Millimeter scale long grating coupler

Abstract

A millimeter scale weak grating coupler comprising a silicon waveguide having bars of overlay material of length (a) disposed periodically at a period (∧) adjacent the silicon waveguide whereby a uniform grating output is achieved.

Claims

1. A millimeter scale weak grating coupler comprising a waveguide having a plurality of bars of overlay material of length (a) disposed periodically at a uniform period (∧) adjacent a surface and along a length of the waveguide, wherein the length (a) of each of the bars and gaps between the bars vary along the length of the waveguide; wherein a duty cycle defined by the length (a)/period (∧) varies along the waveguide; wherein the duty cycle of (a/∧) varies monotonically along the length of the waveguide configured to provide a substantially uniform output profile; and wherein the overlay material has an index of refraction that is between an index of refraction of the waveguide and an index of refraction of a material disposed adjacent the waveguide and the plurality of bars of overlay material.

2. The grating coupler of claim 1, wherein the length (a) of each of the bars decreases and gaps between the bars increases along the length of the waveguide, whereby grating strength of the coupler decrease along the length of the waveguide.

3. The grating coupler of claim 1, wherein the duty cycle of (a/∧) increases along the length of the waveguide.

4. The grating coupler of claim 1, wherein the duty cycle decreases along the waveguide as a grating strength decreases.

5. The grating coupler of claim 1, further comprising a stop layer disposed between the overlay material and the waveguide.

6. The grating coupler of claim 1, wherein a dimension of one or more bars of overlay material along at least one axis is varied across the plurality of bars.

7. A method of forming a grating coupler comprising: a. depositing on a wafer a stop layer; b. depositing a grating layer on the stop layer; c. patterning desired gratings; and d. etching, based on the patterning, the grating layer to create a waveguide including the desired gratings, whereby bars of the remaining grating layer of width “w” and length “a” are disposed periodically at a uniform period “∧” on the wafer, the length (a) of each of the bars and gaps between the bars varying along a length of the gratings, wherein a duty cycle defined by the length (a)/period (∧) varies along the waveguide; wherein the duty cycle of (a/∧) is varied monotonically along the length of the waveguide configured to provide a substantially uniform output profile, and wherein the remaining grating layer comprises a material having an index of refraction that is between an index of refraction of the waveguide and an index of refraction of a material disposed adjacent the waveguide and the bars of the remaining grating layer.

8. The method of claim 7, wherein the length (a) of each of the bars decreases and gaps between the bars increases along the length of the waveguide, whereby a grating strength of the gratings decreases along a surface of the wafer.

9. The method of claim 7, wherein the duty cycle of (a/∧) increases along a surface of the wafer.

10. The method of claim 7, further comprising patterning and etching a waveguide from the wafer whereby the duty cycle of (a/∧) increases along the waveguide moving away from a light source.

11. The method of claim 7, wherein depositing the stop layer comprises depositing Al.sub.2O.sub.3 or SiO.sub.2, or both.

12. The method of claim 7, wherein the wafer comprises Silicon On Insulator (SOI) and wherein depositing the grating layer comprises depositing Si.sub.3N.sub.4.

13. The method of claim 7, wherein a material forming the stop layer is selected such that it will not etch during the etching step, stopping the etch from penetrating a waveguide formed in the wafer.

14. The method of claim 7, wherein etch chemistry and process parameters of the etching step are selected such that an etch rate of the stop layer is lower than an etch rate of the grating layer.

15. The method of claim 7, further comprising depositing a cladding material on the grating coupler.

16. The method of claim 15, wherein the grating layer has an index of refraction that is between an index of refraction of the wafer and an index of refraction of the cladding material.

17. The method of claim 7, further comprising analytically mapping duty cycles of the gratings to a required strength set forth by a predetermined function so as to produce a profile of duty cycles per period for an entire length of the gratings.

18. The method of claim 17, wherein the predetermined function is dependent on an emission intensity profile or phase profile as a function of a direction of propagation of light through a waveguide formed in the wafer adjacent the gratings.

19. A millimeter scale weak grating coupler comprising a waveguide having a plurality of bars of overlay material of length (a) disposed periodically at a uniform period (∧) adjacent a surface and along a length of the waveguide, wherein the length (a) of each of the bars and gaps between the bars vary along the length of the waveguide; wherein a duty cycle defined by the length (a)/period (∧) varies along the waveguide; wherein the duty cycle of (a/∧) varies monotonically along the length of the waveguide configured to provide a substantially uniform output profile; and wherein the overlay material comprises Si.sub.3N.sub.4.

20. A method of forming a grating coupler comprising: a. depositing on a wafer a stop layer; b. depositing a grating layer on the stop layer; c. patterning desired gratings; and d. etching, based on the patterning, the grating layer to create a waveguide including the desired gratings, whereby bars of the remaining grating layer of width “w” and length “a” are disposed periodically at a uniform period “∧” on the wafer, the length (a) of each of the bars and gaps between the bars varying along a length of the gratings, wherein a duty cycle defined by the length (a)/period (∧) varies along the waveguide; wherein the duty cycle of (a/∧) is varied monotonically along the length of the waveguide configured to provide a substantially uniform output profile, and wherein the remaining grating layer comprises Si.sub.3N.sub.4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects and advantages of the invention will be apparent to those skilled in the art based on the following detailed description in conjunction with the appended figures, of which:

(2) FIG. 1 shows a cross section of a device in accordance with an exemplary embodiment.

(3) FIG. 2 shows a tilted Scanning Electron Microscopy picture of the gratings overlaying the silicon waveguide.

(4) FIG. 3 illustrates a grating having a constant period (∧) but a varied duty cycle (a/∧) that has a relatively high value at the beginning of the grating near the light source and a relatively low value at the end of the grating away from the light source.

(5) FIG. 4(a) illustrates the grating strength for several fabricated duty cycles for devices with a 120 nm Si.sub.3N.sub.4 overlay grating layer.

(6) FIG. 4(b) illustrates a comparison of the spatial distribution of light from a Si.sub.3N.sub.4 grating overlay with constant duty cycle (50%) and from the designed Si.sub.3N.sub.4 grating with custom duty cycle.

(7) FIG. 5 illustrates the grating strength converted to duty cycle.

(8) FIG. 6 illustrates simulation of grating's sensitivity to process variations. (a) Strength of grating formed by etching a 250 nm×450 nm silicon waveguide (orange) and by etching a 120 nm silicon nitride overlay on the same silicon waveguide (blue). The period of both gratings is 650 nm. Cross section and spatial mode distribution for a silicon waveguide (b) and for the same waveguide with a silicon nitride overlay (c). One can see that the silicon waveguide tightly confines the light, thus the silicon-nitride overlay only slight perturb the mode.

(9) FIG. 7 illustrates grating uniform emission design by apodizing the grating's duty cycle. (a) Grating's strength dependence on duty cycle extracted from 3D FDTD simulations. (b) The desired super-Gaussian emission profile (blue) and its corresponding grating's strength (orange).

(10) FIG. 8 illustrates platform fabrication steps. (a) Deposition of 8 nm of Al.sub.2O.sub.3 and 120 nm of silicon nitride layers. Defining the grating using E-beam. (b) Etching of silicon-nitride layer and stopping on the Al.sub.2O.sub.3 layer. (c) Defining the waveguides using E-beam (450 nm wide), etching, and stopping on the thermal oxide layer. Later, device is cladded with 1 μm of PECVD SiO.sub.2. (d) Falsed-colored tilted Scanning Electron Microscopy picture of the silicon-nitride grating overlay after the waveguide etch.

(11) FIG. 9 illustrates near-field and far-field measurements and simulations for silicon-nitride/silicon platform. (a) and (c) Far-field measurement and simulations for 1 mm grating with a constant and apodized duty cycle, respectively. (b) and (d) Near-field grating emission profile of constant and apodized duty cycle, respectively. As expected, although both gratings' length is the same, the larger effective aperture of the apodized grating enabled smaller beam divergence.

(12) FIG. 10 illustrates the dependence of the gap between waveguides and coupling length.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(13) An exemplary embodiment of a method and device for obtaining a grating coupler with a uniform output profile is described below with respect to FIGS. 1-5. Those skilled in the art will appreciate that the steps and devices described are for exemplary purposes only and are not limited to the specific processes or devices described.

(14) Overview

(15) A long grating with uniform output profile is provided by using a platform based on Silicon and Si.sub.3N.sub.4 and uniform grating output is achieved by varying the duty cycle along the length of the gratings. Using the Si.sub.3N.sub.4 as a low index material overlay, the index contrast between the grating layer and the surrounding cladding are simultaneously reduced while also moving the grating perturbation further away from the mode that travels in the Silicon waveguide thus achieving low grating strength. The overlay also increases the fabrication robustness since it is straightforward to deposit such a layer uniformly and the grating strength is less sensitive to the layer thickness compared to conventional etching into the Silicon. The uniform grating output is engineered by first creating a normalized flat-top output. Then, the grating strength required for a flat-top function is found using the relationship:

(16) $\begin{array}{cc}2\alpha \left(z\right)=\frac{F^2\left(z\right)}{1-{\int }_{z_0}^z{F}^2\left(z\right)dz}& \mathrm{Equation}\left(1\right)\end{array}$
where α is the grating strength and F is the flat-top function, or any function for the desired emission. Finally, for each period, the grating strength is converted to duty cycle as reflected in FIG. 5. This process results in roughly varying the duty cycle from high at the beginning of the grating to low at the end, which in turn varies the output from weak to strong, flattening the output as the optical power in the waveguide decays along its length.
Aspects

(17) The present disclosure includes at least the following aspects:

(18) Aspect 1: A millimeter scale weak grating coupler comprising a silicon waveguide having a plurality of bars of overlay material of length (a) disposed periodically at a period (∧) adjacent the silicon waveguide.

(19) Aspect 2: The grating coupler of aspect 1, wherein a duty cycle of (a/∧) is uniform along the top of the waveguide.

(20) Aspect 3: The grating coupler of aspect 1, wherein a duty cycle of (a/∧) is varied along the top of the waveguide.

(21) Aspect 4: The grating coupler of aspect 3, wherein the duty cycle increases along the silicon waveguide as a grating strength decreases.

(22) Aspect 5: The grating coupler of any one of aspects 1-4, further comprising a stop layer disposed between the overlay material and the waveguide.

(23) Aspect 6: The grating coupler of any one of aspects 1-5, wherein a dimension of one or more bars of overlay material along at least one axis is varied across the plurality of bars.

(24) Aspect 7: The grating coupler of any one of aspects 1-6, wherein the overlay material has an index of refraction that is between an index of refraction of the waveguide and an index of refraction of a cladding material disposed adjacent the waveguide.

(25) Aspect 8: The grating coupler of any one of aspects 1-7, wherein the overlay material comprises Si.sub.3N.sub.4.

(26) Aspect 9: A method of forming a grating coupler comprising: depositing on a Silicon On Insulator (SOI) wafer a stop layer; depositing a grating layer on the stop layer; patterning desired gratings; and etching, based on the patterning, the grating layer to create the desired gratings, whereby bars of the remaining grating layer of width “w” and length “a” are disposed periodically at a period “∧” on the wafer.

(27) Aspect 10: The method of aspect 9, wherein a duty cycle of (a/∧) is uniform along the top of the wafer.

(28) Aspect 11: The method of aspect 9, wherein a duty cycle of (a/∧) is varied along the top of the wafer.

(29) Aspect 12: The method of any one of aspects 9-11, further comprising patterning and etching a waveguide from the wafer whereby the duty cycle of a/∧ increases along the waveguide moving away from a light source.

(30) Aspect 13: The method of any one of aspects 9-12, wherein the stop layer comprises Al.sub.2O.sub.3 or SiO.sub.2, or both.

(31) Aspect 14: The method of any one of aspects 9-13, wherein the grating layer comprises Si.sub.3N.sub.4.

(32) Aspect 15: The method of any one of aspects 9-14, wherein a material forming the stop layer is selected such that it will not etch during the etching step, effectively stopping the etch from penetrating the waveguide layer.

(33) Aspect 16: The method of any one of aspects 9-14, wherein etch chemistry and process parameters of the etching step are selected such that an etch rate of the stop layer is lower than an etch rate of the grating layer.

(34) Aspect 17: The method of any one of aspects 9-16, further comprising depositing a cladding material on the grating coupler.

(35) Aspect 18: The method of aspect 17, wherein the grating layer has an index of refraction that is between an index of refraction of the wafer and an index of refraction of the cladding material.

(36) Aspect 19: The method of any one of aspects 9-18, further comprising analytically mapping duty cycles of the gratings to a required strength set forth by a predetermined function so as to produce a profile of duty cycles per period for an entire length of the gratings.

(37) Aspect 20: The method of aspect 19, wherein the predetermined function is dependent on an emission intensity profile or phase profile as a function of the direction of propagation.

(38) Device Structure

(39) As described herein, a low strength grating which is robust to fabrication variation can be achieved using a platform based on both silicon and Si.sub.3N.sub.4. FIG. 1 shows a cross section of a device in accordance with an exemplary embodiment. As shown the Al.sub.2O.sub.3 stop layer and Si.sub.3N.sub.4 grating layer are provided on top of the Silicon on Insulator (SOI) wafer. FIG. 2 shows a tilted Scanning Electron Microscopy picture of the gratings overlaying the silicon waveguide. In exemplary embodiments, a uniform grating output can be achieved by varying the duty cycle a/∧ along the length of the gratings. Using the Si.sub.3N.sub.4 as a low index material overlay, the index contrast between the grating layer and the surrounding cladding are simultaneously reduced while the grating perturbation is also moved further away from the mode that travels in the Silicon waveguide thus achieving low grating strength. The overlay also increases the fabrication robustness since it is straightforward to deposit such a layer uniformly and the grating strength is less sensitive to the layer thickness compared to conventional etching into the Silicon. A thin stop layer protects the silicon during the Si.sub.3N.sub.4 etch, since etching the silicon will increase the grating strength. A uniform grating output is engineered by first creating a normalized flat-top output. Then, similar to the process described by Waldhausl et al. in “Efficient Coupling into Polymer Waveguides by Gratings,” Appl. Opt. 36, 9383 (1997), the strength per period corresponding to the flat-top function is found. Finally, for each period, the grating strength is converted to duty cycle. As illustrated in FIG. 2, this process results in roughly varying the duty cycle from high at the beginning of the grating to low at the end, which in turn varies the output from weak to strong, flattening the output profile as the optical power in the waveguide decays along its length.

(40) Device Fabrication

(41) A multilayer deposition process is used to form the silicon nitride gratings and underlying waveguides. Starting with a Silicon On Insulator (SOI) wafer with a 250 nm silicon device layer and a 3 μm buried oxide layer, a very thin (3-5 nm) stop layer of Al.sub.2O.sub.3 is deposited followed by another deposition of 120 nm Si.sub.3N.sub.4 grating layer. A thin stop layer protects the silicon during the Si.sub.3N.sub.4 etch, since etching the silicon will increase the grating strength. After using electron-beam lithography (Elionix) to pattern the gratings, the Si.sub.3N.sub.4 film is etched to the Al.sub.2O.sub.3 stop layer (see FIGS. 1-2). The waveguides are then patterned and etched and the process finishes with cladding the wafer by depositing SiO.sub.2 on the devices. Light is coupled to the waveguides using edge couplers and lensed fibers at 1550 nm. The grating output is imaged using an IR camera, which is used to measure the light output from the grating.

(42) The inventors have experimentally demonstrated low grating strength of 3.5 [1/mm] at 50% duty cycle with good agreement to simulations, which is a much lower grating strength than the 150 [1/mm] grating strength of a simulated typical silicon shallow etch gratings (220 nm Si, 2 μm box, 25 nm etch, period 0.6 μm). The grating strength for several fabricated duty cycles is plotted in FIG. 4(a) for devices with a 120 nm Si.sub.3N.sub.4 overlay grating layer. As illustrated, the 50% duty cycle is the strongest and the gratings strength decreases thereafter, as expected. The higher strength of the experimental gratings compared to the simulation results could be due to the thin stop layer. The Al.sub.2O.sub.3 stop layer was only 3 nm thick, and the Si.sub.3N.sub.4 etch penetrated it slightly and created a shallow grating of 2-3 nm deep in the silicon waveguide layer. This increased the overall grating strength.

(43) By varying the duty cycles a/∧ along the gratings length to match a flat-top function, it is possible to achieve a much more uniform near-field output than that of a constant duty cycle over a grating having a length of one millimeter or less. The grating strength is calculated for several gratings with different duty cycles by fitting their near-field output to an exponent. Then, the grating strength required for a flat-top function is found using Equation (1) above, where α is the grating strength and F is the flat-top function. In the last step, duty cycles of the gratings are analytically mapped to the flat-top required strength, producing a profile of duty cycles per period for the entire gratings length. FIG. 4(b) shows a comparison of the spatial distribution of light from a Si.sub.3N.sub.4 grating overlay with constant duty cycle (50%) and from the designed Si.sub.3N.sub.4 grating with custom duty cycle. As illustrated, the designed grating (Si.sub.3N.sub.4 custom duty cycle) has an almost uniform intensity as a function of length along the grating compared to the diminished intensity as a function of length along the grating shown for the constant duty cycle grating (SiN 0.5 constant duty cycle).

(44) The techniques disclosed herein demonstrate control over the strength of the grating and the near-field output profile of the beam. A Si.sub.3N.sub.4 overlay is used on the SOI substrate to fabricate a near-uniform grating output over 1 mm or less with low grating strength measured over various duty cycles. By engineering the duty cycle of the gratings, it is shown that using different grating strengths along the grating length increases the gratings near-field output uniformity. Those skilled in the art will appreciate that the techniques described herein provide a path for integrating gratings in Optical Phased Arrays with very narrow beam divergence and high resolution.

(45) Long Grating and Custom Output Profile

(46) FIG. 6 illustrates a contrast in existing grating design in Silicon only to one with Silicon as waveguide and Silicon-Nitride overlay for the grating. Using Silicon for both guiding the light and gratings lead to gratings with large scattering coefficient (strong gratings), which limits their length. (b) and (c) compare the light distribution for both illustrating that most light does not interact with the Silicon Nitride overlay, leading to weak (and long) gratings.

(47) FIG. 7 illustrates examples for designing gratings with uniform emission (the blue curve in (b)). We find the grating's strength for each duty cycle (shown in (a)) and the required strength per grating period seen in orange in (b). By combining both we can find the duty cycles required along the grating's length to realize the required grating's output profile.

(48) FIG. 8 illustrates a process flow and SEM image.

(49) FIG. 9 illustrates results for our grating, we compare a gratings with constant duty cycle to one with custom duty cycle showing the near- and far-field profiles.

(50) FIG. 10 illustrates the dependence of the gap between waveguides and coupling length. For an array it is better to have long coupling length as this indicates low cross talk. The figure show that it is advantageous to use Silicon waveguides as they allow for smaller gaps for the same coupling length.