Polarization rotator assembly including a subwavelength composite portion

A polarization rotator assembly for rotating a polarization mode of an electromagnetic signal is provided. The polarization rotator assembly has a waveguiding structure of co-extensive first and second layers defining, successively, an input portion, a subwavelength composite portion and a polarization rotating portion. The subwavelength composite portion is formed by the first and second layers, where the second layer defines a subwavelength pattern. The polarization rotator portion is geometrically configured to rotate the polarization mode of the electromagnetic signal.

FIELD OF THE INVENTION

The present invention relates to the field of integrated photonic waveguides, and more particularly concerns a polarization rotator assembly for rotating a polarization mode of an electromagnetic signal propagating therealong.

BACKGROUND OF THE INVENTION

Over the past decade, integrated photonics has made important progress in implementing optical and electro-optical devices in silicon for use in various technological applications in fields such as telecommunications, sensing and signal processing. Integrated photonic relies on optical waveguides to implement devices such as optical couplers and switches, wavelength multiplexers and demultiplexers, and polarization splitters and rotators. In particular, integrated photonics based on silicon is a promising candidate for compact integrated circuits due to its compatibility with silicon electronics and standard complementary metal-oxide-semiconductor (CMOS) fabrication methods. The high refractive index contrast between the silicon core and silicon dioxide enables the propagation of highly confined optical modes, which allows scaling integrated photonic waveguides down to submicron level.

One consequence of this high refractive index contrast is that integrated silicon photonic waveguides experience large modal structural birefringence between the two orthogonal transverse electric (TE) and transverse magnetic (TM) fundamental modes of the guided light. Because of this birefringence, integrated photonic waveguides typically exhibit a polarization-dependent behavior. Moreover, since silicon photonic waveguides generally have submicron dimensions and very stringent fabrication tolerance requirements, completely eliminating structural birefringence can prove to be an extremely demanding task.

In order to achieve polarization-independent performance, one may implement a polarization diversity scheme. Generally, polarization diversity is accomplished by using polarization splitters and rotators. In this approach, the two orthogonal TE and TM polarization modes are split in two distinct paths of a polarization diversity circuit. By further rotating the polarization state in one of the paths of the polarization diversity circuit to the orthogonal polarization state, the two paths may be operated in parallel on identical high refractive index contrast waveguide structures. For example, in fundamental-mode silicon waveguides having a certain width and height, it is generally desired to convert the TM polarized signal into a TE polarized signal. Then, as a result of this conversion, only optical functions for the TE modes need to be fabricated and polarization dependence may be eliminated or reduced by using a single polarization (i.e. TE) implementation.

In order for the polarization diversity approach to be practical, on-chip polarization splitters and rotators are desired. However, designing and fabricating integrated waveguide-type polarization rotators can be challenging.

U.S. Pat. No. 7,792,403 to Little et al. (hereinafter LITTLE) discloses a waveguide structure that includes a polarization rotator for rotating the polarization of an electromagnetic signal, preferably by about ninety-degrees. In general, the polarization rotation of the electromagnetic signal by the polarization rotator disclosed in LITTLE is achieved via the geometrical parameters of the polarization rotator. In one embodiment (see, e.g.,FIGS. 1 and 2in LITTLE), the polarization rotator includes an input end, an output end and a midsection extending therebetween and along which polarization rotation is achieved. The midsection has a first and a second level of differing heights and the polarization rotator is referred to as a “bi-level” polarization rotator. The first level of the midsection has a width that decreases along the length of the first level, while the second level has a substantially constant width along the length of the second level.

Waveguide structures such as the one shown in LITTLE can be subject to stringent fabrication tolerances. In particular, it is desirable for the electromagnetic signal to reach the polarization rotation portion in the TM polarization mode in order to be properly rotated. However, vertical taper shapes used to transition between waveguides of different heights can be particularly sensitive to mask alignment during fabrication, and fabrication errors can lead to an undesired pre-rotation of the polarization mode of the guided electromagnetic signal.

There therefore exists a need in the art for an improved polarization rotator assembly for rotating the polarization of light in silicon-based photonic integrated circuits.

SUMMARY

In accordance with one aspect of the invention there is provided a polarization rotator assembly for rotating a polarization mode of an electromagnetic signal.

The polarization rotator assembly includes a waveguiding structure having co-extensive first and second layers. The waveguiding structure has a first height corresponding to the first layer and a second height corresponding to a superposition of the first and second layers. The waveguiding structure has a waveguiding axis and includes successively therealong:an input portion formed by the first layer and having a first width;a subwavelength composite portion formed by the first and second layers, where the second layer defines a subwavelength pattern having a characteristic feature size which is less than half an effective wavelength of the electromagnetic signal when propagating therein; anda polarization rotator portion comprising at least the first and second layers and geometrically configured to rotate a polarization mode of the electromagnetic signal.

Embodiments of the invention may be particularly well adapted for use in submicron silicon-based, fundamental-mode waveguide structures exhibiting polarization-dependent characteristics arising from the large structural modal birefringence between the TE and TM fundamental modes.

Other features and advantages of the invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the invention, there is provided a polarization rotator assembly. The polarization rotator assembly allows rotating a polarization state or mode of an electromagnetic signal as the electromagnetic signal propagates therethrough.

Polarization rotator assemblies according to embodiments of the invention can be generally useful in silicon-based integrated photonics or other high index contrast photonics applications, preferably as part of on-chip polarization-diversity circuits implemented for eliminating the polarization dependence in devices based on photonic waveguides. In particular, embodiments of the invention may be particularly well adapted for use in submicron silicon-based, fundamental-mode waveguide structures exhibiting polarization-dependent characteristics arising from the large structural modal birefringence between the TE and TM fundamental modes. In such embodiments, the polarization rotator assembly is preferably operatively configured to rotate the polarization of an electromagnetic signal by ninety degrees. More precisely, to convert a TM polarized signal to its orthogonal counterpart, namely a TE polarized signal or vice versa. The electromagnetic signal may be a telecommunication signal encoded with information according to one of many known modulation schemes or may be embodied by any other optical beam whose polarization is to be rotated. It will be readily understood that polarization rotator assemblies as described herein may be used in different contexts than those mentioned above without departing from the scope of the present invention.

Referring toFIGS. 1,2A and2B, there are shown perspective, top and side elevation views of a polarization rotator assembly20, in accordance with an embodiment of the invention.

The polarization rotator assembly20first includes a waveguiding structure21, which is substantially planar and includes two co-extensive layers22and24. The waveguiding structure21allows guiding of an electromagnetic signal along a waveguiding axis36. The thickness profile of the waveguiding structure is characterized by a first height h1, corresponding to the first layer22, and a second height and h2, which correspond to the superposition of the first and second layers22and24. The first and second layers22and24therefore define a level difference Δh=h2−h1therebetween. The heights h1and h2may be selected so that the ratio of h2and h1is of the order of two. Additionally or alternatively, the heights h1and h2may be selected so as to achieve substantially fundamental-mode (e.g. TE and TM) operation for a given waveguide width.

It will be understood that the first and second layers22and24form the core of the waveguiding structure21, inside which the electromagnetic signal is guided. In the illustrated embodiment, the waveguiding structure21is a strip waveguide, but other appropriate structures could be used in other embodiments including a ridge waveguide and a rib waveguide. The core material forming the first and second layers22and24is preferably silicon having a refractive index of about 3.5 at a wavelength of 1.55 μm, but other core materials could be envisioned including silicon nitride, silicon carbide, indium phosphide, gallium arsenide, high-index polymers and the like. In some embodiments both the first and second layers may be made of a same material, whereas in other embodiments they may each be made of different ones of the materials listed above.

The first and second layers22and24of the polarization rotator assembly20may be defined using any common, preferably CMOS-compatible, photolithographic processes. As known in the art, such processes may involve thin-layer deposition, selective photoresist mask etching and patterning, and oxidation. For example, the polarization rotator assembly20may be formed using two masks and two etching steps. Optionally, a cladding material (not shown) may be deposited over the polarization rotator assembly20. The cladding material is preferably silicon dioxide (silica) having a refractive index of 1.45 at a wavelength of 1.55 μm, but other appropriate materials could alternatively or additionally be used.

Broadly described, the waveguiding structure21of the polarization rotator assembly20illustrated inFIGS. 1,2A and2B includes an input portion26, a subwavelength composite portion28, a buffer zone30, a polarization rotator portion32and an output portion34, which extend successively along the waveguiding axis36. The polarization rotator assembly20of this embodiment may also be conceptually divided into six distinct sections, labeled A to F, which extend between the input portion26and the output portion34.

In operation, an electromagnetic signal propagating along the propagation axis36preferably enters the polarization rotator assembly20via the input waveguide portion26, which defines section A of the polarization rotator assembly20. Preferably, the electromagnetic signal is already polarized into one of the TM and TE polarized modes upon entering the input waveguide portion26. As known in the art, in a polarization diversity scheme, the TE and TM polarization may first be spatially separated in two different waveguides. One of the TE and TM polarized signals may then be rotated through ninety degrees to yield two parallel circuits propagating in the same polarization mode. For example, the polarization rotator assembly20shown inFIGS. 1,2A and2B may be configured to receive a TM polarized signal and convert the same into a TE polarized signal.

The geometrical parameters of the input waveguide portion26(e.g. height and width) may be selected to ensure substantially single-mode propagation along the polarization rotator assembly20and to facilitate matching between the polarization rotator assembly20and other connecting waveguide elements disposed on the upstream side thereof. In the example ofFIGS. 1,2A and2B, the input portion26is formed solely by the first layer22and has first width w1, which is constant along the waveguiding axis36, therefore defining a rectangular shape.

With continued reference toFIGS. 1,2A and2B, after passing through the input portion26, the electromagnetic signal enters the subwavelength composite portion28for propagation therealong. The subwavelength composite portion28is formed by the first and second layers22and24, and the second layer24defines a subwavelength pattern.

As used herein, the term “subwavelength” refers to the fact that the size of the characteristic features or inhomogeneities (typically, corrugation periodicity) of the subwavelength pattern are markedly smaller than half of the wavelength of the electromagnetic signal propagating thereinside. When the wavelength of the electromagnetic signal propagating within the subwavelength composite portion is large compared to the characteristic feature size thereof, the structure can be treated as an effective homogeneous material. This condition is generally met when the characteristic feature size of the subwavelength pattern (typically the periodicity of the corrugations) is less than half the wavelength of the electromagnetic signal propagating therein.

In the illustrated embodiment ofFIGS. 1,2A and2B, the subwavelength pattern is a one-dimensional corrugated grating and therefore includes a series of corrugations38aformed by the second layer22and distributed along the waveguiding axis36and transverse thereto. The series of corrugations38ais interleaved with a series of gaps38bwhere portions of the second layer are absent. The corrugations38aare typically made of a core material such as silicon, and the gaps38bbetween the corrugations38amay be air or be filled by a cladding material such as silica.

It will be understood that, in this embodiment, the characteristic feature size of the subwavelength pattern corresponds to the length of one corrugation38aand one adjacent gap38b, the sum of which represents the period of the pattern. Hence, in order for the pattern to be considered “subwavelength”, the transverse size and separation of corrugations should be on a subwavelength scale along the length of the subwavelength composite portion28, to ensure that resonance and filtering effects typically observed with Bragg gratings or other periodic structures are suppressed. The subwavelength composite portion28therefore acts as a homogeneous medium with an effective refractive index whose value is between those of the corrugations (e.g. core material) and the separation between them (e.g. air or cladding material).

The subwavelength pattern may be formed by selective etching or deposition of the second layer24. In the illustrated embodiment ofFIGS. 1,2A and2B, the series of corrugations is shown as having a fixed period and a fixed duty cycle. As mentioned above the period of the corrugations corresponds to the length, along the propagation axis26, of one corrugation and one adjacent gap. The term “duty cycle” is understood to refer to the ratio of the corrugation length to the period of the subwavelength pattern. It will however be understood that in other embodiments, the period and/or the duty cycle of the series of corrugations may be variable. For example,FIG. 3Aillustrates a pattern of corrugations38awhere the period of the series of corrugations is variable, shown here as increasing progressively by way of example. The duty cycle may also be selected in order to tailor the effective refractive index neffof the subwavelength composite portion28. For example, referring toFIG. 3B, a duty cycle variation along the waveguiding axis36may be implemented in order to taper a difference in refractive index between the input and output ends of the subwavelength composite portion28. In the illustrated example, the duty cycle is shown as increasing from 0.4 to 0.6 while the period remains fixed. The choice of the period and duty cycle may also be influenced by other factors of fabrication and design rules.

It is to be noted, however, that the subwavelength pattern of the subwavelength composite portion28need not be periodic, as long as the characteristic feature size thereof remains below the diffraction limit. By way of example,FIG. 3Cshows a series of corrugations having an aperiodic, even random, profile, as may be used for the subwavelength pattern in some embodiments of the polarization rotator assembly.

Additionally, the subwavelength pattern may be defined by features differing from the transversally disposed series of gaps and corrugations illustrated inFIGS. 1,2A,2B, and2A to3C. For example, Referring toFIGS. 4A to 4C, in other embodiments the subwavelength pattern of the subwavelength composite portion28may be embodied by: longitudinal corrugations44parallel to the waveguiding axis36(FIG. 4A); a periodic or aperiodic array of arbitrarily-shaped pillars46of height Δh formed by the second layer24and projecting upwardly from the first layer22(FIG. 4B); or a periodic or aperiodic array of arbitrarily-shaped holes48of depth Δh extending through the second layer24(FIG. 4C). It will be understood that any combination of two or more of the above cases may be envisioned, as well as subwavelength corrugations patterned on more than one layer, combined with one or more layers without corrugations.

Referring back toFIGS. 1 and 2A, the subwavelength composite portion28can be seen as tapering down from the first width w1to a second width w2. This width reduction can be designed such that, at each point along the axis, the periodicity of the pattern remains small enough to satisfy the subwavelength condition. However, in other embodiments the width of the subwavelength composite portion28could remain constant, as for example shown in the illustrated embodiment ofFIGS. 11,12A and12B.

Optionally, the subwavelength pattern may include a wedge-shaped section29forming a longitudinally widening taper along the waveguiding axis36, shown in section B of the illustrated embodiment ofFIGS. 1,2A and2B. In the illustrated embodiment, the wedge-shaped section29defines a width taper to facilitate matching of the TM polarized signal between the input portion26and the subwavelength composite portion28. The wedge-shaped section29of the subwavelength composite portion28may advantageously allow reducing optical losses at the junction between sections A and B without inducing a rotation of the polarization of the electromagnetic signal when considering the fabrication standard deviation inherent to mask alignment. Also advantageously, the wedge-shaped section29may compensate for the minimum feature size allowed by the fabrication process, which may limit the minimum and maximum achievable values for the duty cycle of the subwavelength pattern. The slope of the width taper may be fixed or not, and may be designed for generating an adiabatic taper or not. It will be understood that providing a width taper at one end of the subwavelength composite portion is optional and need not be included in some embodiments.

It will thus be understood that the subwavelength composite portion28advantageously acts as a vertical mode converter between two waveguide elements defining a level difference Δh therebetween. In addition, the polarization of the electromagnetic signal propagating in the subwavelength composite portion28remains substantially unaffected by fabrication tolerance issues since the polarization rotation is negligible therealong.

Still referring toFIGS. 1,2A and2B, the waveguiding structure21of the polarization rotator assembly20may optionally further include a buffer zone30, formed by the superposition of the first and second layers and therefore of height h2. The buffer zone30is disposed between the subwavelength composite portion28and the polarization rotator portion32. The buffer zone30corresponds to the section D of the illustrated polarization rotator assembly20. The buffer zone30may be provided to avoid an undesirable overlap of the sections C and E during the fabrication of the polarization rotator assembly20, which could arise as a result of vertical or horizontal misalignment (e.g. of the order of 50 nm) of the masks used to define the first and second layers22and24. In this regard,FIG. 6illustrates a top view of a schematic representation of a mask configuration suitable for patterning the two vertically-spaced layers of an embodiment of the polarization rotator assembly. However, it will be understood that the buffer zone30need not be provided and may thus be omitted in other embodiments as for example shown inFIGS. 11,12A and12B.

Referring toFIGS. 7,8A and8B, in some embodiments, the buffer zone may include an input width taper44that extends into the output end of the subwavelength composite portion28. This input width taper44may provide a smoother transition between sections C and D of the polarization rotator assembly20, as well as reduce optical losses at the junction between sections C and D.

Referring back toFIGS. 1,2A and2B, the waveguiding structure21of the polarization rotator assembly20also includes a polarization rotator portion32. The polarization rotator portion32includes the first and second layers22and24and is geometrically configured to rotate the polarization mode of the electromagnetic signal.

The polarization rotator portion may have any configuration which allows the rotation of at least one polarisation mode of the electromagnetic signal propagating in the waveguiding structure21. In the illustrated embodiment ofFIGS. 1,2A and2B, the polarisation rotation is based on the level difference Δh between the first and the second heights h1and h2. In the illustrated embodiment, the electromagnetic signal is receiving by the polarization rotating portion as it exits the buffer zone30, although in other embodiments the electromagnetic signal may propagate directly from the subwavelength composite portion28to the polarization rotation portion32.

Preferably, the polarization rotation portion32is configured to rotate the polarization of the electromagnetic signal by ninety degrees. Further preferably, the polarization rotation portion32is configured to convert a TM polarized signal to TE polarized signal which is its orthogonal counterpart or vice versa.

In the embodiment shown inFIGS. 1,2A and2B, the polarization rotation of the electromagnetic signal is achieved via a change in the geometry of the first and second layers22and24along the polarization rotating portion32. In the illustrated embodiment, the polarization rotating portion32has an input end40aand an output end40b. The width of the second layer24along the waveguiding axis36decreases from the input end40ato the output end40b, whereas the width of the first layer22along the waveguiding axis36increases from the input end40ato the output end40bof the polarization rotating portion32. It will be understood that in other embodiments, the width of the first layer22may alternatively be kept constant. In addition, other configurations of polarization rotators geometrically configured for polarization rotation based on the level difference Δh between the first and second layers22and24may be envisioned without departing from the scope of the present invention.

Referring toFIGS. 9,10A and10B an alternative configuration for the polarization rotator portion32is shown. In this example, the polarization rotation portion32includes a polarization-rotating path52and a polarization-maintaining path50, both at the second height h2. Wing-shaped ribs54made of a waveguiding layer of a height h0smaller than h1extend on either side of the first and second layers22and24. The polarization rotating portion32is geometrically configured to split the electromagnetic signal into two signal polarization components respectively guiding along the polarization maintaining and rotating paths50and52. For example the electromagnetic signal may be split into the TE mode, which remains guided along the waveguiding axis26and the polarization-maintaining path50, while the TM mode is coupled into the polarization-rotating path52which rotates it by 90 degrees, both paths therefore output light in the TE mode.

Of course, numerous examples of polarization rotating structures based on similar principles can be found in the art.

Finally, waveguiding structure21of the polarization rotator assembly20preferably includes the output portion34for receiving the electromagnetic signal exiting the polarization rotating portion. The output portion34defines the section F of the illustrated polarization rotator assembly20. As with the input portion26, the geometrical parameters of the output waveguide portion34(e.g. height and width) may be selected to ensure substantially single-mode propagation along the polarization rotator assembly20and to facilitate matching between the polarization rotator assembly20and other connecting waveguide elements disposed on the downstream side thereof. In the embodiment ofFIGS. 1,2A and2B, the output waveguide portion34is defined by the first layer22and therefore has a same height h1and width w1to ensure good propagation of the fundamental TE mode without significant excitation of higher-order modes. The output portion may also include a subwavelength composite structure acting as a mode converter to provide a transition between waveguiding structures at different heights. For example, in the polarization rotating assembly illustrated inFIGS. 9,10A and10B, both the polarization maintaining and the polarization rotating paths end at a the second height h2; an output portion including a vertical mode converter (not shown) at each path output may be used in order to couple light back into an output waveguide of height h1. The vertical output converter may for example be embodied by a structure similar to the one of the subwavelength composite portion, used in reverse.

One skilled in the art will understand that the enclosed drawings are not drawn to the typical scale of such devices. The polarization rotator assembly may have dimensions and proportions according to requirements and limitations of a particular application. For example, polarization rotators assembly having a configuration similar to the one shown inFIGS. 1,2A and2B were fabricated where h1=220 nm and h2=380 nm, resulting in a level difference Δh=160 nm. In this embodiment, the width and height, characterizing the waveguiding structure21along the input portion were 500 nm and 220 nm respectively, to ensure single-mode propagation for each TE and TM polarizations. In section C, the width of the corrugations (i.e. the width of the second layer24) decreased along the length of the subwavelength composite portion28in the same manner as the width of the first layer22, such that at the end of section C, the subwavelength composite structure28has a second width of 220 nm. The period of the subwavelength corrugated grating was 300 nm. The duty cycle in section B is 0.40, that is, the length of the corrugations (along the waveguiding axis) was 120 nm and their separation 180 nm. Preferably, the length of the corrugations is selected so as to correspond to the minimum feature size allowed by the fabrication process utilized. For example, the first corrugation at the entrance of the input end of the subwavelength composite portion28corresponds to a 120×120×160 nm3pillar. Moreover, the duty-cycle of the subwavelength pattern increased gradually from 0.40 to 0.60, thus creating a smoother transition for the TM polarized signal entering the subwavelength composite structure from a waveguide having a height of 220 nm and a width of 500 nm to a waveguide having a height of 380 nm and a width of 220 nm.

FIGS. 5A to 5Care schematic representations of various vertical taper geometries on integrated photonic waveguides. InFIG. 5A(PRIOR ART), the vertical taper is a conventional vertical taper equivalent to the one provided in LITTLE. InFIGS. 5B and 5C, the vertical taper is respectively embodied by a subwavelength pattern without and with a wedge-shaped section. The parameters indicated inFIGS. 5A to 5C, having the following values: w1=500 nm, w2=220 nm, La=5 μm, Lb=40 μm. The first layer22, has been considered as having a height h1=220 nm and the second layer24has a height h2=380 nm. The table below provides information regarding the TM mode insertion loss (IL) and the polarization extinction ratio (PER) for the structure shown inFIGS. 5A to 5Csubjected to typical mask transversal misalignment of 50 nm of the first layer22relative to the second layer24. The data provided in the table were obtained via a finite-difference time-domain (FDTD) numerical simulation. One would observe that the conventional vertical taper depicted inFIG. 5Asuffers from a non-negligible IL of −1.8 dB and experience a PER degradation well below 15 dB, a level that could be seen as a device with good PER performance. The design shown inFIG. 5Bwith subwavelength corrugations shows a near perfect PER insensitivity to mask misalignment, although the −0.7 dB IL could still be considered non-negligible for some application. The design shown inFIG. 5Cpresents a good trade-off between PER and IL minimal degradation.

It will be understood that the polarization rotator assembly20according to embodiments of the invention is generally reciprocal, that is, the electromagnetic signal could alternatively enter and exit the polarization rotator assembly20via the output and input waveguide portions34and26, respectively, thus going through a reverse polarization rotation. Similarly, the polarization rotator assembly20may also be used to convert a TE polarized signal to a TM polarized signal.

By combining a subwavelength composite portion acting a vertical mode converter with a two-level adiabatic polarization rotating portion, embodiments of the present invention may provide a polarization rotator assembly20exhibiting a reduced sensitivity to mask misalignment and other fabrication tolerance issues.

Of course, numerous modifications could be made to the embodiments described above without departing from the scope of the present invention.