This invention is directed to photonic waveguide devices and more particularly to parasitic coupling reduction and footprint reduction within such photonic waveguide devices through the use of waveguide supercells.
Arrayed waveguide gratings (AWGs) are important components in coarse wavelength divisional multiplexing (CWDM) and dense wavelength division multiplexing (DWDM) systems to increase the transmission capacity in optical communications by multiplexing or demultiplexing multiple optical wavelengths onto the same optical fiber. AWGs can also be used as spectroscopic sensors, optical add-drop multiplexers, optical routers, wavelength filters, and colorless filters. Accordingly, to data AWGs have been implemented in different material systems, including but not limited, silica-on-silicon (Si), silica-on-insulator (SOI), indium phosphide (InP), gallium arsenide (GaAs), polymer, and glass.
AWGs implemented in silica-on-Si are widely used in commercial telecommunication systems due to their high performance. However, the minimum practical bend radius is large with this type of waveguide which results in devices with significant footprints, where typical die footprints can be 50-80 mm long by 20-30 mm wide and even compact silica-on-Si AWGs reported in the prior art are 20-30 mm long by 5-10 mm wide. In contrast, SOI with a higher refractive index contrast allows the use of smaller radius waveguide bends allowing significant reduction in the AWG footprint. However, such SOI AWGs reported within the prior art are sensitive to fabrication variations which result in phase errors between the arrayed waveguides leading to degraded channel crosstalk performance.
In contrast, silicon nitride (SiN) waveguides offer a promising platform for the realization of high performance AWGs. As SiN waveguides have a lower refractive index contrast than SOI waveguides they provide improved tolerance to fabrication errors when compared to SOI and crosstalk is generally lower in SiN AWGs than in SOI AWGs. However, with conventional AWG designs, the waveguides forming the array must be separated by a distance large enough to suppress parasitic coupling between the adjacent waveguides and thus minimize waveguide crosstalk. The inventors, for example, previously established that a large 10 μm gap was necessary to reduce the waveguide crosstalk to an acceptable level. This waveguide separation thereby limits optimization of the AWG footprint and contributes to insertion loss due to the low coupling efficiency between the input/output coupler regions and the central region comprising the arrayed waveguides.
Accordingly, it would be beneficial to provide photonic circuit designers with a methodology allowing the waveguide separation to be reduced whilst limiting the cross-coupling between adjacent waveguides with the array waveguide portion of the AWG, thereby reducing waveguide crosstalk, and reducing channel crosstalk. It would be further beneficial for these techniques for limiting cross-coupling between adjacent waveguides to be compatible with other photonic waveguide circuits topologies and components such that cross-coupling can be limited in any photonic circuit where two or more waveguides must run parallel to one another for significant distances.
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 photonic waveguide devices and more particularly to parasitic coupling reduction and footprint reduction within such photonic waveguide devices through the use of waveguide supercells.
In accordance with an embodiment of the invention there is provided a waveguide device comprising:
In accordance with an embodiment of the invention there is provided a method of improving channel crosstalk performance of an array waveguide grating (AWG) device comprising:
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 photonic waveguide devices and more particularly to parasitic coupling reduction and footprint reduction within such photonic waveguide devices through the use of waveguide supercells.
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 would be understood by one of skill in the art that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the 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 “wavelength division multiplexer” (WDM MUX or MUX) as used herein may refer to, but is not limited to, an optical device for combining (multiplexing) 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. For example, such a MUX may exploit an array waveguide grating (AWG) wherein N input ports each carrying optical signals at a different predetermined wavelength are combined to a single output port.
A “wavelength division demultiplexer” (WDM DMUX or DMUX) as used herein may refer to, but is not limited to, an optical device for splitting (demultiplexing) 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. For example, such a DMUX may exploit an array waveguide grating (AWG) wherein a single input port carrying optical signals is split into N outputs each carrying optical signals at a different predetermined wavelength.
An “optical router” as used herein may refer to, but is not limited to, an optical device comprising a plurality of input ports and a plurality of output ports wherein optical signals at an input are routed to an output in dependence upon their wavelength. For example, such an optical router may exploit an array waveguide grating (AWG) comprising N input ports and M output ports wherein each input port of the N input ports carries optical signals at predetermined wavelengths which are coupled to outputs ports of the M output ports in dependence upon their wavelength and which input port they are coupled to.
“Waveguide crosstalk” as used herein refers to, but is not limited to, optical cross-coupling between adjacent and non-adjacent optical waveguides.
“Channel crosstalk” as used herein refers to the total accumulated optical crosstalk within an optical channel of a wavelength division demultiplexer (DMUX), e.g. an array waveguide grating (AWG) DMUX, arising from all sources including, but not limited to, crosstalk and phase noise within the AWG.
Within the embodiments of the invention described below the optical waveguides exploit a silicon nitride core with silicon oxide upper and lower cladding, a SiO2—Si3N4—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 multi-core optical fiber.
Further, whilst the embodiments of the invention are described and depicted with respect to a waveguide employing a core embedded within cladding, a so-called buried waveguide, it would be evident that other waveguide geometries such as rib waveguide, diffused waveguide, ridge or wire waveguide, strip-loaded waveguide, slot waveguide, and anti-resonant reflecting optical waveguide (ARROW waveguide), photonic crystal waveguide, suspended waveguide, alternating layer stack geometries, sub-wavelength grating (SWG) waveguides and augmented waveguides (e.g. Si—SiO2—Polymer). Further, whilst the embodiments of the invention are described and depicted with respect to a step-index waveguide it would be evident that other waveguide geometries such as graded index and hybrid index (combining inverse-step index and graded index) may be employed.
Whilst embodiments of the invention are described and depicted with respect to passive channel (3D) waveguides within array waveguide gratings (AWGs) it would be evident that the waveguide supercell methodologies described and depicted may be employed within other passive waveguide structures including, but not limited to, Mach-Zehnder interferometers, arrays of Bragg gratings (see for example Menard et al. in WO/2015/131270 entitled “Methods and Systems for Wavelength Tunable Optical Components and Sub-Systems”), optical time delay devices (see for example U.S. Pat. No. 5,852,687 “Integrated Optical Time Delay Unit” and Yegnanarayanan et al. in “Compact Silicon Based Integrated Optical Time Delays” (IEEE Phot. Tech. Lett. Vol. 9, No. 5, pp. 634-535)), optical ring resonators (see for example Tran et al. in “Ultra-Low-Loss Silicon Waveguides for Heterogeneously Integrated Silicon/III-V Photonics” (Applied Sciences, Vol. 8, 1139, pp. 1-13)), optical sensors and monolithically integrated optical sensors (see for example Antoine et al. in “Design of Slow-Light Subwavelength Grating Waveguides for Enhanced On-Chip Methane Sensing by Absorption Spectroscopy” (IEEE Journal of Selected Topics in Quantum Electronics, VOL. 25, NO. 3. pp. 5200308)) and optical beam steerers (see for example Van Acoleyen et al. in “Off-Chip Beam Steering with a One-Dimensional Phased Array on Silicon-on-Insulator” (Optics Letters, Vol. 34, No. 9, pp. 1477-1479)).
Whilst embodiments of the invention are described and depicted with respect to passive channel (3D) waveguides within array waveguide gratings (AWGs) it would be evident that the waveguide supercell methodologies described and depicted may be employed within active waveguide structures including, but not limited to, Mach-Zehnder interferometers, optical beam steerers (see for example Doylend et al. in “Two-Dimensional Free-Space Beam Steering with an Optical Phased Array on Silicon-on-Insulator” (Optics Express, Vol. 19, No. 22, 21595)), dynamic dispersion compensating AWGs, arbitrary filters (see for example Fontaine et al. in “Active Arrayed-Waveguide Grating with Amplitude and Phase Control for Arbitrary Filter Generation and High-Order Dispersion Compensation” (IEEE European Conference on Optical Communication 2008. Paper Mo.4.C.3)), and waveguide arrays (see for example Ma et al. in “High-Resolution Compact On-Chip Spectrometer Based on an Echelle Grating with Densely Packed Waveguide Array” (IEEE Photonics Journal, Vol. 11, No. 1, 4900107)).
As noted above within prior art AWGs the requirement for low waveguide crosstalk between adjacent waveguides results in a large waveguide separation within the central phased array portion of the AWG, thereby resulting in large footprint devices which impacts a variety of factors including, but not limited to, insertion loss, integration of other passive and active waveguide elements, cost, and packaging complexity. Accordingly, it would be beneficial to exploit one or more methodologies which reduce the optical cross-coupling between waveguides as their separation is reduced thereby reducing this waveguide crosstalk. Beneficially, reducing the overall footprint of the AWG also reduces other aspects such as phase noise thereby yielding improved channel crosstalk performance for the AWG.
Accordingly, the inventors exploit the concept of a waveguide supercells wherein both the coupling between waveguides within each waveguide supercell is reduced (referred to by the inventors as intra-coupling) and the coupling between waveguides within adjacent waveguide supercells is reduced (referred to by the inventors as inter-coupling).
Referring to
It would be evident that if optical signals are now coupled to the output array 150 then these optical signals will propagate and combine at the other end of the AWG 100 such that it now acts as a wavelength division multiplexer (MUX).
Now referring to
Accordingly, within the prior art the width of each channel waveguide 260 in the second constant width portion 260B is the same. Hence, for extended regions of such parallel nominally identical waveguides optical coupling occurs between each thereby degrading the performance of the AWG. The inventors thereby introduce the concept of waveguide supercells such as an embodiment of waveguide supercells as depicted in
It is known that the normalized power coupling, waveguide crosstalk, from one waveguide to another waveguide within a pair of waveguides is determined by Equation (2) where P2 represents the power in the second waveguide coupled from the first waveguide with initial power P1, Δβ represents the propagation constant difference between the pair of waveguides, κ represents the coupling strength, and L represents the propagation distance over which the pair of waveguides are coupled. Within the prior art using silica-based optical waveguides, such as SiO2—Ge:SiO2—SiO2 for example, of equal width (i.e. w1=w2) then reduction of optical coupling relied upon reducing the coupling strength K by separating the waveguides sufficient far apart. However, if the width of the pair of waveguides are different such that the effective indices of the two waveguides are now different then such that waveguide crosstalk can be reduced to the same levels achieved with large waveguide separations for identical width waveguides with higher coupling strengths arising from reduced waveguide separation. Generally, this is reduced to the assumption that if the phase mismatch is large relative to the coupling strength (i.e. Δβ>>κ) then the coupling will be low and hence the waveguide crosstalk low. Hence, waveguide crosstalk between waveguides within the AWG array will be lower, which in turn, will contribute to improved channel crosstalk performance for the AWG.
Accordingly, with reference to a waveguide supercell 300 in
However, it is also a design consideration to consider optical coupling between the waveguides with identical widths in adjacent waveguides supercells since waveguide crosstalk can occur between the nearest and next-nearest waveguides within a waveguide array such as the array of waveguides within the central portion of the AWG, e.g. array of waveguides 130 in
Although analysis of waveguide crosstalk between a pair of waveguides is relatively straight forward to analyse using Equation (2) the scaling up of this to offer a crosstalk solution for a large array of waveguides does not easily converge to a single solution. Accordingly, the inventors decision within the embodiments of the invention presented below to use supercells comprising two waveguides per supercell for which a solution convergent upon a set of design parameters can be established according to Equation (2). For example, the two waveguides of width w1 between adjacent waveguide supercells cannot be modelled easily using Equation (2) as the intervening intermediate waveguide of w2 adjusts the coupling. Accordingly, the waveguides within each waveguide supercell and the array of waveguide supercells create multiple options for inter-coupling and intra-coupling. For example, a design methodology may be implemented with a first process establishing waveguide crosstalk below a target level within a waveguide supercell and then a second process establishing waveguide crosstalk below the target level between the multiple waveguide combinations within adjacent supercells. In some instances, a third process may be required to consider large waveguide supercell groupings.
In order to calculate the coupling ratio within and between waveguide supercells the inventors employed an Eigenmode Expansion (EME) solver solution. This being selected as the computational cost of the method scales exceptionally well with the device length, making it more efficient for the design and optimization of long devices compared to other finite difference time domain (FDTD) based methods. Accordingly, the inventors chose as a result of this analysis an initial waveguide supercell comprising a first waveguide of width 800 nm and a second waveguide of width 900 nm. The gap between these waveguides being adjacent waveguides 1.5 μm within the waveguide supercell. Accordingly, within
By comparison, a reference design comprising a pair of identical 800 nm waveguides with a gap of 1.5 μm was also modelled, the results of which are depicted in
In order to observe how alternating the waveguide widths decrease parasitic coupling the field distributions for the two structures were plotted. Accordingly, the waveguide supercell with 800 nm and 900 nm waveguides at 1.5 μm separation is plotted in
In order to demonstrate the performance improvement facilitated by the proposed waveguide supercell structure the inventors designed and fabricated two 1×8 AWGs. The first was a conventional design using an array of waveguides of identical width and the second exploited the waveguide supercell design.
mλ
c
=n
eff(w1)ΔLw1=neff(w2)ΔLw2 (3)
Another aspect of the design is the grating order m which is determined by the design of the output FPR coupler and the arrayed waveguides. A schematic of the output star coupler is shown in
However, despite the effective indexes between two waveguides of the waveguide supercell that are 800 nm and 900 nm wide being different, i.e. neff (w1)≠neff (w2), they must have the same grating order as defined by Equation (4). Accordingly, the length difference between the adjacent waveguides of the waveguide supercell must be designed with different value (ΔLw1≠ΔLw2). As such, the optimization of length difference is a tradeoff between the footprint and the channel crosstalk. A large length difference for a higher grating order leads to a larger footprint and potentially increased losses. However, with respect to Equation (4) a smaller length difference for a lower grating order requires reduced separation either between the arrayed waveguides and/or between output channels. This may cause greater coupling between adjacent apertures and thus higher level of channel crosstalk with the AWG. It would be evident that these design tradeoffs would not exist for other photonic circuits such as those described above, for example, not relying upon phase difference between adjacent waveguides. Accordingly, within the implemented initial 1×8 100 GHz AWG devices to demonstrate the waveguide supercell concept the length difference was established as 472.56 μm for the adjacent 800 nm wide waveguides (ΔLw1) and 461.31 μm for the adjacent 900 nm wide waveguides (ΔLw2).
For both the reference AWG and the AWG with a waveguide supercell (AWG-SC), an array of 40 waveguides with separations 1.5 μm between adjacent waveguides was employed yielding the structures depicted in
In order to characterize the anti-coupling effect of the waveguide supercell, the inventors also designed a pair of Mach-Zehnder interferometers (MZIs). One had a directional coupler formed from identical 800 nm-wide waveguides whilst the other employed the 800 nm/900 nm waveguide pairing of the waveguide supercell. Both MZIs having a coupling gap of 1.5 μm and a 600 μm length imbalance between the two arms.
The AWGs and MZIs were implemented on a SiO2—Si3N4—SiO2 waveguide platform with a nominal Si3N4 waveguide core thickness of 440 nm. The waveguide fabrication comprising:
Depositing a lower cladding of SiO2 with a thickness of 3.2 μm upon the Si wafer; Depositing the core of Si3N4 to a thickness of 440 nm; Electron beam lithography to define the waveguide patterns; Dry etching to remove unwanted Si3N4; Depositing an upper cladding of SiO2 with a thickness of 3.4 μm to conformally coat the Si3N4.
Referring to
Within the exemplary embodiments presented here optical signals were coupled to the MZIs and AWGs through surface grating couplers (SGCs). The normalized transmission spectrums of the MZIs are depicted in
The corresponding transmission spectra for the 1×8 100 GHz AWGs of the AWG-SC design and convention design are depicted in
The channel crosstalk across all channels for the AWG-SC was approximately −18 dB, as shown in
Now referring to
Referring to
Accordingly, each polarization combiner 1410 receives optical signals having a TE polarisation at the wavelength of a predetermined output waveguide of an AWG-SC, e.g. AWG-SC 1320A, and other optical signals having a TM polarisation at the wavelength of the same predetermined output waveguide of the other AWG-SC, AWG-SC 1320B. In this scenario the upper output of polarization splitter 1310 couples TE polarisation signals to the upper AWG-SC, AWG-SC 1320A, and TM polarisation signals to the lower AWG-SC, AWG-SC 1320B. It would be evident to one of skill in the art that alternatively the TE polarisation may be coupled to the lower AWG-SC, AWG-SC 1320B, and the TM polarisation to the upper AWG-SC, AW-SC 1320A. Whilst within schematic 1400 the interconnection between the pair of AWG-SCs 1320A and 1320B and the array of polarization combiners 1410 has been depicted with straight transitions (e.g. as might be implemented with right angled mirrors) it would be evident that these transitions may be implemented with circular waveguide sections or waveguides having polynomials to define position versus length such that they are continuous, for example, in their first and second derivatives.
Alternatively, as depicted in
Further, as depicted in
Optionally, within other embodiments of the invention the MMI may be implemented with other photonic circuit polarization combiner designs including, but not limited to a plasmon based polarization combiner, an asymmetric directional coupler, a 2×2 MMI, and an MZI employing MMIs. Optionally, within other embodiments of the invention the pair of stub waveguides may be ultra-short or zero length such that the output of the MMI 1110 is essentially directly coupled to the input FPR 1020. It would be evident to one skilled in the art that the positions for the TE and TM polarisations at the output of the output FPR 1040 may be TE to the left stub waveguide of the pair of stub waveguides and TM to right stub waveguide of the pair of stub waveguides or reversed.
Accordingly, AWG-SCs such as depicted in
Within embodiments of the invention the polarization splitter may be a fiber optic device coupled to the pair of AWG-SCs or it may a monolithically integrated polarization splitter such as a plasmon based polarization splitter, an asymmetric directional coupler, a 2×2 MMI, and an MZI employing MMIs, for example. In a similar manner each polarization combiners 1410 may be a fiber optic device coupled to the pair of AWG-SCs or it may a monolithically integrated polarization combiner such as a plasmon based polarization combiner, an asymmetric directional coupler, a 2×2 MMI, and an MZI employing MMIs, for example.
Now referring to
However, in some instances to reduce sensitivity of the resulting AWG-SC to manufacturing tolerances, e.g. photolithographic definition of waveguide widths, etching, film thicknesses, film indices etc. it may be beneficial to replace the waveguide set within an AWG-SC or other waveguide device exploiting WG-SCs with the exemplary configuration of waveguides as depicted in schematic 1600 in
Accordingly, within schematic 1600 there are depicted a series of waveguide supercells (WG-SC), WG-SC(1) 1620(1) to WG-SC(M) 1620(M) each comprising a pair of waveguides. Accordingly, WG-SC(1) 1620(1) comprises Waveguide(1) 1610(1) and Waveguide(2) 1620(2) whilst WG-SC(M) 1620(M) comprises Waveguide(N−1) 1610(N−1) and Waveguide(N) 1620(N). However, each waveguide now varies along its length as depicted in
Accordingly, each waveguide varies through a taper from an initial width at the first free propagating region (FPR) to another width at the second FPR. This taper may be of a relatively short length disposed at a point along each waveguide or it may be a long taper such that the taper occurs over a portion of the curved/bent waveguide regions between the first and second FPRs. Within the limit the taper may be the whole waveguide from first FPR to second FPR.
As noted below each waveguide supercell may comprise R waveguides where R≥2 and R is an integer. In this each Waveguide(S), 1≤S≤R, may taper from an initial width WS to a final width WT, where S=1, 2, . . . , R and T=R, 1, . . . , R−1. It would be evident that other combinations would be possible without departing from the scope of the invention.
However, within other embodiments of the invention a pair of waveguide tapers may be employed such as depicted in
As noted below each waveguide supercell may comprise R waveguides where R≥2 and R is an integer. In this each Waveguide(S), 1≤S≤R, may taper from an initial width WS to an intermediate width WT before transitioning back to its final width WS, where S=1, 2, . . . , R and T=R, 1, . . . , R−1. It would be evident that other combinations would be possible without departing from the scope of the invention.
It would be evident that whilst
Channel crosstalk within the AWG depends upon the resolution of the recombined field distribution at the output channels, which is a function of the number of arrayed waveguides. Within the designs employed this was limited to 40. Increasing the number of arrayed waveguides results in a more reliable recreation of the field distribution of the input channel. However, it also increases sensitivity to phase errors caused by variation in the fabrication process and increases the device footprint. Within the prior art exploiting a SOI platform it has been suggested that the optimum number of arrayed waveguides should be 3.5 times the number of output channels, which for an 8 channel AWG-SC would be 28. The suppressed neighborhood waveguide crosstalk within the waveguide supercells improves the recombined field distribution at the output channels which significantly improves the channel crosstalk performance and also allows for a denser waveguide array of the AWG-SC. The denser waveguide array incidentally also reduces the AWG-SC device footprint, which helps to minimize the portion of the insertion loss which is tied to the propagation losses that result from the arrayed waveguides.
The channel crosstalk of an AWG is a function of the overall phase noise for all waveguides within the array of the AWG. As the superlattice concept allows the packing density of these waveguides to be increased without increasing waveguide crosstalk, the overall footprint reduction reduces the overall phase noise within the array of waveguides. Accordingly, the waveguide superlattices improve the channel crosstalk performance for an AWG.
Within previous work by the inventors a 1×8 AWG on the same SiO2—Si3N4—SiO2 platform showing similar performance required a 10 μm gap between the 800 nm waveguides within the arrayed waveguides, rather than the 1.5 μm separation of the AWG-SC according to an embodiment of the invention, with a die footprint of 4.7 mm×1.4 mm. Accordingly, an AWG-SC according to an embodiment of the invention as presented above exploits approximately 40% of the footprint of the conventional prior art AWG.
Within the embodiments of the invention described and depicted above a waveguide supercell has been described and depicted as comprising a pair of waveguides which are then replicated within the array of waveguides within the AWG. However, it would be evident to one of skill in the art that within other embodiments of the invention other counts for the number of waveguides within a waveguide supercell may be employed, such as 3, 4, 5 etc. Where three waveguides are employed with widths w1, w2, and w3 where w1<w2<w3 then these may be employed in one of several sequences w1, w2, w3 (set 1), w1, w3, w2 (set 2), w2, w1, w3 (set 3), w2, w3, w1 (set 4), w3, w2, w1 (set 5), and w3, w1, w2 (set 6). Within an embodiment of the invention a specific sequence may be repeated within the waveguide supercell. Within another embodiment of the invention the waveguide supercell may exploit a repeating sequence of a subset of the potential subsets, for example three subsets of the six potential subsets when using three different waveguide widths (e.g. a repeating sequence of sets 1, 2, 3 or repeating sequence of sets 1, 4, 6 for example). Within another embodiment of the invention the waveguide supercell may exploit a randomized sequence of a subset of the potential subsets, for example three subsets of the six potential subsets when using three different waveguide widths (e.g. a pseudo-randomized sequence of sets 1, 2, 3 or pseudo-randomized sequence of sets 1, 4, 6 for example).
Within other embodiments of the invention the variations in effective index of the waveguides within the waveguide supercell may be achieved using one or other techniques according to the waveguide system being employed. Within embodiments of the invention multiple lithography and deposition steps may be exploited to provide waveguides with varying thickness discretely or in combination with width variations. Within other embodiments of the invention multiple lithography and deposition steps may be exploited to provide waveguides with varying composition either discretely with constant width and thickness cross-sections or with variations in width and/or thickness of the cross-section. For example, these techniques may be exploited with waveguides exploiting SiO2—Si3N4—SiO2; SiO2—SiOXNY—SiO2; SiO2—Ge:SiO2—SiO2; Si—SiO2; polymeric materials, InGaAsP, InP, GaAs, III-V materials, II-VI materials, Si, and SiGe material systems or waveguides exploiting buried waveguides, rib waveguides, ridge or wire waveguides, strip-loaded waveguides, slot waveguides, ARROW waveguides, photonic crystal waveguides, suspended waveguides, alternating layer stack geometries, and augmented waveguides for example.
Within other embodiments of the invention different dopant cross-sections, i.e. film thickness and/or width, may be employed when forming diffused waveguides for example. Within other embodiments of the invention different dopant profiles, for example, from ion implantation rather than ion diffusion, may be employed to generate waveguides of different effective indices discretely or in combination with other techniques.
Within other embodiments of the invention rather than employing constant width waveguides of different widths within the array of waveguides the waveguides may have different widths, but these widths may vary along the length of the waveguides within the array of waveguides. Such variations may be periodic or aperiodic. The width variation(s) may be only applied to a subset of the waveguides within a waveguide supercell. The width variation(s) may be applied over only a portion of the length of the waveguide between the first FPR and the second FPR.
Within embodiments of the invention described above a gap between adjacent waveguides of adjacent waveguide supercells (1.5 μm) is the same as that between the pair of waveguides (1.5 μm) within each waveguide supercell. Accordingly, within embodiments of the invention the gap between adjacent waveguides of adjacent waveguide supercells may be the same as that between any pair of waveguides within each waveguide supercell. Within other embodiments of the invention the gap between adjacent waveguides of adjacent waveguide supercells may be different to that between adjacent pairs of waveguides within each waveguide supercell.
Within embodiments of the invention the waveguides may be designed to have a predetermined birefringence between TE and TM polarisations. This may be a zero birefringence in some embodiments of the invention, constrained with a predetermined range within other embodiments of the invention or unconstrained within other embodiments of the invention.
Within embodiments of the invention the photonic waveguide devices exploiting waveguide supercells may be designed to be athermal. Within other embodiments of the invention one or more methods of temperature compensation may be employed such as active heaters, coolers, etc. or tunable optical launch to the first FPR for example. Within other embodiments of the invention the design of the photonic device may be such that temperature drifts within the device are accommodated within the overall circuit design and/or performance specification.
Within embodiments of the invention described above the AWGs employing waveguide supercells are transmissive with two FPRs coupled at either end of the array of waveguides. Within other embodiments of the invention the array of waveguides may terminate with reflectors such that the AWG is folded back onto itself such that the first and second FPR are the same FPR. These reflectors may be wide Bragg grating based reflectors (over an operating wavelength range of the AWG), thin film filters (over an operating wavelength range of the AWG), mirror facets on the waveguides, a mirrored facet of the die within which the AWG is formed, or a facet of the die within which the AWG coated with a coating having a high reflectivity over the operating wavelength range of the AWG.
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 Application of World Intellectual Property Organization Patent Application PCT/CA2021/050484 filed Apr. 12, 2021; which itself claims the benefit of priority from U.S. Provisional Patent Application 63/015,841 filed Apr. 27, 2020; the entire contents of each being incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/CA2021/050484 | 4/12/2021 | WO |
Number | Date | Country | |
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63015841 | Apr 2020 | US |