Patent Application
20160116680
The subject matter herein relates generally to an optical device that is configured to optically couple with another element, such as an optical fiber or laser, through a grating coupler.
Recently, more and more industries have begun to use optical devices and, in particular, optical devices developed through silicon photonics. For example, photonic integrated circuits (PICs) may be used for various applications in optical communications, instrumentation, and signal-processing fields. A PIC may use submicron waveguides to interconnect various on-chip components, such as optical switches, couplers, routers, splitters, multiplexers/demultiplexers, modulators, amplifiers, wavelength converters, optical-to-electrical signal converters, and electrical-to-optical signal converters. One advantage that PICs have is the potential for large-scale manufacturing and integration through known semiconductor fabrication techniques (e.g., complementary metal-oxide-semiconductor (CMOS)).
A PIC may be optically coupled to an external optical fiber or a light source such that the PIC may receive light from the optical fiber or the light source and/or direct light into the optical fiber. However, it can be challenging to optically couple the optical fiber and the PIC in an efficient manner, such as greater than 50% efficiency. For instance, the optical fiber has a cross-sectional area that is much larger than the cross-sectional area of the submicron waveguide of the PIC. Thus, the mode-field cross-sectional area must be significantly reduced as the light transitions from the optical fiber to the PIC or vice versa.
The two most common light-coupling solutions are in-plane coupling and out-of-plane coupling. In-plane coupling, which may also be referred to as edge coupling or butt coupling, includes orienting the optical fiber such that an end of the optical fiber is aligned with a central axis of the waveguide. In other words, the end of the optical fiber is “in-plane” with the waveguide. Although in-plane coupling can be efficient and can effectively reduce the mode-field diameter, PICs that utilize in-plane coupling can be difficult to manufacture, package, and test for quality control.
In out-of-plane coupling, the optical fiber is not aligned with the central axis or the plane of the waveguide. Instead, the axis of the optical fiber is almost normal to the plane of the waveguide. Out-of-plane coupling may be accomplished through grating couplers. A grating coupler includes a planar grating that is oriented almost normal to the axis of the optical fiber. The grating is configured to scatter the light in a manner that propagates the light in the desired direction (i.e., into the waveguide of the PIC or into the optical fiber).
Grating couplers are generally more tolerant to misalignment and lend themselves to less packaging complexity. At least for certain applications, however, PICs that include grating couplers are typically less efficient than PICs that include in-plane coupling. Moreover, aligning the PIC and the optical fiber can still be challenging. For example, it is often necessary to orient the optical fiber such that the optical fiber is not perfectly normal with respect to the grating. For example, the optical fiber is typically positioned within about 9.0° to about 12.0° with respect to normal. For some applications, it may be difficult to reliably position the optical fiber in this orientation.
Accordingly, there is a need for a light-coupling structure having a grating coupler that is capable of coupling with a light beam that is effectively normal with respect to the grating coupler.
In an embodiment, a light-coupling structure is provided. The light-coupling structure includes a grating coupler that is configured to optically couple with an optical element. The grating coupler has a diffraction grating that extends parallel to a grating plane. The grating coupler is configured to diffract a light beam into first and second diffracted portions when the light beam is directed from the optical element to the grating coupler and is effectively normal to the grating plane. The first and second diffracted portions propagate away from each other. The light-coupling structure also includes first and second intermediate waveguides that are optically coupled to the grating coupler and configured to receive the first and second diffracted portions, respectively, from the grating coupler. The light-coupling structure also includes a common waveguide that is coupled to the first and second intermediate waveguides at a waveguide junction. The first and second diffracted portions propagate within the first and second intermediate waveguides, respectively, and are combined in-phase at the waveguide junction.
In some embodiments, the light beam is effectively normal with respect to the grating plane when the light beam is within about 6.0° of being normal with respect to the grating plane.
In some embodiments, the first and second intermediate waveguides are formed from a waveguide layer. The waveguide layer also forms a light-coupling portion that extends alongside the diffraction grating. The diffraction grating is configured to direct the first and second diffracted portions into the light-coupling portion. The first and second diffracted portions propagate in the opposite directions within the light-coupling portion. Optionally, the grating coupler includes a cladding layer that extends alongside the waveguide layer. The diffraction grating may be embedded within the cladding layer such that a portion of the cladding layer extends between the diffraction grating and the waveguide layer. Optionally, the diffraction grating is separated from the waveguide layer by a cladding sub-layer.
In some embodiments, the diffraction grating has a grating period that is less than a wavelength of the light beam. For example, the diffraction grating may have a grating period that is less than 1000 nanometers.
In an embodiment, an optical device is provided that includes a grating coupler that is configured to optically couple with an optical element. The grating coupler has a diffraction grating that extends parallel to a grating plane. The grating coupler is configured to diffract a light beam into first and second diffracted portions when the light beam is directed from the optical element to the grating coupler and is effectively normal to the grating plane. The first and second diffracted portions propagate away from each other. The optical device also includes first and second intermediate waveguides that are optically coupled to the grating coupler and configured to receive the first and second diffracted portions, respectively, from the grating coupler. The optical device also includes a common waveguide that is coupled to the first and second intermediate waveguides at a waveguide junction. The first and second diffracted portions propagate within the first and second intermediate waveguides, respectively, and are combined in-phase at the waveguide junction to form a guided portion. The optical device also includes an optical circuit that is optically coupled to the common waveguide. The optical circuit is configured to process the guided portion in a designated manner.
Optionally, the optical device is a photonic integrated circuit. Optionally, the optical circuit includes a modulator.
In some embodiments, the optical device 100 is an integrated device that includes a silicon photonics chip. At least a portion of the optical device 100 may be fabricated with processes that are used to manufacture semiconductors. For example, the optical device 100 may be manufactured using processes that produce complementary metal-oxide-semiconductor (CMOS) devices and/or silicon-on-insulator (SOI) devices. In particular embodiments, the entire optical device 100 is manufactured using CMOS or SOI processes. The optical device 100 may be incorporated into a larger system or device.
As shown in
The optical device 100 includes a first light-coupling structure 106 that is optically coupled to an optical circuit 108 and/or a second light-coupling structure 110. The second light-coupling structure 110 is optically coupled to the second optical element 104. The optical circuit 108 and the light-coupling structure 110 are illustrated generically in
In an exemplary embodiment, the light-coupling structure 106 is an in-coupling structure that receives a light beam 120 from the first optical element 102, and the light-coupling structure 110 is an out-coupling structure that provides the modulated light to the second optical element 104. In some embodiments, however, the optical device 100 may be configured to propagate light in the opposite direction from the light-coupling structure 110 to the light-coupling structure 106.
The light-coupling structure 106 includes a grating coupler 112 and first and second intermediate waveguides 114, 116. The grating coupler 112 is optically coupled to the optical element 102 such that the light beam 120 received from the optical element 102 is separated into first and second diffracted portions that are directed in opposite first and second directions (represented as arrows 115, 117). As such, the grating coupler 112 may be described as a one-dimensional (1D) grating coupler. The first and second diffracted portions of the light beam 120 are directed into the first and second intermediate waveguides 114, 116, respectively. The first and second diffracted portions are transmitted along the respective first and second intermediate waveguides 114, 116 and joined or re-coupled at a waveguide junction 130 or an optical combiner (e.g., multimode interference structure). The waveguide junction 130 is configured to join the first and second diffracted portions in-phase such that the first and second diffracted portions form a combined light within a common waveguide 132. The combined first and second diffracted portions are referred to as a guided portion or a combined portion. The guided portion may then propagate along the common waveguide 132 to a coupling-transition region 134. The coupling-transition region 134 includes a device waveguide 136 that directs the guided portion to the optical circuit 108.
As described herein, the light-coupling structure 106 is configured to receive the light beam 120 from the first optical element 102. Unlike conventional grating couplers, the light beam 120 may be effectively normal or perpendicular to a grating plane 122, such as within 6.0° of a normal axis 124. The grating plane 122 may represent a plane that extends parallel to one or more layers of the light-coupling structure 106. For example, the grating coupler 112 includes a grating 126 having a variation or modulation in refractive index that extends parallel to the grating plane 122. The refractive index variation may be periodic throughout or include a plurality of portions that vary at different frequencies.
Due to tolerances in the manufacturing of the optical device 100 and/or the optical element 102, it may be difficult to position the optical element 102 such that the light-propagating axis 128 is perfectly normal with respect to the grating coupler 112 or the grating plane 122. Embodiments set forth herein may orient the light-coupling structure 106 and/or the optical element 102 relative to each other such that the light-propagating axis 128 may be effectively normal with respect to the grating coupler 112 or the grating plane 122. Embodiments herein may be “effectively normal” if the light-propagating axis 128 is 6.0° or less from being perfectly normal with respect to the grating coupler 112 or the grating plane 122. In particular embodiments, the light-propagating axis 128 is effectively normal if the light-propagating axis 128 is 5.0° or less, 4.0° or less, or 3.0° or less from being perfectly normal with respect to the grating coupler 112 or the grating plane 122. In more particular embodiments, the light-propagating axis 128 may be 2.5° or less, 2.0° or less, 1.5° or less, 1.0° or less, or 0.5° or less from being perfectly normal with respect to the grating coupler 112 or the grating plane 122. The cone shown with respect to the normal axis 124 and the grating plane 122 may represent permitted tolerances of the light-propagating axis 128.
Embodiments set forth herein may be unlike conventional light-coupling structures that intentionally tilt an optical fiber relative to the normal axis of the grating coupler. Conventional light-coupling structures typically tilt the light-propagating axis with respect to the normal axis by 9° or more to, among other things, increase the coupling efficiency. Despite the light-propagating axis 128 being effectively normal with respect to the grating coupler 112 or the grating plane 122, embodiments may be capable of achieving a reasonable coupling efficiency. For example, in some embodiments, a coupling efficiency between the optical element 102 and the grating coupler 112 and/or the optical device 100 may be at least 50% when the light-propagating axis 128 is effectively normal with respect to the grating plane 122. In particular embodiments, the coupling efficiency may be at least 60% or at least 70%. In more particular embodiments, the coupling efficiency may be at least 75% or at least 80%.
In some embodiments, the optical device 100 and/or the light-coupling structure 106 includes a plurality of substrate layers that are stacked over each other. For example, the light-coupling structure 106 may include a series of substrate layers having different refractive indices that are configured to control light as set forth herein. By way of example, the substrate layers may include one or more layers of silicon oxide, one or more layers of silicon nitride, one or more layers silicon oxynitride (SiON), one or more layers of silicon rich oxide, one or more layers of a silicon substrate, and one or more buried oxide layers. As described herein, the optical device 100 and/or the light-coupling structure 106 may be manufactured using semiconductor fabrication processes. For example, the substrate layers may be provided using processes that are used in CMOS and/or SOI technologies.
In the illustrated embodiment, the waveguide layer 172 is shaped to include a light-coupling portion 146 and the first and second intermediate waveguides 114, 116. The light-coupling portion 146 is stacked with respect to the cladding layer 171 and the diffraction grating 126. The first and second intermediate waveguides 114, 116 are coupled to opposite sides or ends 150, 152 of the light-coupling portion 146 or the grating coupler 112. Optionally, the light-coupling portion 146 may have an area that is at least equal to the grating coupler 112. For example, the grating coupler 112 extends along a first dimension 180 and a second dimension 182. The first and second dimensions 180, 182 are perpendicular to each other and may define an area of the grating coupler 112.
As described above, when the light beam 120 (
Each of the first and second path segments 158, 160 has a designated length that is measured from the corresponding mode-conversion segment to the waveguide junction 130. The first and second path segments 158, 160 may also have a designated path shape or contour. For example, the first and second path segments 158, 160 are substantially S-shaped. In an exemplary embodiment, the lengths of the first and second path segments 158, 160 are effectively equal and the first and second path segments 158, 160 may have identical shapes. As such, the first and second path segments 158, 160 may be effectively symmetrical with respect to a plane 161 that extends between the waveguide junction 130 and a center of the grating coupler 112. The plane 161 may extend parallel to the normal axis 124 (
As shown, the waveguide junction 130 may be a Y-junction. The first and second path segments may 158, 160 may extend into the waveguide junction 130 at an angle 162. The angle 162 may be, for example, less than 20°. The first and second path segments 158, 160 may combine to form the common waveguide 132. The common waveguide 132 may have a cross-sectional area that is similar or identical to the first and second path segments 158, 160 of the first and second intermediate waveguides 114, 116.
Each of the substrate layers 171-174 may include a single layer or a plurality of sub-layers. For example, the cladding layer 171 may include a first cladding sub-layer 176 that extends between the diffraction grating 126 and the waveguide layer 172 and a second cladding sub-layer 177 that is formed along the diffraction grating 126. For example, after the first cladding sub-layer 176 is formed, the second cladding sub-layer 177 and the diffraction grating 126 may be subsequently formed on top of the first sub-layer 176. The first sub-layer 176 may have a refractive index that is lower than the refractive index of the waveguide layer 172 or the refractive index of the grating material 179. The second sub-layer 177 may include a single layer or multiple sub-layers.
The diffraction grating 126 may be formed in various manners before, after, or concurrently with the first sub-layer 176 and/or the second sub-layer 177. For example, the diffraction grating 126 may be written, impressed, embedded, imprinted, etched, grown, deposited or otherwise formed within the light-coupling structure 106. As shown in
The series of spaced-apart ridges 184 of the diffraction grating 126 may be co-planar with respect to one another. Optionally, the ridges 184 may have square or rectangular cross-sections. For example, each ridge 184 may have a height (or depth) 186 and a width (or duty cycle) 188. Adjacent ridges 184 are separated by a gap or spacing 190. The width 188 and the gap 190 may determine a period or pitch 192 of the diffraction grating 126. The period 192 may be uniform for an entirety of the first dimension 180. In alternative embodiments, the period 192 may change for predetermined portions along the first dimension 180 to achieve a desired effect. The height 186, the width 188, the gaps 190, and the material of the diffraction grating 126 include at least some of the parameters that may be configured so that the grating coupler 112 performs as desired.
In particular embodiments, the period 192 of the diffraction grating 126 is less than the wavelength of the light beam 120. The period 192 of the diffraction grating 126 may be determined by the grating coupling equation:
wherein Λ is the period 192, λ is the wavelength of the incoming light, Neff is the effective index of the guided mode in the waveguide layer 172 as well as the diffraction grating 126, η is the refractive index of the second cladding layer 177, and θ is the incident angle of the incoming light with respect to the normal axis. In some embodiments, the incident angle θ may be effectively zero such that the equation can be changed to:
The period 192 may be calculated by satisfying a phase match condition with respect to the waveguide layer 172. The diffraction grating 126 may be characterized as having a sub-wavelength grating period. The light beam 120 may have one or more wavelengths within a predetermined range. For example, the wavelength (or wavelengths) of the light beam 120 may be between 800 nanometers (nm) and 1600 nm. Common wavelengths used in industry may include 850 nm, 1310 nm, and 1550 nm. In particular embodiments, the period 192 may be configured to reduce an efficiency or power of the second order of diffraction. The period 192 may be less than the wavelength of the light beam or incident light. For example, the period 192 may be less than 1250 nm, less than 1125 nm, less than 1000 nm, less than 900 nm, or less than 850 nm. In particular embodiments, the period 192 may be less than 800 nm, less than 775 nm, or less than 750 nm. In more particular embodiments, the period 192 may be less than 725 nm or less than 700 nm. The period 192 may be based on other parameters of the diffraction grating 126, such as the refractive indices of the different materials that form the diffraction grating 126.
To illustrate values that may be used by embodiments set forth herein, the height 186 may be about 250 nm, the width 188 may be about 300 nm, the refractive index of the ridges 184 may be about 3.5, the refractive index of the material of the cladding layer 171 extending between the ridges 184 may be about 1.45, and the period 192 may be about 755 nm. The wavelength of the light may be about 1310 nm. However, the above values and other values noted herein are provided only to illustrate exemplary values that may be used by one or more embodiments and it should be understood that other values may be used depending upon circumstances and/or the desired application.
The diffraction grating 126 is configured such that the effectively normal light beam 120 is diffracted by the diffraction grating 126 to form first and second diffracted portions 202, 204. The first and second diffracted portions 202, 204 are directed toward the waveguide layer 172 at an angle that allows the first and second diffracted portions 202, 204 to couple with the waveguide layer 172. As shown in
The light beam 120 may be configured such that the light beam 120 includes only one polarization, either transverse electric (TE) mode or transverse magnetic (TM) mode. In an exemplary embodiment, the optical device 100 (
Although
As shown in
The coupling-transition region 134 also includes an inverse taper portion 210 of the device waveguide 136. The inverse taper portion 210 is positioned adjacent to the end portion 206 of the common waveguide 132 and extends parallel to the common waveguide 132. The inverse taper portion 210 and the end portion 206 are positioned and shaped relative to each other such that the guided portion of the light is directed into the inverse taper portion 210. For instance, as shown in
It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Dimensions, types of materials, orientations of the various components, and the number and positions of the various components described herein are intended to define parameters of certain embodiments, and are by no means limiting and are merely exemplary embodiments. Many other embodiments and modifications within the spirit and scope of the claims will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
As used in the description, the phrase “in an exemplary embodiment” and the like means that the described embodiment is just one example. The phrase is not intended to limit the inventive subject matter to that embodiment. Other embodiments of the inventive subject matter may not include the recited feature or structure. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means—plus-function format and are not intended to be interpreted based on 35 U.S.C. §112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.
