Patent Publication Number: US-11650371-B2

Title: Grating coupler and integrated grating coupler system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority benefit of U.S. Non-provisional patent application Ser. No. 16/774,897, filed on Jan. 28, 2020, the contents of which are incorporated herein by reference in their entireties and the benefits of each are fully claimed. 
     FIELD OF THE INVENTION 
     This invention generally relates to grating couplers for optical chips, also called photonic integrated circuits (PICs), and more particularly to grating coupler system connecting one active chip and one passive optical chip. 
     BACKGROUND OF THE INVENTION 
     The target application is a hybrid integration of an active optical chip such as one containing InP waveguides and a passive chip such as one containing silicon and/or silicon nitride waveguides. The expected properties are, large tolerance to mis-alignment, easiness of bonding processes, and high coupling efficiency. 
     Silicon photonics offer many advantages of which the fabrication cost is the most important factor. Furthermore, high refractive index contrast between the silicon waveguide and the surrounding silicon dioxide layers offer tight bending with low loss possible, leading to higher density and complexity PICs. Silicon nitride waveguides offer similar low cost capabilities, with lower optical loss property. On the other hand, there is no reliable optical gain or emission capability with direct current injection. Therefore, hybrid integtation of active PICs (such as InP, GaAs, or GaN-based ones) with passive silicon photonics PICs become very important to achieve low cost, full functionality, and high density PICs. 
     However, optically connecting two waveguides precise requires precise alignment typically with sub-micron accuracy, due to narrow waveguides and thus fast diverging beam on both sides. There is a need to connect two optical chips with larger tolerance with high coupling efficiency. 
     SUMMARY OF THE INVENTION 
     Some embodiments of the present disclosure are based on recognition that two-dimensional long period grating on a passive waveguide from an optical chip creates shallow angle emission towards the substrate side, diffracted at the chip facet (second end) at a steeper angle, manipulated to form a narrow beam, and then coupled to the passive optical chip through a grating coupler. 
     In accordance to some embodiments, a novel grating coupler system is realized by a grating coupler having first and second ends for coupling a light beam to a waveguide of a chip including a substrate configured to receive the light beam from the first end and transmit the light beam through the second end, the substrate having a first refractive index n1; a grating structure having grating curves (lines) arranged on the substrate, the grating structure having a second refractive index n2, wherein the grating curves (lines) have line width w and height d and are arranged by a pitch Λ, wherein the second refractive index n2 is greater than first refractive index n1; and a cladding layer configured to cover the grating structure, wherein the cladding layer has a third refractive index n3, wherein the third refractive index n3 is different from the second refractive index n2, wherein the cladding layer is arranged so as to reflect the light beam diffracted from the grating structure toward below the cladding layer. The two-dimensional grating curves (lines) comprise a series of arcs which are part of ellipse lines, whose pitch is gradually decreased in two dimensions, such that the diffracted beam is shaped or narrowed to have a focused spot on the second grating, which is typically a silicon grating. 
     In accordance with another embodiment of the present invention, a grating coupler having first and second ends for coupling a light beam to a waveguide of a chip includes a substrate configured to receive the light beam from the first end and transmit the light beam through the second end, the substrate having a first refractive index n1; a grating structure having grating curves arranged on the substrate, the grating structure having a second refractive index n2, wherein the grating curves have line width w and height d and are arranged by a pitch Λ, wherein the second refractive index n2 is greater than the first refractive index n1, wherein the grating curves are arranged to diffract the light beam to form a narrowing beam in a two orthogonal axes perpendicular to a light propagation direction of the light beam; and a cladding layer configured to cover the grating structure, wherein the cladding layer has a third refractive index n3, wherein the third refractive index n3 is different from the second refractive index n2. 
     Further, another embodiment of the present invention is based on recognition that an integrated grating coupler system includes a grating coupler formed on a first chip, the grating coupler having first and second ends for coupling a light beam to a waveguide of a second chip, wherein the grating coupler comprises a substrate configured to receive the light beam from the first end and transmit the light beam through the second end, the substrate having a first refractive index n1; a grating structure constructed to have grating curves arranged on the substrate, the grating structure having a second refractive index n2, wherein the grating curves have line width w and height d and are arranged by a pitch Λ, wherein the second refractive index n2 is greater than first refractive index n1; and a cladding layer configured to cover the grating structure, wherein the cladding layer has a third refractive index n3, wherein the third refractive index n3 is less than the second refractive index n2. The cladding layer can be of the same material as the substrate, or SiO 2 , Si 3 N 4 , or polymer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The presently disclosed embodiments will be further explained with reference to the attached drawings. The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the presently disclosed embodiments. 
         FIG.  1    shows a cross-sectional view of an integrated grating coupler system according to embodiments of the present invention; 
         FIG.  2 A  shows the top view of the two-dimensional grating curves which includes the thickness of each grating line, according to embodiments of the present invention; 
         FIG.  2 B  shows the top view of center lines of the two-dimensional grating curves to embodiments of the present invention; 
         FIG.  3    shows a two-dimensional grating structure, where the grating is arranged to be asymmetric with respect to the light propagation direction of the light beam such that the reflected light from the grating curves is prevented from coupling to the first end of the waveguide, according to embodiments of the present invention; 
         FIG.  4    shows a side view of the multi-step grating structure, according to embodiments of the present invention; 
         FIG.  5    shows a side view of the gratings wherein the asymmetric gratings are formed by multiple steps, according to embodiments of the present invention; 
         FIG.  6 A  show a cross-section view of an example structure a grating coupler including a first chip (light beam transmission side) and a second chip (receiving the transmitted beam from the first chip), according to embodiments of the present invention; and 
         FIG.  6 B  shows the top view of the grating coupler system of  FIG.  6 A , according to embodiments of the present invention. 
     
    
    
     While the above-identified drawings set forth presently disclosed embodiments, other embodiments are also contemplated, as noted in the discussion. This disclosure presents illustrative embodiments by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of the presently disclosed embodiments. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the following description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing one or more exemplary embodiments. Contemplated are various changes that may be made in the function and arrangement of elements without departing from the spirit and scope of the subject matter disclosed as set forth in the appended claims. 
     Specific details are given in the following description to provide a thorough understanding of the embodiments. However, understood by one of ordinary skill in the art can be that the embodiments may be practiced without these specific details. For example, systems, processes, and other elements in the subject matter disclosed may be shown as components in block diagram form in order not to obscure the embodiments in unnecessary detail. In other instances, well-known processes, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. Further, like reference numbers and designations in the various drawings indicated like elements. 
     Furthermore, embodiments of the subject matter disclosed may be implemented, by use of at least in part, or combinations of parts of the structures described below. 
     Optical coupling between two optical chips constitute the most important part of hybrid PICs. The easiness of alignment and high coupling efficiency are very important factors. Grating couplers offer these capabilities. In some cases, conventional elliptic grating curves create a collimating beam, i.e., beam shape is almost constant along the propagation axis. However, this is not sufficient when the emission area is large and narrower beam width, or focusing, is necessary to couple into the second grating efficiently. According to embodiments of the present invention, it provides shapings of grating curves, such that the beam is formed to be of a desired shape at the surface of the second grating, resulting in higher coupling efficiency. 
     There are multiple factors in achieving high coupling efficiency for this configuration. 
       FIG.  1    shows a cross-sectional view of an integrated grating coupler system  100  according to the invention. The first optical chip (first chip)  105  is made on an InP substrate  110 , containing an InGaAsP waveguide layer  130 , InP cladding layer  120 , and a first grating  140 . The second optical chip (second chip)  145  comprises of a silicon substrate  150 , a buried SiO 2  layer (also called a BOX layer)  160 , a silicon (Si) waveguide layer (also called silicon-on-insulator, or SOI)  170 , and a SiO 2  cladding layer  180 , and the second grating  190  etched onto the silicon waveguide layer. The diffracted light in the first optical chip  105  propagates through the InP substrate  110  and the first optical chip facet  195 , and is coupled into the grating  190  on the second optical chip  145 . Further, an example of a top view of an integrated grating coupler system is shown in  FIGS.  6 A and  6 B . 
     Here, the grating pitch Λ is the distance between the rising edges of the grating, w is the line width of the main tooth, and d is the thickness of the grating. The grating pitch Λ does not have to be constant, and can be a function of the propagation distance from the end of the input waveguide, expressing a chirped grating. The grating pitch Λ also depends on the angle from the primary propagation distance, to form elliptic lines. In the first optical chip, grating diffracts light towards the substrate as a shallow angle, which is further diffracted at the chip fact to a steeper angle. The beam is shaped and is shone on the grating in the second chip and is guided to its waveguide. The operating wavelength of 1530-1570 nm, the typical grating pitch Λ is 5-15 μm, and the typical grating line width w is 10-60% of the grating pitch, depending on whether sub-gratings are included, or how the sub-gratings are designed. The typical grating thickness d is 0.2-1 μm. 
       FIG.  2 A  shows an example illustrating a top view of a grating structure  295 , wherein the shaded region  240  is the area whose cladding layer thickness is greater than the surrounding area,  220  is the etched grating regions, and  230  is the input waveguide. 
     It should be noted that a distance L gr  between a straight end of the first end to a first grating line is arranged so that substantial amount of the intensity of the light beam can reach to the first grating curve (line) without unwanted diffractions of the light beam. For instance, the distance L gr  may be a range of nλ g  (n: a multiplier; λ g : wavelength of the light beam in the waveguide), where the multiplier n may be between 10 to 1000, more preferably, 50-500. 
       FIG.  2 B  shows a top view of center lines of the grating curves of the etched regions  220 , wherein the grating curves are expressed as 
                     q   ⁢   λ     =         xn   c     ⁢   cos   ⁢     ϕ   c       -         n   eff     (       x   2     +     y   2       )       1   2       +       Δ   x     ⁢     x   2       +       Δ   y     ⁢     y   2                 (   1   )               
where x and y are the directions parallel to and perpendicular to the light propagation in the grating structure, respectively, q=m, m+1, m+2 . . . (m&gt;0) is the integer corresponding to each grating line, λ is the wavelength, n c  is the refractive index of the substrate, ϕ c  is the angle from the waveguide surface normal, n eff  is the effective refractive index of the waveguide, ΔΔ x  and Δ y  are the coefficients of grating chirp, expressing the narrowing or focusing effect in x and y direction. Negative values of Δ x  and Δ y  mean that the pitch or spacing of the curves decrease as the curves move away from the origin (0, 0), i.e., the end of the input waveguide. Note that Eq. (1) does not necessarily express ellipse lines unless both Δ x  and Δ y  are zero, however, they can be very well approximated by ellipse lines. The actual grating curves are part of the Eq. (1), such that they form protrusions toward a light propagation direction of the light beam as shown in  FIG.  2 A .
 
     The diffracted light from this grating can be manipulated in two dimensions, i.e., in the two orthogonal axes each perpendicular to the diffracted beam propagation direction. With a proper choice of the negative values for Δ x  and Δ y , the diffracted beam can be narrowed as it propagates. In the case where Δ x  or Δ y  is equal to zero, i.e., the distance between the grating curves stays constant, the diffracted beam stays collimated in the corresponding direction. 
     Also, when they the absolute values of Δ x  and Δ y  are small, then grating curves expressed by Eq. (1) have distances decreasing at a fixed rate. This value may be determined, typically between 0.2% and 2% of the pitch, so as to have enough narrowing effect (to form a narrowing beam within the area of the grating  640 , see  FIGS.  6 A and  6 B ) but not to have too close focusing distance. 
     An integrated grating coupler system may contain semiconductor lasers on the same substrate, however, semiconductor lasers are very sensitive to any reflection. It may cause mode hopping or laser linewidth fluctuation. Therefore, it is very important to minimize any reflection from the optical components inside or outside of the cavity, including grating couplers which tend to show small amount of back reflection. 
       FIG.  3    shows a schematic of a two-dimensional grating, where the two-dimensional grating is asymmetric with respect to the light propagation direction of the light beam  310 . In this case, the two-dimensional grating is arranged so that the reflected light from the gratings curves is prevented from coupling to the first end of the waveguide. In other words, the axes  320  of the curves  300  (long axes in the case of nearly ellipse curves) cross the waveguide line  310  with non-zero angle α 0 . 
     The cladding layer can be a non-semiconductor material. Contrary to using semiconductor cladding layer which usually requires costly crystal regrowth, dielectric (SiO 2  or Si 3 N 4 ) or polymer materials do not require regrowth, so the fabrication is easier and cost is lower. 
     However, the refractive index of dielectric or polymer materials are typically between 1.4 and 2.3, while that of the waveguide layer is between 3.0 and 3.6 at the wavelength of 1.3-1.6 μm where most optical communications take place. Therefore, the refractive index difference between the waveguide layer and the cladding layer becomes larger than when a semiconductor is used in the cladding layer. This creates a situation where higher-order (n=2, 3, 4, 5 . . . ) diffraction players a larger role, and reduces the coupling efficiency to another grating coupler (or another optical component), typically made on Si substrate. Therefore, it is very important to minimize the higher-order diffraction. 
     Each order of diffraction is highly correlated to the Fourier component of the diffraction grating. For example, rectangular diffraction grating contains large amount of third-order and fifth-order Fourier component, so the third- and fifth order diffraction is very high. Therefore, it is important to effectively soften the rising and falling edge of the grating. 
       FIG.  4    shows the side view of the gratings, wherein the waveguide layer  420 , sandwiched by a substrate  410  and a cladding layer  430 , is formed by a grating  460  with more than two height levels or steps. This can be formed by multiple photolithography and etching processes. Since the typical grating pitch for InP grating couplers is 8-12 μm, the formation of the multiple-step grating is feasible even with processes capable the minimum feature size of ˜0.5 μm. 
     In addition, the cross-sectional shape of the grating can be asymmetric as shown in  FIG.  5   . With respect to the direction of the light propagation  540 , the grating can have a sharper sizing etch and slower falling edge, which creates an effective blazing grating effect. This way, the input light is more effectively directed to the downward direction  550 . 
       FIG.  6 A  show a cross-section view of an example structure a grating coupler including a first chip (light beam transmission side) and a second chip (receiving the transmitted beam from the first chip). As described above the grating lines are arranged according to Equation (1) with respect to  FIGS.  2 A and  2 B , having predetermined distances and curvatures.  FIG.  6 B  shows the top view of the grating coupler system of  FIG.  6 A , where the grating lines are curved. θ 1  and θ 2  are the angles for the concentric grating lines for the first and second chips, respectively. One way to narrow the lateral beam divergence is to use curved gratings, such as elliptic grating.  FIGS.  6 A and  6 B  show a cross-sectional view and a top of the grating coupler system  600 , respectively, wherein the first optical chip  610  and second optical chip  630  have elliptical gratings  620  and  640 , respectively. In one example, an InP waveguide  615  with around 1 μm width is connected to the elliptic grating  620  with at least 10° in full width. A silicon waveguide  635  with 0.5 μm width is also connected to an elliptic silicon grating  640 . 
     The above-described embodiments of the present invention can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers. Such processors may be implemented as integrated circuits, with one or more processors in an integrated circuit component. Though, a processor may be implemented using circuitry in any suitable format. 
     Also, the embodiments of the invention may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     Use of ordinal terms such as “first,” “second,” in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     Although the present disclosure has been described with reference to certain preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the present disclosure. Therefore, it is the aspect of the append claims to cover all such variations and modifications as come within the true spirit and scope of the present disclosure.