Patent Application
20230358975
This invention is directed to silicon photonics and more particularly to silicon photonic building blocks exploiting microelectromechanical systems (MEMS) for control and/or tuning providing polarisation rotators, analog and digital phase shifters with MEMS actuation and passband filters.
Optical networking is a means of communication that uses signals encoded in light to transmit information in various types of telecommunications networks. These include limited range local-area networks (LAN) or wide-area networks (WAN), which cross metropolitan and regional areas as well as long-distance national, international and transoceanic networks. Optical networks typically employ optical amplifiers, lasers, modulators, optical switches and wavelength division multiplexing (WDM) to transmit large quantities of data, generally across fiber-optic cables. Because it is capable of achieving extremely high bandwidth, it is an enabling technology for the Internet and telecommunication networks that transmit the vast majority of all human and machine-to-machine information today. Optical networks are also employed in other applications such as storage area networks and data centers for optical interconnections at rack/server level but these techniques can extend to optical interconnections within a server, between circuits on a circuit board etc.
Optoelectronic integrated circuits exploiting hybrid or monolithic integration offer solutions for the different optical components required. To date hybrid integration approaches have been dominant with semiconductor emitters and detectors with bulk and micro-optic solutions for filters, switches, attenuators, etc. and integrated optical modulators either integrated with the semiconductor emitter or externally coupled. However, silicon photonics offers several benefits making it an attractive material system for monolithic integration. Firstly, the material is transparent to the wavelengths commonly used for optical communication systems (namely 1300-1600 nm), it supports standard Complementary Metal-Oxide Semiconductor (CMOS) processing techniques, and it is CMOS-compatible allowing processing of monolithic opto-electronic devices. Accordingly, silicon photonics offers a material system for optical componentry offering higher speed, increased functionality, lower electrical power and smaller footprint, all at a lower cost. Further, developments of silicon based light-emitting diodes offer a path to optical emitter integration other than hybrid integration of semiconductor devices.
Accordingly, various silicon photonic building blocks are required in order to provide a toolkit for a circuit designer to build optoelectronic integrated circuits (OEICs) in a similar manner as they work with libraries of standard electronic building blocks today. Further, other silicon photonic building blocks are required to address specific aspects of OEICs not present within electronics such as polarisation dependency of the optical waveguides, OEIC building blocks etc.
In addition to silicon photonics and CMOS electronics silicon offers the further ability to integrated microelectromechanical systems (MEMS) elements within the circuits to provide additional functionality. Within the following specification the inventors outline the establishment of several silicon photonic building blocks including polarisation rotators with MEMS based tuning, analog and digital phase shifters with MEMS actuation and passband filters with MEMS tuning.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
It is an object of the present invention to mitigate limitations in the prior art relating to integrated optical microelectromechanical systems and more particularly to establishing structures and methods for implementing phase shifting elements within integrated optical microelectromechanical systems and integrated optical microelectromechanical system based devices exploiting such phase shifting elements.
In accordance with an embodiment of the invention there is provided an optical device comprising:
In accordance with an embodiment of the invention there is provided an optical device comprising:
In accordance with an embodiment of the invention there is provided a method of providing a waveguide polarisation rotator comprising:
In accordance with an embodiment of the invention there is provided an optical device comprising:
In accordance with an embodiment of the invention there is provided a method of providing an optical waveguide phase shift element comprising:
In accordance with an embodiment of the invention there is provided an optical device comprising:
In accordance with an embodiment of the invention there is provided a method comprising: dynamically establishing a bandwidth, a passband shape and a center wavelength of an optical filter; wherein
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:
The present invention is directed to integrated optical microelectromechanical systems and more particularly to establishing structures and methods for implementing phase shifting elements within integrated optical microelectromechanical systems and integrated optical microelectromechanical system based devices exploiting such phase shifting elements.
The ensuing description provides representative embodiment(s) only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the ensuing description of the embodiment(s) will provide those skilled in the art with an enabling description for implementing an embodiment or embodiments of the invention. It being understood that various changes can be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. Accordingly, an embodiment is an example or implementation of the inventions and not the sole implementation. Various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention can also be implemented in a single embodiment or any combination of embodiments.
Reference in the specification to “one embodiment”, “an embodiment”, “some embodiments” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment, but not necessarily all embodiments, of the inventions. The phraseology and terminology employed herein is not to be construed as limiting but is for descriptive purpose only. It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not to be construed as there being only one of that element. It is to be understood that where the specification states that a component feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included.
Reference to terms such as “left”, “right”, “top”, “bottom”, “front” and “back” are intended for use in respect to the orientation of the particular feature, structure, or element within the figures depicting embodiments of the invention. It would be evident that such directional terminology with respect to the actual use of a device has no specific meaning as the device can be employed in a multiplicity of orientations by the user or users.
Reference to terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, integers or groups thereof and that the terms are not to be construed as specifying components, features, steps or integers. Likewise, the phrase “consisting essentially of”, and grammatical variants thereof, when used herein is not to be construed as excluding additional components, steps, features integers or groups thereof but rather that the additional features, integers, steps, components or groups thereof do not materially alter the basic and novel characteristics of the claimed composition, device or method. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional elements.
A “two-dimensional” waveguide, also referred to as a 2D waveguide or a planar waveguide as used herein may refer to, but is not limited to, an optical waveguide supporting propagation of optical signals within a predetermined wavelength range which does not guide the optical signals laterally relative to the propagation direction of the optical signals.
A “three-dimensional” waveguide, also referred to as a 3D waveguide or a channel waveguide as used herein may refer to, but is not limited to, an optical waveguide supporting propagation of optical signals within a predetermined wavelength range which guides the optical signals laterally relative to the propagation direction of the optical signals.
A “microelectromechanical system” or “microelectromechanical systems” (MEMS) as used herein may refer to, but is not limited to, a miniaturized mechanical and electro-mechanical element which is manufactured using techniques of microfabrication. For example, the MEMS may be implemented in silicon.
A “wavelength division demultiplexer” (WDM DMUX) as used herein may refer to, but is not limited to, an optical device for splitting multiple optical signals of different wavelengths apart which are received on a common optical waveguide, e.g. a waveguide forming part of a photonic integrated circuit or an optical fiber.
A “wavelength division multiplexer” (WDM MUX) as used herein may refer to, but is not limited to, an optical device for combining multiple optical signals of different wavelengths together onto a common optical waveguide, e.g. a waveguide forming part of a photonic integrated circuit or an optical fiber.
A “Mach-Zehnder interferometer” (MZI) as used herein may refer to, but is not limited to, an optical device exploiting phase imbalance between two arms disposed between an input 1×2 or 2×2 3 dB coupler and an output 2×1 or 2×2 3 dB coupler to provide for programmable modulation, attenuation, optical switching or wavelength filtering functions.
Section 1: Polarisation Rotator
As noted above silicon photonics offers a promising technology for reducing the cost structure of the various optical components employed within optical networks as it allows for leveraging the economies of scale of the microelectronics industry as well as the monolithic integration of electronics, e.g. CMOS. However, whilst the single mode optical fibers linking nodes within these networks offer low loss polarization independent transmission lines with low polarization dependent loss and polarization mode dispersion (e.g. ≤0.1ps/km for Corning™ SMF-28) the same is not true for the integrated optical waveguides upon substrates forming the tunable transmitters, tunable receivers, routers, reconfigurable optical add/drop multiplexers (ROADMs), wavelength division multiplexers (WDMs) and optical filters.
Accordingly, within the prior art significant research has been directed to techniques for mitigating polarisation dependent effects of the substrate based optical waveguides through fabrication processes, complex waveguide geometries etc. to provide polarisation independent optical waveguides. In parallel, other research has taken an alternate approach to exploit polarization diverse designs that handle the TE and TM polarizations wherein the increased circuit complexity of duplicate processing with high volume silicon manufacturing is expected to offer lower final circuit costs by exploiting standard fabrication and processing flows rather than bespoke fabrication processes, non-standard process flows, etc. with lower yields.
These issues are significant for existing telecommunication systems but become critical for coherent optical communication systems where data is encoded on both TE and TM polarisations.
An important photonic building block therefore is a polarisation rotator. This allows a received polarisation, e.g. TM, to be converted to another polarisation, e.g. TE, wherein it is processed by the photonic circuit comprising the optical waveguides. In this manner, received TE and TM signals may be parallel processed in the TE polarisation by a photonic circuit rather than requiring that the photonic circuit have parallel paths processing the TE and TM signals thereby reducing material constraints, fabrication constraints, etc.
Within the prior art polarization rotators generally use two methods to perform the rotation from one optical mode to the other optical mode. These are the adiabatic mode evolution and mode interference. Adiabatic mode evolution adiabatically converts the input fundamental TM mode to a higher order TE mode and then convert it to the fundamental TE mode using an appropriate mechanism. Mode interference allows complete transfer of power between the fundamental hybrid modes based upon the beating of these two modes which are tilted by 45 degrees with respect to the eigenaxis. Amongst, the structure employed in mode-interreference are longitudinally periodic modified structures, bend structures, and single section waveguides with asymmetric core structures.
However, adiabatic polarization rotators usually require a long device length to achieve high efficiency. Moreover, in order to exploit the hybrid-modes of the waveguides for polarization rotation, usually an asymmetry is required in the waveguide structure. Within the prior art this asymmetry has been achieved by modifying the thickness of the waveguide, breaking the symmetry of the waveguide cross-section by using a stair-like geometry, changing the material of the upper cladding etc. However, such geometrical constraints and fabrication complexities result in designs unsuitable for mass productions. Accordingly, the inventors have established a novel design wherein the fundamental hybrid modes interfere with each other such that at the appropriate length, the input TE mode is converted to the TM mode and vice-versa. In contrast to the prior art complexities of design and/or manufacturing the novel architecture is implemented with a single etch step. Further, as will become evident the inherent variations of the manufacturing process can be compensated for using electrostatic MEMS tuning.
In contrast to the prior art the novel polarisation rotator established by the inventors exploits mode-interference. As noted above, in contrast to prior art mode interference polarisation rotators, the novel polarisation rotator does not require partial etching of the waveguide core, a different top cladding material or exposing the waveguide core to air. Similarly, the novel polarisation rotator does not introduce hybridization in the waveguides by modifying its shape or thickness or both. In contrast, the inventive polarisation rotator exploits partial side cladding removal. With respect to
Accordingly, referring to
Accordingly, initial embodiments of the invention were implemented using the ONO (SiP2—Si3N4—SiO2) waveguide structure with a core thickness of 435 nm and a top-width, Wwg, of 435 nm. Accordingly, fabrication began with the deposition of 3.2 μm of SiO2 (SiO2) on a Si wafer followed by that of the Si3N4 (SiN). The SiN waveguide pattern was then defined using optical lithography followed by dry etching wherein the fabricated SiN core has a trapezoidal shape with a side-wall angle of approximately 80°. In the final step the wafer was covered with another 3.2 μm of SiO2 to form the top cladding, which was etched after patterning with electron beam lithography. The side-angle of the etched cladding based on this fabrication process was 86°. For this waveguide geometry, which is governed by the fabrication process, if the side-clad is etched from one side of the waveguide as shown in first cross-section 100A in
Now referring to
As the mode beating length is dependent upon the mode indices of the two modes then the performance of the polarisation rotator is sensitive to the width of the SiN waveguide and the side-cladding. Accordingly, for high volume manufacturing upon commercial silicon foundries it would be beneficial for a tuning mechanism to be implementable in conjunction with the polarisation rotator structures to allow for tuning the device to compensate for errors after fabrication. Within the prior art a common tuning mechanism for optical devices is thermo-optic tuning. Thermo-optic tuning has been used to produce phase-shift in devices that produce polarization rotation with a polarization extinction ratio range of 40 dB. However, thermal tuning requires high electrical power consumption and provides undesired thermal cross-talk to adjacent elements of the photonic circuit.
Accordingly, the inventors have established a novel tuning mechanism which exploits electrostatic MEMs actuators thereby avoiding the limitations of thermal tuning. Referring to
For cladding widths lower than 157 nm, the first mode is more like a quasi-TM mode and then second mode is more like a quasi-TE mode. However, upon perturbing it, it is evident that tuning of the first two modes is possible to become hybrid with the polarization fractions close to 50%. The values of the gap between the oxide block and polarisation rotator in nanometers are shown in the boxes in
Referring to
For example, whilst the designs described and depicted with respect to
Section 2: Analog and Digital Mems Based Phase Shifters
Within photonic circuit building blocks such as Mach-Zehnder interferometers (MZIs) a defined phase balance or imbalance is required in order to allow for either symmetric drive or asymmetric drive. As noted above in respect of Section 1 a common approach within the prior art to inducing a static phase shift within an optical waveguide is via the thermo-optic effect. However, as noted this requires high power consumption and one or more of complex control algorithms and complex manufacturing to accommodate/eliminate thermal crosstalk between multiple photonic circuit elements within the same photonic circuit. Accordingly, the inventors have established a series of analog and digital microelectromechanical system (MEMS) based methods for controlled phase shift within optical waveguides and therein within optical circuit elements such as in integrated optical components such as MZIs for example. Beneficially, such novel solutions reduce electrical power consumption, eliminate thermal crosstalk issues, and provide for solutions that can be latched thereby eliminating the requirement for continuous electrical signals applied to the tuning elements.
2A: Overview
Within this Section and with respect to
In common with the design methodology described and depicted in
Accordingly, the phase shift produced in an optical waveguide, which for the following embodiments is described and discussed with respect to a MZI but may be a phase shift or perturbation within other photonic waveguide elements or circuits can be controlled through different configurations of MEMS actuators. Within the following embodiments of the invention the MEMS actuator 500C is described and depicted as being an electrostatic MEMS actuator. However, it would be evident that other MEMS actuators may be employed without departing from the scope of the invention. Exemplary embodiments of the invention described and depicted below in respect of
Electrostatic comb drive MEMS actuator (hereinafter comb drive) fabrication can be complex, and the voltage range obtained for controlled tuning of the perturbation element can be, typically, within a range of 10 V to 20 V with the displacement range typically on the order of 50 nm to 250 nm. Accordingly, embodiments of the invention have also been developed using alternative parallel plate actuation-based designs which rely upon closing of the air gap between the optical waveguide to be perturbed (i.e. the arm of the MZI upon a fixed portion of the circuit) and the perturbation element (upon a movable portion of the MEMS) completely or closing the air gap to a predetermined gap, e.g. 250 nm, using built-in mechanical stoppers. Since these parallel plate actuators work upon a pull-in phenomenon where discrete displacement occurs beyond a pull-in voltage then the inventors refer to these designs as “digital actuators”. Accordingly, at 0V the actuator is at an initial default position and above the pull-in voltage the actuator is fixed in displacement.
Further, as described and depicted below a long waveguide section with a single perturbation element as depicted in first and second schematics 600A and 600B respectively in
Alternatively, the digital MEMS design allows for multiple actuators of equal length or multiple actuators of different lengths such that for example one actuator may provide π/2 phase-shift, another π/3, another π/4 etc. However, it would be evident that the lengths of the multiple actuators could be design with lengths in a binary configuration where the length of a perturbation element establishes π/N where N=2n for n=0, 1, 2, 3 etc. Such a binary configuration can increase the resolution of phase shift applied to the device. For example, if a digital MEMS tunable configuration with zero gap actuators shown in
An important aspect of the fabrication of devices according to embodiments of the invention is the air gap in the perturbation region as shown in cross-sectional 500A view of
2B: Analog MEMS Tunable Perturbation Elements
Initial MEMS tunable MZI designs established by the inventors according to embodiments of the invention exploited comb drive based MEMS actuators which offered continuous displacement versus voltage characteristics, i.e. what the inventors refer to as analog actuators. An initial analog MEMS based design is depicted in
These designs were simulated using static structural analysis for a device thickness of 10 μm as employed within the commercial MEMS technology employed by the inventors. These results are depicted in
As expected, the lower stiffness spring system provides lower actuation voltage for a 3 μm displacement in comparison to the higher stiffness spring. However, the tuning voltage range provided by a softer spring is ˜8 V in comparison to ˜15 V for a device with stiffer spring for tuning from 3.00 μm to 3.25 μm. However, as electrostatic actuation method consumes negligible power since there is no current through the MEMS during actuation the higher voltage design is not disadvantaged per se relative to the lower voltage design.
However, the inventors deemed it beneficial to further increase the tuning voltage range and accordingly, non-linear spring designs with a single silicon beam anchored only in the center were analysed as depicted in
2C: Digital MEMS Tunable Perturbation Elements
As the analog actuators based upon comb drive actuation from the preceding analysis in Section 2.B were limited in their tuning voltage range for producing the requisite range of motion of the perturbation element and accordingly, for example, induced phase shift in an MZI with low resolution the inventors established an alternative novel design methodology of tuning using parallel plate actuators. These actuators rely upon discrete ON and OFF states through electrostatic pull-in phenomena, and accordingly are referred to as digital actuators. As noted above multiple parallel plate actuators adjacent to a common optical waveguide can provide a predictable tuning in the optical waveguide, e.g. MZI, upon actuation of each actuator. Each actuator consists of a MEMS platform designed to accommodate perturbation waveguides of equal lengths as depicted in first and second schematics 1500A and 1500B in
Accordingly, the first and second schematics 1500A and 1500B, hereinafter referred to as Design 1, provide the following advantages:
However, Design 1 also suffered perceived disadvantages of:
A design variant of the Design 1 concept was established as depicted in
However, Design 2 also suffered perceived disadvantages of:
Accordingly, the inventors established a further design methodology, referred to as Design 3, where mechanical stoppers were incorporated to minimize stiction and eliminate any contact between the MEMS parts that are different potentials. Such a design being depicted in
However, Design 3 suffers a perceived disadvantage of:
This led to further design variants being considered resulting in the design concept depicted in
The digital MEMS design concepts presented and described with respect to
Accordingly, both of these design categories are presented in
2D: Binary MEMS Tunable Perturbation Elements
The digital MEMS actuator designs discussed in Section 2C provide actuators supporting high resolution tuning through discrete actuation of each actuator. However, to further enhance the resolution of the tuning (i.e. phase shift) obtained upon use of these digital actuators, the inventors as noted above propose exploiting different perturbation element lengths on different platforms. Further, such lengths could be scaled by a multiple of two between elements thereby enabling a binary combination of the multiple actuators. Such a binary combination of discrete tuning elements can increase the degree of control over the induced perturbations, e.g. phase shift, multifold relative to a number of equal length perturbation elements. Further, as discussed in Section 2C MEMS actuators designed for embodiments of the invention were designed for fabrication upon the commercial MEMS fabrication process selected by the inventors and were also categorized on the basis of having either a zero tuning gap or a 250 nm tuning gap. Referring to
Similar adjustments were made to the 250 nm gap digital MEMS actuator design described above in Section 2C to yield the 250 nm gap binary MEMS actuator for the commercial MEMS fabrication process selected by the inventors. The binary configuration in this instance as depicted in
It would evident that the design depicted in
2E: Optical Analysis
The inventors have established several design approaches for the tuning of an optical waveguide using perturbation elements exploiting digital actuators and/or analog actuators individually or in combination. Within the following overview several design approaches are presented with respect to the tuning of an oxide-nitride-oxide (ONO) waveguide structure with a silicon oxide lower cladding, a silicon nitride waveguide core and an upper silicon oxide cladding, i.e. a SiO2—Si3N4—SiO2 waveguide structure. However, it would be evident to one of skill in the art that other design methodologies may be employed without departing from the scope of the invention either for an ONO waveguide structure or for other waveguide structures. For example, materials with higher refractive indices than the optical waveguides may be employed to increase the perturbation strength per unit length or allow larger gaps to be employed, materials with lower refractive indices than the optical waveguides may be employed to decrease the perturbation strength, materials with complex refractive indices may be employed, etc.
Accordingly, considering an ONO waveguide structure without additional materials being added to the fabrication process then in a first option the optical waveguide is formed within the ONO stack and the perturbation element may be similarly another element formed within the ONO stack upon the moving Si MEMS platform of the MEMS actuator. Alternatively, the perturbation element may be simply an oxide layer on top of the Si MEMS platform such as depicted in
Accordingly, referring to
The structures depicted in
2F: Microfabrication Sequence for Near Zero Gap Implementation of ONO Waveguide—ONO Perturbation Element
As discussed above the integration of MEMS actuators with silicon nitride based optical waveguides for perturbation through gap closing of a perturbation element presents fabrication challenges. The commercial process flow can provide an ONO stack or oxide with an 86° etch profile. The silicon nitride etch angle remains at 80° and the etch angle for silicon is inverted 91°. As noted these fabrication limitations can lead to a minimum gap of 475 nm between the ONO waveguide core and the perturbation element. In order to compensate for these fabrication limitations, the inventors established a MEMS tunable perturbation geometry with the ONO facet for the optical waveguide with another ONO facet for the perturbation element such as depicted in
The ONO etch to get this initial tuning gap can be achieved through photolithography eliminating alignment issues between the silicon oxide layer and the silicon nitride layer. Accordingly, the manufacturing sequence established by the inventors which is compatible with the commercial MEMS fabrication processes and tolerances exploits a highly selective vapor HF etch to selectively etch excess silicon oxide around the silicon nitride core in the tuning gap region. This helps reduce the tuning gap further enabling larger phase shifts per unit length. In order to implement this a chromium hard mask is used for this step. A cross-sectional view 2600A of the tuning gap region for a design according to embodiments of the invention with slightly overhanging silicon nitride during this step is shown in
A detailed process flow proposed for microfabrication of the MEMS tunable ONO waveguides with silicon nitride overhangs in the perturbation region is presented in
2G: Summary
Accordingly, within Section 2 novel MEMS based tuning elements for inducing perturbations within optical waveguides have been described and depicted with respect to
Accordingly, embodiments of the invention provide fast and low power MEMS based solutions for tuning optical components with controlled phase shift or other perturbations.
Section 3: Serially Coupled Ring Resonator Assisted Mach-Zehnder Interferometer Tunable Bandpass Filters
The ever-increasing demand for bandwidth in data communication and telecommunication systems has resulted in the development of dense wavelength division multiplexing (DWDM) at 200 GHz, 100 GHz and 50 GHz channel spacings to support networks with 40, 80 and 160 channels of 10 Gb/s (OC-192) data on the C-band (1529 nm-1568 nm) and L-bands (1569 nm-1610 nm). However, such networks require planning and structured deployment. Accordingly, there is increasing interest in gridless networks, also known as elastic optical networks (EONs), where the channel spacing and bandwidth can be adjusted dynamically. Accordingly, EONs would allow operators to dynamically maximize the available bandwidth and limit spectrum wastage. However, in front of each optical detector there must be an optical filter to isolate the channel that optical detector receives. With DWDM networks such filters were typically static in wavelength and fixed in optical bandwidth (e.g. designed for a specific 200 GHz, 100 GHz or 50 GHz channel) requiring planned deployment, inventory management etc. In some instances, tunable optical filters are deployed allowing selection of a channel from a number of channels but again the optical bandwidth was fixed, and the tuning range/tuning speed limited in many technologies employed.
Accordingly, to be useful in EONs, the optical filters should be tunable both in optical bandwidth and center frequency. For example, dynamically allocating 40 Gb/s to specific nodes rather than 10 Gb/s requires a different optical bandwidth even if the same centre wavelength is used. Additionally, these filters should have low insertion loss, a flat-top response, a box-like passband, high extinction ratio and high side-band rejection.
Within the prior art multiple design to implement optical filters with an optimized passband response have been proposed and the evolution of optical communications to EONs has seen increasing interest in reconfigurable bandpass filters (BPF) with tunable bandwidth and wavelength. Amongst, these designs ring resonators are the most commonly employed filtering components in these filters as they are easy to fabricate and have a small footprint. One approach to implementing a BPF is the Ring Assisted Mach-Zehnder interferometer (RA-MZI) wherein one or more ring resonators (RRs) are embedded in one or both of arms of a Mach-Zehnder interferometer (MZI) as this configuration offers a more boxlike passband response when compared to simply cascading RRs. However, as the number of RR elements increases in these RA-MZI filters, the tuning mechanism to achieve the optimum filter shape for the filter becomes more and more complex.
A simpler tuning requirement is offered by a prior art filter architecture using an unbalanced MZI and two cascaded RRs. Accordingly, the inventors have established based upon this architecture novel bandpass filters with desired performance parameters exploiting different coupling configurations between the RRs and MZI Amongst these, a second order filter with two RRs in series and in parallel to the MZI was analyze yielding to the inventor's knowledge the first implementation of a BPF using a serially coupled Ring Resonators and MZI (SR-MZI) configuration in which two RRs are connected in series to the MZI. Moreover, the inventors observed that the response of this SR-MZI filter offers several advantages compared to previous configurations; specially in terms of the shape of the bandpass response and the degrees of freedom to optimize the various performance parameters. Further, the inventors have established a novel MEMS based tuning mechanism for such an SR-MZI allowing the tuning to be performed with low power and without thermal crosstalk considerations with other elements of a photonic circuit within which the tunable BPF is integrated.
In common with the polarisation rotator and phase shifter devices described and depicted in respect of Sections 1 and 2 the inventors have analysed and fabricated novel tunable BPFs based upon a commercial CMOS compatible MEMS microfabrication process and ONO (SiO2—Si3N4—SiO2) waveguide structures. Accordingly, using MEMS elements the inventors have established tunability of the filter bandwidth and filter shape by varying the coupling between the RRs themselves and the RR(s) with the MZI.
3A: Device Design
3A1: Analytical Modelling of Various RA-MZI Configurations
Referring to first schematic 2800A in
The field transmission and coupling coefficients between the MZI and RRs are represented by t and K, respectively. The loss in the RRs is represented by α and the phase change is θ=−iβL, where, L is the circumference of the RRs and β is propagation constant of the ring waveguide. The complex electric field, Et, at the output of the cascaded rings second schematic 2800B in
E
t
=E
i×((t−a exp(iθ))2/(1−αt*exp(iθ))2) (1)
E
OUT=0.5EIN×[exp(iθMZI)+(Et/Ei)] (2)
Referring to
E
t
=E
i×(A/(1−t13−t23+t12)2) (3)
A=(K22√{square root over ((1−K12))}exp(i(θ1+θ2)−K22√{square root over ((1−K32))}exp(iθ2)))×(√{square root over ((1−K12))}−exp(iθ2)√{square root over ((1−K12)(1−K22)(1−K32))}−exp(iθ1)√{square root over ((1−K32))}+exp(i(θ1+θ2))√{square root over ((1−K22))})−K12K22K32 exp(i(θ1+θ2))+(K12(1−K22)exp(i(θ1+θ2)−K12√{square root over ((1−K22)(1−K32))}exp(iθ1))(1−t13−t23+t12))+√{square root over ((1−K12)(1−K22))}(1−t13−t23+t12)2 (4)
t
13=√{square root over ((1−K12)(1−K32))}exp(iθ1) (5)
t
23=√{square root over ((1−K22)(1−K32))}exp(iθ2) (6)
t
12=√{square root over ((1−K12)(1−K22))}exp(i(θ1+θ2)) (7)
Equation (3) can be substituted in Equation (2) to get the expression for the electric field, Et, at the output of the RA-MZI filter in first schematic 2900A in
Referring to
E
a
=−K*E
i
+t*α exp(iθ/2)Eb (8)
E
b
=t
1*α exp(iθ/2)Ea−K1*α1 exp(iθ1/2)E1b (9)
E
1a
=K
1α exp(iθ/2)Ea−t1α1 exp(iθ1/2)E1b (10)
E
1b=α1 exp(iθ1/2)E1a (11)
E
t
=tE
i
+Kα exp(iθ/2)Eb (12)
Accordingly, the electric field, Et, at the output of the serially coupled RRs in second schematic 3000B in
The expression for Et, in Equation (13) can be substituted into Equation (2) to obtain the output of the filter depicted in first schematic 3000A in
3A.2 Filter Responses
In the various second order RA-MZI configurations discussed above, and depicted in
To compare the performance of each of the architectures of Designs 1 through 3, the coupling coefficients were optimized to achieve a 3-dB bandwidth of 0.14 nm. For example, the coupling coefficient K in Design 1 needs to be 0.82 to provide the desired 3-dB bandwidth.
However, as evident below Design 3 provides an ideal bandpass filter response with flexibility to tune the shape of the response.
The SR-MZI (Design 3) according to embodiments of the invention provides the required bandpass filter response with flexibility to tune both its shape and side-band rejection. The inventors further investigated its performance by studying the impact of K and K2 by varying only one coupling coefficient at a time.
3B. Experimental Results
The inventors implemented filter designs according to embodiments of the invention using ONO waveguides such as described above in respect of Section 2 as fabricated upon a commercial MEMS compatible microfabrication process. This yields trapezoidal SiN cored waveguides with a side-wall angle of approximately 80°. The thickness of the waveguide was 440 nm and the top width, WTOP, was varied from 440 nm to 450 nm and 460 nm to understand the effect of the waveguide width on the filter performance. The fabrication process comprising in an abbreviated sequence:
It should be noted that the initial devices fabricated did not have a metallization layer on top of the cladding and therefore, did not have heaters to tune the response of these filters by tuning the RRs and MZI using the known techniques of the prior art so that compensations for fabrication variations in the filter can be applied.
As there were no heaters on the fabricated devices the inventors fabricated devices with different spacings between the RRs, and RR1 and MZI to validate their simulation models. The coupling coefficients were evaluated using Finite Difference Time Domain (FDTD). Within these the gap between RR1 and the MZI was fixed at 700 nm, 900 nm, and 1100 nm respectively and the gap between the RRs established at 600 nm, 800 nm, and 1000 nm, respectively. Additionally, the wavelength of the filter can be tuned by simultaneously tuning the phase in the two rings and the MZI.
Experimental results for the measured filter response of five devices are presented in
The values of K and K1 were different for each device as shown in
In order to evaluate the fabricated devices optical signals were coupled in and out of the photonic circuits using grating couplers. The MZI in the fabricated SR-MZIs employs 3-dB multimode interference (MMI) couplers at the input and output. The extinction ratio of the filters is limited by the splitting ratio of these MMI couplers which can be further optimized for a better performance. The extinction ratio in the theoretical response was also decreased to match the measured response. Accordingly, it should also be noted that the grating couplers provided an optimum response around a wavelength of 1600 nm for the TE mode whilst
Furthermore, the inventors observe that the measured FSR in the experimental devices is slightly higher than the theoretical one which implies that the refractive indices used in the simulation are higher than the actual values. Moreover, some of these devices exhibit a slightly higher bandwidth than expected. It is expected that, due to fabrication variations, the coupling coefficients might differ from the theoretical values presented. However, the inventors found that the shift in coupling coefficients for a variation of ±20 nm in waveguide thickness or width was not significant. The measured bandwidths for the devices whose spectra are presented in
3C: Tuning of SR-MZI Filters
As evident from the analysis in Section 3A the bandwidth, shape, and wavelength of SR-MZI filters according to embodiments of the invention can be tuned to implement full reconfigurability. The bandwidth and shape of the filter can be tuned simply by changing the strength of coupling between the two rings, and between RR1 and MZI, respectively. On the other hand, the wavelength of the filter can be tuned by simultaneously adjusting the phases of the two rings and the MZI.
Referring to
The coupling between the RRs or RR and MZI reduces with increased power dissipated from the heaters. These heaters can be used to thermally tune the bandwidth, shape and wavelength in the filter as described above. However, as noted above with respect to Sections 1 and 2 thermal actuated elements result in complex control algorithms to compensate for thermal crosstalk within the same photonic circuit element, e.g. the five heaters within the SR-MZI, as well as thermal crosstalk from other photonic circuit elements.
Accordingly, the inventors also have established a design methodology according to embodiments of the invention as depicted in
Accordingly, using first and second MEMS actuators 39100 and 39200 respectively the first and second movable platforms 3910 and 3930 can be moved relative to each other and the fixed platform 3950 allowing the coupling strengths between the MZI 3990 and RR1 3980 and between RR1 2980 and RR2 3970 to be adjusted. Optionally, the first MEMS actuator 39100 and RR1 3970 may formed upon a movable platform nested within second movable platform 3930 or vice-versa.
Also forming part of the first movable platform 3910 and RR2 3970 is a first phase shift element 3920 and forming part of the second movable platform 3930 and RR1 3980 is a second phase shift element 3940. The MZI 3990 further includes a third phase shift element 3960. Each of the first to third phase shift elements 3920, 3940 and 3960 may exploit thermal tuning as outlined above in respect of
Whilst the embodiments of the invention described and depicted above in respect of
Whilst the embodiments of the invention described and depicted above in respect of
Further, whilst embodiments of the invention have been described with respect to electrostatic actuation it would be evident that other actuation means/mechanisms may be employed within other embodiments of the invention including, but not limited to, piezoelectric, magnetic, and thermal.
Embodiments of the invention may further incorporate other MEMS elements allowing additional functionality or features to be implemented. For example, MEMS elements may grip or lock the MEMS actuator such that long term actuation of the actuator is not required. For example, a gripping structure may be actuated to allow the actuator to move and then once set to the desired point the gripping structure de-actuated to re-grip. Alternatively, a tooth or teeth on the MEMS actuator may be selectively engaged with other teeth upon a locking actuator so that the locking actuator is engaged to separate its teeth from those on the actuator, the actuator adjusted, and then the locking actuator de-actuated to relock its teeth with those on the actuator.
Within the embodiments of the invention described above the optical waveguides have been described as exploiting a silicon core upon a silicon dioxide SiO2 cladding, i.e. a Si—SiO2 waveguide structure. However, it would be evident that embodiments of the invention may also be employed in conjunction with other waveguide materials systems. These may include, but not be limited to:
Additionally, waveguide structures without upper claddings may be employed. However, it would be evident to one skilled in the art that the embodiments of the invention may be employed in a variety of waveguide coupling structures coupling onto and/or from waveguides employing material systems that include, but not limited to, SiO2—Si3N4—SiO2; SiO2—Ge:SiO2—SiO2; Si—SiO2; ion exchanged glass, ion implanted glass, polymeric waveguides, indium gallium arsenide phosphide (InGaAsP), InP, GaAs, III-V materials, II-VI materials, Si, SiGe, and single mode optical waveguides and multimode optical waveguides.
Whilst the embodiments of the invention have been described and depicted with respect to silicon material system supporting monolithic integration of the optical waveguides and MEMS actuators it would be evident that other embodiments of the invention may employ discrete actuators or hybrid integration methodologies.
Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.
The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.
Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.
This application claims the benefit of priority as a 371 National Phase Entry of PCT/CA2021/050940 filed Jul. 9, 2021; which itself claims the benefit of priority from U.S. Provisional patent application 63/050,351 filed Jul. 10, 2020.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2021/050940 | 7/9/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63050351 | Jul 2020 | US |
