Patent Application
20160209593
The present invention relates to optical telecommunications and, in particular, to polarization-spill wavelength filters based on ring/racetrack resonators.
Optical waveguides, e.g. silicon photonic waveguides, have great potential as platforms for photonic integrated circuits (PIC) and are widely used in optical networks and for environmental sensing. However, geometric shapes of waveguide cores and anisotropy of materials can cause polarization mode dispersion (PMD) in optical waveguides, thereby increasing polarization dependence of optical waveguide components and deteriorating the performance of optical functional devices such as wavelength filters.
There is therefore a desire to build photonic integrated circuits with polarization diversity. Traditional designs of a polarization diversity circuit usually includes a polarization splitter to separate input light into Transverse Electric (TE) and Transverse Magnetic (TM) components for guiding these orthogonal, components through two separate waveguides. One such approach is to rotate one component 90 degrees by a first polarization rotator in one optical path, so that components in both waveguides are propagating in the same polarization mode and functional devices of one polarization can be used. After the functional processing, a second polarization rotator rotates one component so that both components are merged by a polarization combiner into an output fiber. Another approach uses two optical functional devices in respective polarization modes.
The following presents a simplified summary of some aspects or embodiments of the invention in order to provide a basic understanding of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some embodiments of the invention in a simplified form as a prelude to the more detailed description that is presented later.
In general, the present disclosure relates to a polarization-splitting wavelength filter that includes two ring/racetrack resonators having couplers for coupling the TE and TM polarizations.
One inventive aspect of the disclosure is a polarization-splitting wavelength filter comprising an input waveguide for receiving an optical input with both a Transverse Electric (TE) polarization and a Transverse Magnetic (TM) polarization, a first ring/racetrack resonator disposed along the input waveguide wherein the first ring/racetrack resonator includes a first coupler for coupling the TE polarization of a first wavelength and a second ring/racetrack resonator disposed along the input waveguide, wherein, the second ring/racetrack resonator includes a second coupler for coupling the TM polarization of a second wavelength.
Another inventive aspect of the disclosure is a method of performing polarization-splitting and wavelength filtering. The method entails receiving an optical Input via an input waveguide, the optical input having both a TE polarization and a TM polarization, coupling the TE polarization of a first wavelength into a first ring/racetrack resonator, and coupling the TM polarization of a second wavelength into a second ring/racetrack resonator.
These and other features of the disclosure will become more apparent from the description in which reference is made to the following appended drawings.
The following detailed description contains, for the purposes of explanation, numerous specific embodiments, implementations, examples and details in order in provide a thorough understanding of the invention. It is apparent, however, that the embodiments may be practiced without these specific details or with an equivalent arrangement. In other instances, some well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the embodiments of the invention. The description should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their fell scope of equivalents.
Various embodiments of the present invention are presented in this disclosure. These embodiments relate to a polarization diversity wavelength filter that does not require any separate polarization splitter and/or rotator. In the various embodiments disclosed in this application, the polarization-split wavelength filler employs two or more ring/racetrack resonators.
In the following description, the term ring/racetrack resonator is used in a broader sense to refer to any looped resonator and includes a looped optical waveguide and a coupling mechanism to access the loop. A ring resonator may in a narrower sense refer to a resonator of a circular, elliptical or elongated shape, whereas a racetrack resonator refers to a resonator where the shape is elongated with at least one straight section (typically along the coupling section). While specific embodiments may be described or illustrated with certain specific shapes, it should be understood that loops of other shapes (or other resonator shapes) may be used to achieve the same or substantially similar result.
In a ring/racetrack resonator, light is coupled into the ring/racetrack by a coupling mechanism such as, for example, a directional coupler. In the disclosed embodiments, a polarization-split wavelength filter includes two or more ring/racetrack resonators wherein the coupling mechanism of each ring/racetrack resonator performs functions of both coupling and polarization separation. Accordingly, the polarization-split wavelength filter can be implemented as a single optical device or optical apparatus without the need for any additional polarization splitter and/or rotator.
In the embodiment illustrated by way of example in
The straight waveguide 206 receives an optical input with, both TE and TM polarizations and functions as an optical bus line to the resonators 202, 204.
The flat waveguide portion of the racetrack 202 (or the portion of the racetrack 202 that is adjacent the waveguide 206) and the bus waveguide 206 constitute a first coupler 210, allowing light of a specific wavelength to couple into the racetrack 202, and to further drop out from a second straight waveguide 208 which is located at the other side of the racetrack 202. lire second straight waveguide 208 and the other side of the racetrack 202 constitute a second coupler 212 of the racetrack 202. Each coupler 210, 212 in the first racetrack 202 is designed for TE polarization, which allows only light m the TE polarization to couple into the racetrack 202, and to transmit to drop port 220.
The functionality of the TM polarization splitting and wavelength filtering is implemented by a second racetrack 204, which is disposed at a different location on the same bus waveguide 206. The flat waveguide portion of the racetrack 204 (or the portion of the racetrack 204 that is adjacent the waveguide 206) and the bus waveguide 206 constitute a first coupler 214 of the racetrack 204, allowing the light in a specific wavelength to couple into the racetrack 204, and to further drop out from a third straight waveguide 216 which is located at the other side of the racetrack 204. The third straight waveguide 216 and the other side of the racetrack 204 constitutes a second coupler 218 of the racetrack 204. Each coupler 214, 218 in the second racetrack 204 is designed for light propagation in the TM polarization, which permits only light in the TM polarization to couple into the racetrack 204, and to drop out from the third straight waveguide 216 via the TM mode drop port 222. The wavelengths coupled by the first ring/racetrack resonator 202 and the second ring/racetrack resonator 204 may be the same or different, depending on different applications.
In the embodiment illustrated by way of example in
In the embodiment illustrated by way of example in
In the embodiments depicted in
Both the ring/racetrack structure and the coupling mechanism are dependent on PMD, which would influence significantly the coupling wavelength and coupling ratio. PMD may be caused by the geometric shape of the waveguide geometric shape, stress imposed on the waveguide and/or the anisotropy of materials used to make the waveguide. For silicon nano-wire devices, the PMD is particularly due to its non-square cross-sectional waveguide structure. In at least some embodiments, each ring/racetrack resonator is designed to filter a specific wavelength in one polarization.
In at least some embodiments, each ring/racetrack resonator is designed to filter a specific wavelength λres designed so that when the light waves in the loop of the ring/racetrack build up a round trip phase shift that equals an integer times 2π, the waves interfere constructively and the cavity is in resonance. λres can be generally represented by the following equation:
where L is the length of the ring or racetrack and neff is the effective index. The spacing between adjacent resonances, known as the free spectral range (FSR) is represented as follows:
In a ring/racetrack resonator, neff is usually Influenced by the retractive index of the waveguide cladding and is dependent on the material of the waveguide, shape of the ring/racetrack, and also the polarization mode. Accordingly, to filter the same wavelength, the TM and TE ring/racetracks are usually built with different lengths.
In at least some embodiments, each coupler in the ring/racetrack resonators is also designed to perform the function of a polarization splitter, so that one polarization travels through while the other polarization couples into the ring/racetrack.
The separation of the TE and TM modes may make use of the difference of propagation constants between the two modes caused by birefringence. In the case of a directional coupler, the coupling length Lx, or the cross-over length where power is localized in the coupling waveguide, is dependent on the polarization and can be used to separate the two modes.
In the embodiment depicted by way of example in
In the embodiment depicted by way of example in
In the embodiments depicted In both
As illustrated by
While various embodiments refer to directional couplers for coupling light from the input waveguide to the ring/racetrack resonators, it should be understood that other couplers may be used, such as multi-mode interferometers (MMI), MMI-based Y-branch couplers, or other polarization-sensitive couplers or polarization-insensitive couplers. For example, in the case of an MMI coupler, the MMI coupler may be designed to split TE and TM propagation modes based on the self-image lengths of TE and TM which are different in the MMI slab area.
In other implementations, a polarization-split wavelength filter or a polarization diversity wavelength filter can be built using more than two ring/racetrack resonators, where each ring/racetrack resonator is used to couple and filter one particular wavelength of one polarization. Depending on different applications, the rings/racetracks can be placed at various positions along the input/output waveguide and may or may not come in pairs. It should be understood that depending on specific applications, an optical device with any number of ring/racetrack resonators and that work with different wavelengths can be implemented without any separate polarization splitter(s).
Various optical materials cm be used to build the polarization diversity wavelength filter or the polarization-split wavelength filter such as, for example, silicon, silica-on-silicon, InP, SiON, Si3N4, polymer or other suitable optical materials.
The first ring/racetrack resonator may be designed by determining a racetrack/ring length for the first ring/racetrack resonator based on the first wavelength. Likewise, the second ring/racetrack resonator may be designed by determining a racetrack/ring length for the second ring/racetrack resonator based on the second wavelength.
It is to be understood that the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a device” includes reference to one or more of such devices, i.e. that there is at least one device. The terms “comprising”, “having”, “including” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of examples or exemplary language (e.g. “such as”) is intended merely to better illustrate or describe embodiments of the invention and is not intended to limit the scope of the invention unless otherwise claimed.
While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented.
In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.
