HK1237885B - Photonic chip grating couplers - Google Patents

Description

Photonic chip grating coupler

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/258,221, filed on November 20, 2015, and entitled “PHOTONIC CHIP GRATING COUPLERS,” the entire contents of which are incorporated herein by reference.

Technical Field

The present disclosure relates to the field of optical communications, and in particular to an optical coupler.

Background Art

Silicon photonics technology, which combines optical, electrical, and optoelectronic components on the same chip, offers low-cost, low-power, and high-speed optical solutions for data communications, telephony, and, in particular, optical communications. By integrating optical, electrical, and optoelectronic components on the same substrate, transceiver channels and transmission speeds can be adjusted.

Summary of the Invention

At least one aspect relates to a coupling device for coupling light from a source to a chip. The coupling device includes a beam splitter having an input port, a first output port, and a second output port, the beam splitter being configured to receive an incident light beam at the input port and output a first light beam having substantially transverse magnetic (TM) polarization from the first output port and a second light beam having substantially transverse electric (TE) polarization from the second output port. The coupling device includes a first grating coupler formed on the chip, which includes a first horn portion, the first horn portion including a first grating having a first plurality of arcuate grating lines. The coupling device includes a first reflector, the first reflector being arranged to reflect the first light beam to the first grating coupler such that a first incident plane of the first light beam is substantially perpendicular to an axis of the first horn portion, the TM polarization direction is within the first incident plane, and the first light beam is incident on the first plurality of gratings at an angle relative to a plane normal of the first grating coupler.

In some embodiments, the coupling device includes a second grating coupler formed on the chip, including a second horn portion, the second horn portion including a second grating having a second plurality of arcuate grating lines; and a second reflector, the second reflector being arranged to emit a second light beam to the second grating coupler such that a second incident plane of the second light beam is substantially parallel to an axis of the second horn portion, a direction of TE polarization is substantially perpendicular to the second incident plane, and the second light beam is incident on the second grating at an angle relative to a plane normal of the second grating coupler.

In some embodiments, the first horn portion and the second horn portion comprise silicon.

In some embodiments, the axis of the first horn portion is an axis of symmetry of the first horn portion within the plane of the first horn portion.

In some embodiments, the first plurality of arcuate grating lines are substantially asymmetric about the first horn portion.

In some embodiments, the second plurality of arcuate grating lines are formed substantially as elliptical arcs.

In some embodiments, the first horn portion includes a narrow end coupled to a waveguide, and the first grating directs incident light toward the narrow end of the horn portion and into the waveguide.

In some embodiments, the second horn portion includes a narrow end coupled to a waveguide, and the second grating directs incident light toward the narrow end of the horn portion and into the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a schematic diagram of an example photonic chip used in optical communications.

FIG. 1B shows a schematic diagram of the example photonic chip shown in FIG. 1A coupled to an optical coupler.

FIG2 is a schematic diagram showing the optical components in an optical coupler.

3A and 3B illustrate cross-sectional views of the first prism/reflector, TE grating coupler, second prism/reflector, and TM grating coupler shown in FIG. 2 .

4A-4D show schematic diagrams of example grating couplers coupled to waveguides.

FIG5 shows a cross section of an example photonic chip with a grating coupler.

The same reference numbers and symbols in different drawings indicate the same elements.

DETAILED DESCRIPTION

The various principles introduced above and discussed in more detail below may be implemented in any of a variety of ways, as the principles are not limited to any particular implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.

A major challenge in silicon photonics technology is efficiently coupling light from lasers or optical fibers into and out of photonic chips. Silicon photonic integrated circuits are typically designed to operate on and process a single (fundamental) waveguide mode, where the fundamental waveguide mode relates to the polarization direction of the light to be processed. Light from, for example, an optical fiber or laser and entering a photonic chip can have transverse electric (TE) and/or transverse magnetic (TM) polarization. There are some solutions to efficiently couple TE polarized light between the fundamental mode of an optical fiber or laser and a single waveguide guiding light on a photonic chip. However, there is a lack of more efficient solutions for coupling TM polarized light to the fundamental mode of a waveguide.

Standard TM couplers typically have high optical losses and do not use the fundamental mode output light in the waveguide. As a result, an additional polarization rotator is required to rotate the polarization of the TM coupler output light into the fundamental mode. Adding optical components such as polarization rotators can introduce additional losses and reduce the link energy budget of the communication system.

FIG1A shows a schematic diagram of an example photonic chip 100 used in optical communications. In particular, the photonic chip 100 can be used to process both optical and electrical signals. The photonic chip 100 may include one or more optical/optoelectronic components 102 and one or more electronic components 104. In some embodiments, the optical/optoelectronic components 102 may include, but are not limited to, photodetectors, lasers, waveguides, splitters, filters, multiplexers, demultiplexers, lenses, reflectors, polarizers, speed reducers, optical and/or electro-optical modulators, amplifiers, attenuators, and the like. In some embodiments, modulators other than optical or electro-optical modulators may be used, such as, but not limited to, acousto-electric modulators, magneto-optical modulators, mechanical-optical modulators, thermo-electric-optical modulators, or combinations thereof. In some embodiments, the modulator may utilize techniques such as quadrature amplitude modulation (QAM) and phase shift keying (PSK) to modulate the carrier signal. Other types of modulation may be used. The optical components may be used to process optical signals received via optical fibers, or to process optical signals generated on the photonic chip 100. In some embodiments, the photonic chip 100 may further include a grating coupler 106 that allows for coupling of optical signals between the optical component 102 on the photonic chip 100 and an off-chip optical fiber or laser. For example, light between the grating coupler 106 and the other optical components 106 may communicate using a waveguide 112. The grating coupler 106 is further discussed below in Figures 2-5. In some embodiments, the electronic components 104 may include analog and digital electronic components, such as, but not limited to, voltage and/or current amplifiers, transconductance amplifiers, filters, digital signal processors, analog-to-digital converters, digital-to-analog converters, and the like. The electronic components 104 and the optical components 102 may be used to implement a variety of electro-optical functional blocks, such as, but not limited to, transmitters, receivers, switches, modulators, repeaters, amplifiers, and the like. Although Figure 1A shows optical, electro-optical, and electronic components fabricated on the same chip, in some embodiments, these components may be mounted on separate chips interconnected by electrical interconnects (e.g., wired couplings, copper pillars, etc.) and/or optical interconnects (e.g., waveguides, optical fibers, etc.).

FIG1B shows a schematic diagram of the example photonic chip 100 shown in FIG1A coupled to an optical coupler 108. In particular, the optical coupler 108 can serve as an interface between the photonic chip 100 and one or more optical fibers 110. The optical coupler 108 can typically be located on the grating coupler 106 ( FIG1A ) of the photonic chip 100. The optical coupler 108, coupled to the grating coupler 106, can facilitate bidirectional optical signal communication between the optical fibers 110 and the optical components 102 on the photonic chip 100. In some embodiments, instead of or in addition to the optical fibers 110, the optical coupler 108 can facilitate coupling a light source (e.g., a laser) to the photonic chip 100. The optical coupler 108 can include a housing for enclosing one or more optical components. The housing can also provide a specific position and orientation for the optical components so that light can be received from the optical fibers 110 and the grating coupler 106 at a desired angle and light can be sent to the optical fibers 110 and the grating coupler 106 at a desired angle.

FIG2 shows a schematic diagram of an optical assembly in an optical coupler 200. Specifically, the optical assembly shown in FIG2 can be used in the optical coupler 108 shown in FIG1B. The optical coupler 200 may include a polarization beam splitter (PBS) 202, a first prism/reflector 204, and a second prism/reflector 206. The PBS 202 can split an incident light beam into two light beams of different polarizations. Specifically, the PBS 202 can split an incident light beam having both TM and TE polarizations into one light beam having substantially only TM polarization and another light beam having substantially only TE polarization. As shown in FIG2, a light beam 208 from an optical fiber 210 (e.g., one of the optical fibers 110 shown in FIG1B) is incident on the PBS 202. The incident light beam 208 has both TM and TE polarizations, such that the TM polarization is orthogonal to the TE polarization and the TE polarization is in the plane of incidence. The PBS 202 splits the incident light beam 208 into two light beams: a first light beam 212 having TE polarization and a second light beam 214 having TM polarization. The first light beam 212 is directed toward the first prism/reflector 204, while the second light beam 214 is directed toward the second prism/reflector 206. The first prism/reflector 204, in turn, reflects the incident first light beam 212 toward the on-chip TE grating coupler 216. Similarly, the second prism/reflector 206 reflects the incident second light beam 214 toward the on-chip TM grating coupler 218. In some embodiments, the TE grating coupler 216 and the TM grating coupler 218 can be similar to the grating coupler 106 shown in FIG. 1A .

It can be understood that the optical coupler 200 shows that the light beam output from the optical fiber 210 is directed to the grating couplers 216 and 218 on the chip. At the same time, the optical coupler 200 can also combine the beam lights generated by the grating couplers 216 and 218 and direct the combined light beam back to the optical fiber 210.

3A and 3B illustrate cross-sectional views of the first prism/reflector 204, TE grating coupler 216, second prism/reflector 206, and TM grating coupler 218 shown in FIG2 . In particular, FIG3A illustrates a cross-sectional view of the first prism/reflector 204 and TE grating coupler 216 along the plane of incidence of the first light beam 212, while FIG3B illustrates a cross-sectional view of the second prism/reflector 206 and TM grating coupler 218 along the plane of incidence of the second light beam 214. In some embodiments, the TE grating coupler 216 and the TM grating coupler 218 may be made of silicon, silicon nitride, silicon oxide, polysilicon, or a combination of these materials. The first prism/reflector 204 and the second prism/reflector 206 both constitute a prism or a reflective surface (e.g., a mirror) to reflect incident light at a specific angle to a grating. The first prism/reflector 204 is positioned relative to the TE grating coupler 216 such that the first light beam 212 is incident on the TE grating coupler 216 at a desired angle relative to the normal to the plane of the TE grating coupler 216. Furthermore, the plane of incidence of the light beam is oriented substantially along the longitudinal axis of the TE grating coupler 216. In some embodiments, the plane of incidence of the TE polarized light beam may have an angle of from about -5° to about +5° or from about -10° to about +10° relative to the longitudinal axis of the TE grating coupler 216. Similarly, the second prism/reflector 206 is positioned relative to the TM grating coupler 218 such that the second light beam 214 is incident on the TM grating coupler 218 at a desired angle relative to the normal to the plane of the TM grating coupler 216. However, unlike the TE light beam (whose plane of incidence is substantially along the longitudinal axis of the TE grating coupler), the plane of incidence of the TM polarized light beam is substantially perpendicular to the longitudinal axis of the TM grating coupler 218. In some embodiments, the plane of incidence of the TM polarized light beam can have an angle from about 85° to about 95° or from about 80° to about 100° relative to the longitudinal axis of the TM grating coupler 218 .

In some embodiments, the optical coupler 200 can provide some degree of adjustment (manual or automatic) for the orientation and position of the PBS 202, the first prism/reflector 204, and the second prism/reflector 206. In some embodiments, the first prism/reflector 204 and the second prism/reflector 206 can include micro-electromechanical system (MEMS)-based reflectors or mirrors whose orientation can be adjusted using electrical signals from an on-chip controller. In some embodiments, the optical coupler 200 can also include a calibration lens to calibrate the light in the PBS 202 and/or a focusing lens to match the beam width to the scattering pattern of the grating couplers 216 and 218.

Figures 4A-4D illustrate schematic diagrams of example grating couplers coupled to waveguides. In particular, Figures 4A and 4C illustrate top views of a TE grating coupler 402 and a TM grating coupler 452 coupled to their associated waveguides 404 and 454. Figures 4B and 4D illustrate cross-sectional views of waveguides 404 and 454, respectively. The TE grating coupler 402 shown in Figure 4A can be used to implement the TE grating fiber discussed above with respect to Figures 2 and 3A, while the TM grating coupler 452 shown in Figure 4C can be used to implement the TM grating coupler discussed above with respect to Figures 2 and 3B.

4A and 4B , the TE grating coupler 402 may include a TE horn portion 406 having a narrow end and a wide end. The waveguide 404 may be coupled to the narrow end of the TE horn portion 406, while the TE grating 408 may be formed at the wide end of the TE horn portion 406. The TE grating 408 may include a plurality of lines or ridges 412. The horn shape allows a larger mode of incident light to couple with a relatively narrow mode of the waveguide 404. For reference, the plane of the TE grating coupler 402 is shown in the x-y plane of a three-dimensional Cartesian space represented by the x, y, and z axes. The horn portion 406 may include a longitudinal axis extending from the narrow end to the wide end of the horn portion 406. For ease of reference, FIG. 4A illustrates the longitudinal axis of the horn portion 406 co-incident with the y-axis. In some embodiments, the longitudinal axis of the horn portion 406 may be an axis of symmetry of the horn portion 406. In some embodiments, the TE grating 408 is symmetrical along the longitudinal axis or the axis of symmetry of the horn portion 406. The TE grating may include a plurality of grating lines or ridges 412, for example, shaped or curved into circular, parabolic, or elliptical arcs, separated by a plurality of grooves formed on the surface of the chip on which the grating coupler 402 is mounted. In some embodiments, the TE grating 408 may include a plurality of periodically spaced regions of the same optical index separated by spaced regions of different optical indices. The periodically spaced regions do not necessarily need to be separated by grooves. In some embodiments, the TE grating 408 may be considered to act as an optical antenna, directing or guiding a light beam incident on the grating toward the narrow end of the horn portion 406 and into the waveguide 404.

FIG4A also shows a light beam 410 having TE polarization incident on the TE grating 408. As shown, the electric field (E) is normal or perpendicular to the plane of incidence, along the centerline 414 of the beam 410 and the longitudinal axis of the horn section 406 and parallel to the magnetic field (H). The plane of incidence of the TE polarized light beam 410 is substantially perpendicular (orthogonal) to the x-axis of the first horn section 406. The TE polarized light beam 410 is incident on the TE grating 408 at an angle θ relative to the normal (z-axis) to the plane of the grating coupler 402. Having a non-zero angle θ reduces the risk of light being reflected back to a reflector (e.g., the prism/reflector 204 shown in FIG3A ), which reflects the light beam to the TE grating 408. The TE polarized light beam 410 is oriented with respect to the horn section 406 such that the plane of incidence of the light beam 410 is substantially along the longitudinal axis of the horn section 406. As a result, the electric field (E) of the TE polarization of the TE polarized light beam 410 is oriented substantially perpendicular to the longitudinal axis of the horn section 406 (as shown). Thus, when the TE polarized light beam 410 is incident on the TE grating 408, it has a polarization substantially parallel to the x-y plane, and the incident light 410 is directed from the horn section 406 into the waveguide 404, as shown by the electric field component (E) in FIG4B . In some embodiments, the plane of incidence of the TE polarized light beam 410 may have an angle α of approximately 5-10° relative to the longitudinal axis of the horn section 406.

As described above, FIG4C illustrates a top view of a TM grating coupler 452. The TM grating coupler 452 may include a TM horn portion 456, which may be similar in shape to the TE horn portion 406 shown in FIG4A. A waveguide 454 may be coupled to the narrow end of the TM horn portion 456, and a TM grating 458 may be formed at the wide end of the TM horn portion 456. The TM grating may include a plurality of lines or ridges 462. For reference, the plan view of the TM grating coupler 452 is shown in the x-y plane. The TM horn portion 456 may include a longitudinal axis extending from the narrow end to the wide end of the TM horn portion 456. For ease of reference, FIG4C illustrates the longitudinal axis of the TM horn portion 456 as being aligned with the x-axis.

In some embodiments, unlike the TM grating 408 shown in FIG4A (which is symmetric along the longitudinal axis of the TE horn portion 406), the TM grating 458 can be asymmetric along the longitudinal axis of the TM horn portion 456. In some embodiments, the asymmetry of the TM grating 458 can be partially represented by the angle formed between the radius of the elliptical grating arc of the TM grating 458 in the plane of the TM grating coupler 452 and the longitudinal axis of the TM horn portion 456. This angle is represented by , as shown in FIG4C. In some embodiments, the elliptical shape of the TM grating lines can be modeled by the following equations (1)-(3):

x＝r·cos(φ) (1)

y＝-r·sin(φ) (2)

in

FIG4C also shows a TM polarized light beam 460 incident on the TM grating 458. As shown, the magnetic field (H) is orthogonal or perpendicular to the plane of incidence, which is along the centerline 464 of the beam 460 and parallel to the electric field (E). In a manner similar to the TE incident light beam 410 shown in FIG4A , the TM polarized light beam 460 also forms an angle of incidence θ relative to the plane normal (z-axis) of the TM grating coupler 452. In some embodiments, the angle of incidence formed by the TM polarized light beam 460 relative to the plane normal of the TM grating coupler 452 can be different from the angle formed by the TE incident light beam 410 shown in FIG4A . In addition, the plane of incidence of the TM light beam 460 is substantially perpendicular to the longitudinal axis of the horn section 456. This is unlike the plane of incidence of the TE light beam 410 shown in FIG4A , which is substantially parallel to or along the longitudinal axis of the horn section 406. In some embodiments, the plane of incidence of the TM polarized light beam 460 can have an angle of approximately 85-95° or approximately 80-100° relative to the longitudinal axis of the horn section 456. The orientation of the incident TM polarized light beam 460 relative to the TM coupler 452 causes the electric field (E) of the TM polarized light beam 460 to be oriented perpendicular to the longitudinal axis of the horn section 456. As a result, the TM polarized light 410 is introduced into the waveguide 454 with a polarization substantially parallel to the x-y axis, as shown by the electric field component (E) in FIG4D. This is similar to the polarization of light entering the waveguide 404 coupled to the TE grating coupler 402 shown in FIG4B. Because the polarization of the light beam in the TM waveguide 454 is the same as the polarization of the light in the TE waveguide 404 (i.e., the fundamental mode), no additional components (e.g., polarization rotators) are required to rotate the polarization of the light in the TE or TM waveguides. As a result, losses due to components such as polarizer rotators can be avoided, resulting in improved signal quality.

Figure 5 shows a cross-sectional view of an example photonic chip 500 having a grating coupler. In particular, the cross-sectional view of the photonic chip 500 may represent a cross-sectional view along the longitudinal axis of the TE or TM grating couplers 402 and 452 (shown in Figures 4A and 4C) and along the longitudinal axis of their respective horn portions. Figure 5 shows a silicon-on-insulator (SOI) chip 500, which includes a silicon handle layer or substrate 502, a buried oxide layer 504, and a thin silicon layer 506 cast on the oxide layer 504. The thin silicon layer 506 can be formed to form a grating 512, which has grating lines or ridges 508 separated by grooves 510. In some embodiments, these grating features can be formed using dry etching or wet etching. In some other embodiments, the grating features can be formed using anisotropic etching techniques. However, other well-known techniques for forming semiconductors can also be used. In some embodiments, the thin silicon layer 506 can have a thickness of approximately hundreds of nanometers.

In some embodiments, the width and slope of the grating lines or ridges 508 and grooves 510 can be fixed (uniform grating) or can vary from one groove to another (apodized grating) to optimize the local scattering intensity to increase the overlap with the incident light beam mode and/or to tune the grating 51 to the wavelength of the incident light beam. In some embodiments, the thickness of the thin silicon layer 506 or oxide layer 504 can be selected to optimize the light coupling from the grating 512 to the optical fiber and improve the bandwidth. In some embodiments, a reflective layer (dielectric or metal) can be cast under the grating 512. In some embodiments, materials such as SiN, SiON can be used to form one or more layers in the photonic chip 500. In some embodiments, standard CMOS processes can be used to manufacture the photonic chip 500.

Various modifications to the embodiments described in this disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of this disclosure. Therefore, the claims are not intended to be limited to the embodiments shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles disclosed herein, and the novel features.

Claims (8)

1. A coupling device for coupling light from a source to a chip, comprising: A polarizing beam splitter having an input port, a first output port, and a second output port, the polarizing beam splitter being configured to: receive an incident beam at the input port, and output a first beam having substantially transverse magnetic TM polarization from the first output port and a second beam having substantially transverse electric TE polarization from the second output port, wherein the incident beam has both the TM polarization and the TE polarization such that the TM polarization is orthogonal to the TE polarization, and the TE polarization lies in the plane of incidence of the incident beam; A first grating coupler formed on the chip, the first grating coupler including a first horn portion, the first horn portion including a first grating having a first plurality of arcuate grating lines; and A first reflector is arranged to reflect the first beam onto the first grating coupler such that a first incident plane of the first beam is substantially perpendicular to the axis of the first horn portion, the direction of the TM polarization is in the first incident plane, and the first beam is incident on the first grating at an angle relative to the normal to the plane of the first grating coupler. 2. The coupling device according to claim 1, further comprising: A second grating coupler formed on the chip, the second grating coupler including a second horn portion, the second horn portion including a second grating having a second plurality of arcuate grating lines; and A second reflector is arranged to reflect the second beam onto the second grating coupler such that the second incident plane of the second beam is substantially parallel to the axis of the second horn portion, the direction of TE polarization is substantially perpendicular to the second incident plane, and the second beam is incident on the second grating at an angle relative to the normal to the plane of the second grating coupler. 3. The coupling device according to claim 2, wherein the first horn portion and the second horn portion comprise silicon. 4. The coupling device according to claim 1, wherein the axis of the first horn portion is the axis of symmetry of the first horn portion in the plane of the first horn portion. 5. The coupling device according to claim 1, wherein the first plurality of arcuate grating lines are substantially asymmetrical about the axis of the first horn portion. 6. The coupling device according to claim 2, wherein the second plurality of arcuate grating lines are shaped as substantially elliptical arcs. 7. The coupling device of claim 1, wherein the first horn portion includes a narrow end coupled to the waveguide, and the first grating guides incident light to the narrow end of the horn portion and into the waveguide. 8. The coupling device of claim 2, wherein the second horn portion includes a narrow end coupled to the waveguide, and the second grating guides the incident light to the narrow end of the horn portion and into the waveguide.