “PIC side” while the second curved mirror 150 is placed at the “fiber side”. This arrangement allows for separation of the optical fiber 120 from the PIC 110, thereby gaining high and relaxed alignment tolerances (at three-dimensions). In an embodiment, the positioning and creation of the first and second curved mirrors 140 and 150 is performed on the substrate of the PIC 110 and on the substrate layer 170 using a similar photolithography process such as, but not limited to, grayscale lithography. In an embodiment, the placement on the tilted flat mirror 160, the curved mirror 150, and the fiber trenches are placed using the same lithography mask alignment accuracy. In another embodiment, the placement on the tilted flat mirror 160 and the curved mirror 150 are placed using a first lithography mask alignment accuracy, and the fiber trenches are placed using a second lithography mask alignment accuracy.

[0026] Further, the first and second curved mirrors 140 and 150 are placed and created during fabrication, which ensures high accuracy positioning and accurate reflective mirrors. As a non-limiting example, the fabrication process utilized to create the mirrors may include a Silicon-On-Insulator (SOI), complementary metal-oxide semiconductor (CMOS), and the like.

[0027] The first and second curved mirrors 140 and 150 are fabricated by two different processes and optionally at two different fabrication facilities (fabs), but using the same or substantially similar grayscale lithography process. This ensures high accuracy of the mirrors and their assembly to create the optical coupler. Furthermore, by fabricating and placing the first and second curved mirrors 140 and 150 on the substrates, the optical fiber 120 is separated from the PIC 110, thereby allowing relaxed alignment tolerances in 3-dimenstions. That is, even if the “fiber side” of the optical coupler 100 is not perfectly aligned with the PIC 110, the optical signal is not significantly attenuated.

[0028] The disclosed arrangement of the optical coupler 100 achieves high signal efficiency with a relaxed alignment between the PIC 110 and the light beam source and/or drain due to the specific locations and shape of the first and second curved mirrors 140 and 150 placed against each other. The locations of the first and second curved mirrors 140 and 150 are determined at least with respect to the source/drain light beam. This allows the light beam to be reflected from the first and second curved mirrors 140 and 150. Specifically, the first and second curved mirrors 140 and 150 are shaped in such a way that all light beams from the source are reflected and collimated at a certain angle at a center of the first curved mirror 140 and focused to a drain after the second curved mirror 150. The design of the first and second curved mirrors 140 and 150 is described in further detail with respect to FIG. 5.

[0029] For example, as illustrated in FIG. 1, the first curved mirror 140 reflects a diverging light beam 180-1 from the optical fiber 120 (via the tilted flat mirror 160) into parallel light beams 180-2. The light beams 180-2 reach the second curved mirror 150 which reflects a focused light beam 180-3 back to the PIC's 110 transceiver-115. The same optical path is true for a light beam transmitted by the transceiver-115. The embodiment for designing the coupler 100 is discussed with reference to FIG. 6. It should be noted that all light beams 180 travel to the spacer 130.

[0030] It should be noted the optical coupler 100 discussed with referenced to FIG. 1 allows a connection between a single fiber and the PIC 110. However, in a typical arrangement, a plurality of couplers 100 can be utilized to connect a plurality of optical fiber to the PIC 110.

[0031] As noted above, the optical fiber 120 is attached to the coupler 100 using a fiber trench. This arrangement is further illustrated in FIG. 2 which shows an exemplary and non-limiting top view of the “fiber side” of the disclosed arrangement. FIG. 2 illustrates four (4) fiber trenches 210-1 through 210-4 (hereinafter referred to individually as a fiber trench 210 and collectively as fiber trenches 210, merely for simplicity purposes). Each fiber trench 210 adjoins a tilted flat mirror 220. The fiber trench 210 is shaped as a groove etched in the substrate layer 170. Each of the tilted flat mirrors 220-1 through 220-4 is oriented as the tilted flat mirror (160) shown in FIG. 1. As demonstrated in FIG. 2, optical fibers 230-1 and 230-2 are placed in the fiber trenches 210-3 and 210-4, respectively.

[0032] Also illustrated in FIG. 2, are four curved mirrors 240-1 through 240-4. Each of the curved mirrors 240 is oriented as the second curved mirror (150) shown in FIG. 1. It should be noted that only 2 optical fibers 230-1 and 230-2 are shown in FIG. 2 merely for illustrative purposes. Other numbers of optical fibers may be utilized without departing from the scope of the disclosed embodiments. The exemplary arrangement shown in FIG. 2 can support coupling of four optical fibers to a PIC (not shown). It should be noted that the number of optical fibers that can be supported can be greater than four. It should be further noted that the fiber trenches illustrated in FIG. 2 are shaped as V-grooves, however, any type of groove shape can be utilized, such as square, cylinder, diamond, and the like.

[0033] The process for creating a fiber trench 210 is further described with reference to FIGS. 3A through 3D. FIG. 3A is a side view of the substrate layer 170. At first, only the curved mirror 150 is placed on the substrate layer 170. Then, as shown in FIG. 3B, a groove is etched in the substrate layer 170 to create the fiber trench 210. Finally, an optical fiber 120 is placed in the fiber trench 210 (FIG. 3C). FIG. 3D shows a side view of the substrate layer 170 with the attached optical fiber 120. It should be noted that the arrangement shown in FIG. 3D is flipped relative to the arrangement shown in FIG. 1.

[0034] FIG. 4 is a side view of a chip-to-fiber optical coupler 400 utilized as a waveguide arranged according to an embodiment. In this embodiment, the PIC 410 is flipped and placed on an interposer 420 which serves as a spacer (similar to, e.g., the spacer 130) of the coupler 400. The interposer 420 is an electrical interface routing from one connection to another in order to spread a connection to a wider pitch or to re-route a connection 421 to a different connection.

[0035] Also coupled to the interposer 420 is an integrated circuit (IC) 430 including only electrical elements. The connection between the IC 430 and a PIC circuit board (PCB) 440 is through vias 450. The optical connection between the PIC 410 and the optical fiber 460 is achieved by means of the coupler 400. The coupler 400 is constructed as discussed in greater detail herein above with reference to FIG. 1. That is, the coupler 400 includes a pair of curved mirrors 401 and 402, as well as a tilted flat mirror 403. It should be noted that the single mounting by a standard electronics method (flip-chip on an interposer) provides the PIC 410 with electrical and optical connectivity required for its operation. The interposer 420 is made of a material that is transparent to the wave length of the light beam.

[0036] FIG. 5 is an exemplary diagram utilized to describe the fiber-to-chip optical coupler 500 according to an embodiment. The optical coupler 500 includes a first curved mirror 501, a second curved mirror 502, a tilted flat mirror 503, and a spacer 504. In this example, a drain 510 is an optical fiber and a transmitter 521 of a PIC 520 is the source of the light beam.

[0037] A few adjustable parameters determine the design of the coupler 500: a spacer height, main propagation angles (α, β, γ), the propagation medium type of the spacer 504, and a target tolerance for misalignment.

[0038] The beam's radius is determined by the beam's radius at the source 521, the medium in which the beam propagates, and the wavelength. First, the angle of divergence (θ) is selected as the angle where the intensity of the light beam is 1% of the intensity at the center of the beam. Then, in an exemplary embodiment, the main propagation angles (α, β, γ) are set to meet the following constraints:

α+β>θ

α=β=γ

[0039] Typically, the value of θ is 8°-12°. It should be noted that other constraints may be set to different target tolerances. As noted above, the spacer height L is set respective of the allowed tolerance for rotation and leveling errors. In an exemplary embodiment, L equals 300 μm.

[0040] In an embodiment, the first and second curved mirrors 501 and 502 are designed so that their respective centers are located where the main propagation axis intersects each mirror. Specifically, the mirrors are designed such that the center of the second curved mirror 502 is at a distance D₁ from the source 521. In an embodiment, the distance D₁ is computed as follows:

D₁=L×tan(α)+L×tan(β);

[0041] The center of the first curved mirror 501 is at a distance D₂ from the drain 510. In an embodiment, the distance D₂ is computed as follows:

D₂=L×tan(γ)+L×tan(β)

Further, the lateral distance, in a 0 μm misalignment, between the first and second curved mirrors 501 and 502 is computed as follows:

L×tan(β)

[0042] In an embodiment, the first and second curved mirrors 501 and 502 are shaped in such a way that all light beams from the source 521 are reflected and collimated at the angle β after the first curved mirror 501 and focused to the drain 510 after reflecting from the second curved mirror 502. The surfaces of the first and second curved mirrors 501 and 502 are large enough to cover the divergence axis. It should be noted that all calculations are performed as 0 misalignment conditions.

[0043] The various optical couplers have been discussed herein with a reference to a specific embodiment with the curved mirrors are utilized for propagating light beams. However, the disclosed embodiments can be realized using other reflective or focusing elements, such as optical lenses, zone plates (e.g., Fresnel zone plates), and the like.

[0044] It should be understood that any reference to an element herein using a designation such as “first,” “second,” and so forth does not generally limit the quantity or order of those elements. Rather, these designations are generally used herein as a convenient method of distinguishing between two or more elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements may be employed there or that the first element must precede the second element in some manner. Also, unless stated otherwise a set of elements comprises one or more elements. In addition, terminology of the form “at least one of A, B, or C” or “one or more of A, B, or C” or “at least one of the group consisting of A, B, and C” or “at least one of A, B, and C” used in the description or the claims means “A or B or C or any combination of these elements.” For example, this terminology may include A, or B, or C, or A and B, or A and C, or A and B and C, or 2A, or 2B, or 2C, and so on.

[0045] All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.