CMOS-compatible highly efficient polarization splitter and rotator based on a double-etched directional coupler

Integrating inverse design and partially etched platform: an ultra-compact polarization splitter and rotator as an example

Silicon photonics devices benefit greatly from a partially etched platform and inverse design. Herein, we propose a bi-layer polarization splitter and rotator with a topology pattern and demonstrate it on a silicon-on-insulator platform. Our device exhibits a significantly reduced physical footprint of only 2µm×6µm, compared to traditional directional couplers and tapered waveguides. The device accomplishes the functions of polarization conversion and separation in such a compact design without redundant tapered or bending waveguides. The tested minimum insertion loss with the fabrication batch reaches 0.57 and 0.67 dB for TE and TM modes, respectively. The TE mode demonstrates a wider bandwidth and lower ILs than the TM modes, averaging around 1 dB from 1530 to 1565 nm. The M modes exhibit approximately 2 dB ILs at the same wavelength range, decreasing to about 1 dB between 1565 and 1580 nm. Improved designs and fabrication conditions strongly suggest the potential for further performance enhancement in the device. This successful initiative validates the exceptional performance resulting from the integration of the partially etched platform and inverse design, providing valuable insights for future photonic integrated device designs.

Compact and broadband silicon polarization splitter-rotator using adiabaticity engineering

We propose and demonstrate a short and broadband silicon mode-conversion polarization splitter-rotator (PSR) consisting of a mode-conversion taper and an adiabatic coupler-based mode sorter both optimized by adiabaticity engineering (AE). AE is used to optimize the distribution of adiabaticity parameter over the length of the PSR, providing shortcut to adiabaticity at a shorter device length. The total length of the PSR is 85 µm. The design is compatible with standard silicon photonics platforms and requires only one patterning step. Fabricated PSR has a polarization cross talk of less than -20 dB over the entire O-band for the TE polarization and a polarization cross talk of less than -15 dB from 1267 to 1348 nm for the TM polarization. Overall, the PSR shows low polarization cross talk (-15 dB) over a bandwidth of 81 nm in the O-band. Cross-wafer measurements show that the PSR has good fabrication tolerance.

Fully adiabatic polarization rotator-splitter based on thin-film lithium niobate platform

A Polarization Rotator-Splitter (PRS) based on thin-film lithium niobate (TFLN) is demonstrated in this work. The PRS consists of a partially etched polarization rotating taper and an adiabatic coupler, which enables the input TE₀ and TM₀ to be output as TE₀ from two ports, respectively. The fabricated PRS using standard i-line photolithography achieved large polarization extinction ratios (PERs) of > 20 dB across the whole C-band. Excellent polarization characteristics are maintained when the width is changed by ±150 nm. The on-chip insertion losses of TE₀ and TM₀ are less than 1.5 dB and 1 dB, respectively.

Broadband and CMOS-compatible polarization splitter and rotator built on a silicon nitride-on-silicon multilayer platform

A broadband and CMOS-compatible polarization beam splitter and rotator (PSR) built on the silicon nitride-on-silicon multilayer platform is presented. The PSR is realized by cascading a polarization beam splitter and a polarization rotator, which are both subtly constructed with an asymmetrical directional coupler waveguide structure. The advantage of this device is that the function of PSR can be directly realized in the SiN layer, providing a promising solution to the polarization diversity schemes in SiN photonic circuits. The chip is expected to have high power handling capability as the light is input from the SiN waveguide. The use of silicon dioxide as the upper cladding of the device ensures its compatibility with the metal back-end-of-line process. By optimizing the structure parameters, a polarization conversion loss lower than 1 dB and cross talk larger than 27.6 dB can be obtained for TM-TE mode conversion over a wavelength range of 1450 to 1600 nm. For TE mode, the insertion loss is lower than 0.26 dB and cross talk is larger than 25.3 dB over the same wavelength range. The proposed device has good potential in diversifying the functionalities of the multilayer photonic chip with high integration density.

Broadband integrated polarization splitter and rotator using subwavelength grating claddings

We present a broadband integrated photonic polarization splitter and rotator (PSR) using adiabatically tapered coupled waveguides with subwavelength grating (SWG) claddings. The PSR adiabatically rotates and splits the fundamental transverse-magnetic (TM₀) input to the fundamental transverse-electric (TE₀) mode in the coupler waveguide, while passing the TE₀ input through the same waveguide. The SWGs work as an anisotropic metamaterial and facilitate modal conversions, making the PSR efficient and broadband. We rigorously present our design approaches in each section and show the SWG effect by comparing with and without the SWG claddings. The coupling coefficients in each segment explicitly show a stronger coupling effect when the SWGs are included, confirmed by the coupled-mode theory simulations. The full numerical simulation shows that the SWG-PSR operates at 1500-1750 nm (≈250 nm) wavelengths with an extinction ratio larger than 20 dB, confirmed by the experiment for the 1490-1590 nm range. The insertion losses are below 1.3 dB. Since our PSR is designed based on adiabatical mode evolution, the proposed PSR is expected to be tolerant to fabrication variations and should be broadly applicable to polarization management in photonic integrated circuits.

Fabrication tolerant and broadband polarization splitter-rotator based on adiabatic mode evolution on thin-film lithium niobate

A polarization splitter-rotator device can facilitate on-chip polarization-division multiplexing to enhance the transmission data rate. Here, we propose and experimentally demonstrate a polarization splitter-rotator based on adiabatic mode evolution on the thin-film lithium niobate platform. The measured results for a fabricated device show low insertion losses of <-0.5 dB and large extinction ratios of >20 dB over the 110-nm band. Large fabrication tolerance is also demonstrated with extinction ratios of >15 dB in the wavelength range of 1465-1630 nm for a waveguide width variation of 80 nm.

Development of nano- and microdevices for the next generation of biotechnology, wearables and miniaturized instrumentation

This is a short communication based on recent high-impact publications related to how various chemical materials and substrate modifications could be tuned for nano- and microdevices, where their application for high point-of-care bioanalysis and further applications in life science is discussed. Hence, they have allowed different high-impact research topics in a variety of fields, from the control of nanoscale to functional microarchitectures embedded in various support materials to obtain a device for a given application or use. Thus, their incorporation in standard instrumentation is shown, as well as in new optical setups to record different classical and non-classical light, signaling, and energy modes at a variety of wavelengths and energy levels. Moreover, the development of miniaturized instrumentation was also contemplated. In order to develop these different levels of technology, the chemistry, physics and engineering of materials were discussed. In this manner, a number of subjects that allowed the design and manufacture of devices could be found. The following could be mentioned by way of example: (i) nanophotonics; (ii) design, synthesis and tuning of advanced nanomaterials; (iii) classical and non-classical light generation within the near field; (iv) microfluidics and nanofluidics; (v) signal waveguiding; (vi) quantum-, nano- and microcircuits; (vii) materials for nano- and microplatforms, and support substrates and their respective modifications for targeted functionalities. Moreover, nano-optics in in-flow devices and chips for biosensing were discussed, and perspectives on biosensing and single molecule detection (SMD) applications. In this perspective, new insights about precision nanomedicine based on genomics and drug delivery systems were obtained, incorporating new advanced diagnosis methods based on lab-on-particles, labs-on-a-chip, gene therapies, implantable devices, portable miniaturized instrumentation, single molecule detection for biophotonics, and neurophotonics. In this manner, this communication intends to highlight recent reports and developments of nano- and microdevices and further approaches towards the incorporation of developments in nanophotonics and biophotonics in the design of new materials based on different strategies and enhanced techniques and methods. Recent proofs of concept are discussed that could allow new substrates for device manufacturing. Thus, physical phenomena and materials chemistry with accurate control within the nanoscale were introduced into the discussion. In this manner, new potential sources of ideas and strategies for the next generation of technology in many research and development fields are showcased.

This is a short communication based on recent high-impact publications related to how various chemical materials and substrate modifications could be tuned for nano- and microdevices, where their application for high point-of-care bioanalysis and further applications in life science is discussed.

Ultrahigh extinction ratio and ultra-low insertion loss silicon TE polarizer covering 1260-1675 nm bandwidth

An ultra-broadband TE polarizer with outstanding performance is proposed and demonstrated on a 220 nm-thick silicon-on-insulator platform. The proposed TE polarizer consists of six cascaded directional couplers assisted by subwavelength grating (SWG) structures and two Euler bends. The SWG is introduced to control the coupling strength of the fundamental TE and TM modes. Simulations show that our proposed TE polarizer possesses ultra-low insertion loss (IL < 0.3 dB) for the fundamental TE mode and an ultrahigh polarization extinction ratio (PER > 35 dB) for the fundamental TM mode covering all communication bands from 1260 nm to 1675 nm. The experimental results show that the fabricated TE polarizer has excellent performance of IL < 0.6 dB and PER > 35 dB over a 210 nm bandwidth, which is limited by the measurement equipment. To the best of our knowledge, our proposed TE polarizer is the first single-etched all-silicon TE polarizer with such high PER covering all communication bands.

Compact broadband ( O, E, S, C, L & U bands) silicon TE-pass polarizer based on ridge waveguide adiabatic S-bends

A compact, ultra-broadband and high-performance silicon TE-pass polarizer is proposed and demonstrated experimentally. It is based on partially-etched (ridge) waveguide adiabatic S-bends, input/output tapers and side gratings on a silicon-on-insulator (SOI) platform. A compact footprint and weak back reflections are obtained due to the bent waveguide and the tapers, respectively. An extremely high extinction ratio is achieved by scattering the undesired light in the slab section using the side gratings. The 3D FDTD simulations show a TE loss less than 0.3 dB and an extinction ratio greater than 30 dB over a 500 nm wavelength range (1200 nm to 1700 nm). Measured results show a high TM loss (> 35 dB) and a low TE insertion loss (< 1.5 dB), over a 200 nm wavelength range (1450 nm to 1650 nm). The measured TE loss is < 0.6 dB at a communication wavelength of 1550 nm. The footprint of the optimized design is 65 µm × 20 µm.

Broadband and highly efficient integrated polarization rotator designed by topology optimization

Being a fundamental block for systems that utilize polarization-diversity schemes, such as coherent transceivers, polarization rotators allow the conversion of polarization states. In this work, we present an ultra-compact efficient silicon polarization rotator designed via an inverse design method. By optimizing a topology based on the adjoint method, we designed polarization rotators for several combinations of lengths and widths. Simulation results show that the best optimized device presents a polarization conversion loss of 0.67 dB and cross talk of -18dB for a central wavelength of 1550 nm. These results were achieved for a 7 µm long and 1.2 µm width device. Furthermore, the high coupling efficiency and low cross talk were achieved for a bandwidth exceeding 100 nm. The polarization conversion loss and cross talk were maintained below 0.82 dB and -18dB, respectively, for a band ranging from 1500 nm to 1600 nm.

Efficient polarization splitter-rotator on thin-film lithium niobate

Recently, thin-film lithium niobate coherent modulators have emerged as a promising candidate for the next generation coherent communication system. High performance polarization splitter-rotators (PSRs) are essential to further achieve dual polarization coherent modulators. Here we present a PSR on the lithium niobate on insulator (LNOI) platform with the measured insertion loss less than 1 dB, extinction ratio exceeding 26.6 dB and 19.6 dB for TE0 and TM0 modes, working bandwidth of 1520-1580 nm and total length of 440 µm. In addition, a relatively large fabrication tolerance for waveguide width is also proved. This demonstrated PSR can find its potential application in polarization-division multiplexing (PDM) optical transmitter based on LNOI.

Polarization Splitter-Rotator Based on Multimode Waveguide Grating

We demonstrate a polarization splitter rotator (PSR) based on multimode waveguide grating (MWG) on a silicon-on-insulator (SOI) platform. Bloch mode hybridization in mini-stopband is exploited to achieve high polarization conversion efficiency. The fabricated device yields a high extinction ratio of > 53 dB and > 31 dB, low crosstalk of < −26.4 dB and < −40 dB for the injected TE0 and TM0 mode, with average insertion loss of 1.2 dB and 1.5 dB in the wavelength regime 1552 nm–1562 nm. Such a device shows great design flexibility and an easy fabrication process, serving as a good candidate in integrated polarization diversity circuits, especially for applications requiring spectra manipulation. Additionally, the polarization conversion approach provides opportunities to develop novel polarization management devices.

C + L band polarization rotator-splitter based on a compact S-bend waveguide mode demultiplexer

A novel high-fabrication-tolerance mode demultiplexer (MD) based on an S-bend waveguide is designed, which is used to split TE₁ mode and TE₀ mode, and convert the TE₁ mode to TE₀ mode. Based on the MD, a polarization-rotator-splitter (PRS) is demonstrated. The transmission losses of the fabricated PRS are lower than 0.5 dB and 0.6 dB for TE₀ mode and TM₀ mode, respectively, in the wavelength span of 1520-1630 nm. And the corresponding polarization extinction ratios are larger than 19.5 dB and 17.6 dB, respectively. This MD has the most compact size comparing with other experimentally demonstrated MDs used in PRS.

Silicon Integrated Nanophotonic Devices for On-Chip Multi-Mode Interconnects

Mode-division multiplexing (MDM) technology has drawn tremendous attention for its ability to expand the link capacity within a single-wavelength carrier, paving the way for large-scale on-chip data communications. In the MDM system, the signals are carried by a series of higher-order modes in a multi-mode bus waveguide. Hence, it is essential to develop on-chip mode-handling devices. Silicon-on-insulator (SOI) has been considered as a promising platform to realize MDM since it provides an ultra-high-index contrast and mature fabrication processes. In this paper, we review the recent progresses on silicon integrated nanophotonic devices for MDM applications. We firstly discuss the working principles and device configurations of mode (de)multiplexers. In the second section, we summarize the multi-mode routing devices, including multi-mode bends, multi-mode crossings and multi-mode splitters. The inverse-designed multi-mode devices are then discussed in the third section. We also provide a discussion about the emerging reconfigurable MDM devices in the fourth section. Finally, we offer our outlook of the development prospects for on-chip multi-mode photonics.

Compact silicon 10-mode multi/demultiplexer for hybrid mode- and polarisation-division multiplexing system

To further increase the capacity of the optical transmission system, the hybrid mode- and polarisation-division multiplexing (MDM-PDM) technology has been proved to be an efficient approach by multiplexing dual polarisations for each orthogonal eigen mode. A hybrid (de)multiplexer [(De)MUX] is one of the most important fabrics for the hybrid MDM-PDM networks. A compact silicon 10-mode hybrid (De)MUX is proposed based on three cascaded asymmetric directional couplers (ADCs) based sections, three adiabatic tapers, and a polarisation beam splitter (PBS). The phase-matching conditions can be achieved by varying the widths of the bus waveguides for the TM modes and then by varying the widths of the access waveguides for the TE modes. The simulated results show that a compact total coupling length for TM₁ ~ TM₃ and TE₁ ~ TE₅ modes can be achieved to be 55.4 μm. In addition, the total loss of the proposed hybrid (De)MUX can be reduced benefitting from the fewer tapers compared with the conventional cascaded ADCs. The PBS is also optimised with a compact length of 7.0 μm and high extinction ratios of 32.9 dB and 15.4 dB for the TM₀ and TE₀ modes, respectively.

Sub-wavelength grating-assisted polarization splitter-rotators for silicon-on-insulator platforms

We propose and demonstrate broadband, entirely mode-evolution-based, polarization splitter-rotators (PSR) using sub-wavelength grating (SWG) assisted adiabatic waveguides for two SOI platforms. Our PSRs are more compact than previously demonstrated entirely mode-evolution-based designs. The devices were fabricated using two fabrication processes and, in both cases, the measured spectra show close matches to the simulation results. One of the processes uses standard optical lithography and, hence, this is the first time that an SWG-based PSR has been experimentally implemented using such a process. Finally, measurements for arbitrary input polarizations on an active, automated polarization receiver, that uses one of our PSRs, are also presented.

Transmissive silicon photonic dichroic filters with spectrally selective waveguides

Many optical systems require broadband filters with sharp roll-offs for efficiently splitting or combining light across wide spectra. While free space dichroic filters can provide broadband selectivity, on-chip integration of these high-performance filters is crucial for the scalability of photonic applications in multi-octave interferometry, spectroscopy, and wideband wavelength-division multiplexing. Here we present the theory, design, and experimental characterization of integrated, transmissive, 1 × 2 port dichroic filters using spectrally selective waveguides. Mode evolution through adiabatic transitions in the demonstrated filters allows for single cutoff and flat-top responses with low insertion losses and octave-wide simulated bandwidths. Filters with cutoffs around 1550 and 2100 nm are fabricated on a silicon-on-insulator platform with standard complementary metal-oxide-semiconductor processes. A filter roll-off of 2.82 dB nm-1 is achieved while maintaining ultra-broadband operation. This new class of nanophotonic dichroic filters can lead to new paradigms in on-chip communications, sensing, imaging, optical synthesis, and display applications.

Integrated InP polarization rotator using the plasmonic effect

we report on an integrated InP based polarization rotator scheme using the plasmonic effect. It operates as a half-wave retarder in ridge waveguide structure. The rotation angle of the eigenmode axes of the half-wave retarder waveguide is determined by the position off a bottom corner of a metal layer placed above the waveguide core in the upper cladding region. The simple rotator structure enables an easy and tolerant fabrication process. The length of the fabricated device is less than 50 μm, and a polarization extinction ratio (PER) of 20 dB has been achieved.

Ultracompact, high extinction ratio polarization beam splitter-rotator based on hybrid plasmonic-dielectric directional coupling

A polarization beam splitter-rotator based on asymmetric directional coupling is proposed. An ultrashort cross-polarization coupling length of 7.7 μm is achieved by manipulating the optical field distribution via the plasmonic effect, which is the shortest one reported so far, to the best of our knowledge. At the wavelength of 1.55 μm, the extinction ratios are as high as 50.9 dB and 28.2 dB for fundamental transverse magnetic (TM₀) and transverse electric (TE₀) polarizations, respectively, and the corresponding insertion losses are 1.545 dB and 0.037 dB. In addition, the TM₀-to-TE₀ polarization conversion efficiency is higher than 95% within a bandwidth of 70 nm.

Graphene-assisted ultra-compact polarization splitter and rotator with an extended bandwidth

The high refraction-index contrast between silicon and the surrounding cladding makes silicon-on-insulator devices highly polarization-dependent. However, it is greatly desirable for many applications to address the issue of polarization dependence in silicon photonics. Here, a novel ultra-compact polarization splitter and rotator (PSR), constructed with an asymmetrical directional coupler consisting of a rib silicon waveguide and a graphene-embedded rib silicon waveguide (GERSW), on a silicon-on-insulator platform is proposed and investigated. By taking advantage of the large modulation of the effective refractive index of the TE mode for the GERSW by tuning the chemical potential of graphene, the phase matching condition can be well satisfied over a wide spectral band. The presented result demonstrates that for a 7-layer-graphene-embedded PSR with a coupling length of 11.1 μm, a high TM-to-TE conversion efficiency (>−0.5 dB) can be achieved over a broad bandwidth from 1516 to 1602 nm.

Ultra-broadband silicon polarization splitter-rotator based on the multi-mode waveguide

We propose and demonstrate an ultra-broadband polarization splitter-rotator (PSR) on the Silicon-On-Insulator (SOI) platform. The proposed PSR consists of a straight multi-mode waveguide, an asymmetrical directional coupler and a bent directional coupler. The multi-mode waveguide enables highly-efficient TM₀-TE₁ polarization rotation. The excited TE₁ mode is then converted to be TE₀ mode by the asymmetrical directional coupler. The remained TM₀ mode is filtered out by the bent directional coupler. On the other hand, the incident light of TE₀ mode goes through the PSR with negligible conversion and coupling. Only one-step etching is required for the proposed PSR. The fabricated PSR shows a high extinction ratio > 30.82 dB and a low loss < 0.57 dB at the central wavelength. The extinction ratio is > 20 dB over an ultra-broad wavelength band > 85 nm.

Polarization rotator based on augmented low-index-guiding waveguide on silicon nitride/silicon-on-insulator platform

Using a newly proposed augmented low-index-guiding scheme with silicon nitride/silicon dual-core waveguide, we have designed, fabricated, and characterized a transverse electric (TE) to transverse magnetic (TM) and TM-to-TE compact polarization rotator. The polarization rotation is realized in an asymmetric directional coupler. The measured peak conversion efficiencies for the TE-to-TM and TM-to-TE rotations are approximately 97%. The measured polarization extinction ratio for the TE-to-TM rotation is greater than 20 dB over 50-nm bandwidth, while for the TM-to-TE rotation it is greater than 15 dB over the C-band.

High-extinction-ratio silicon polarization beam splitter with tolerance to waveguide width and coupling length variations

We demonstrate a compact silicon polarization beam splitter (PBS) based on grating-assisted contradirectional couplers (GACCs). Over 30-dB extinction ratios and less than 1-dB insertion losses are achieved for both polarizations. The proposed PBS exhibits tolerance in width variation, and the polarization extinction ratios remain higher than 20 dB for both polarizations when the width variation is adjusted from + 10 to -10 nm. Benefiting from the enhanced coupling by the GACCs, the polarization extinction ratio can be kept higher than 15 dB and the insertion loss is lower than 2 dB for both polarizations when the coupling length varies from 30.96 to 13.76 μm.

300 nm bandwidth adiabatic SOI polarization splitter-rotators exploiting continuous symmetry breaking

Adiabatic polarization splitter-rotators are investigated exploiting continuous symmetry breaking thereby achieving significant device size and losses reduction in a single mask fabrication process for both SOI channel and ridge waveguides. A crosstalk lower than -25 dB is expected over 300nm bandwidth, making the device suitable for full grid CWDM and diplexer/triplexer FTTH applications at 1310, 1490 and 1550nm.

Symmetrical polarization splitter/rotator design and application in a polarization insensitive WDM receiver

In integrated photonics, the design goal of a polarization splitter/rotator (PSR) has been separating the TE0 and TM0 modes in a waveguide. This is a natural choice. But in theory, a PSR only needs to project the incoming State Of Polarization (SOP) orthogonally to its output ports, using any orthogonal mode basis set in the fiber. In this article, we introduce a novel PSR design that alternatively takes the linear combination of TE0 and TM0 (TE0 +/- TM0) as orthogonal bases. By contrast, existing approaches exclusively use TE0 and TM0 as their basis set. The design is based on two symmetric and robust structures: a bi-layer taper and a Y-junction, and involves no bends. To prove the concept, we incorporated it into a four-channel polarization insensitive wavelength division multiplexing (PI-WDM) receiver fabricated in a standard CMOS Si photonics process. 40 Gb/s data rate and 0.7 +/- 0.2 dB polarization dependent loss (PDL) is demonstrated on each channel. Lastly, we propose an improved PSR design with 12 μm device length, < 0.1 dB PDL, < 0.4 dB insertion loss and < 0.05 dB wavelength dependence across C-band for both polarizations. Overall, our PSR design concept is simple, easy to realize and presents a new perspective for future PSR designs.

A silicon-on-insulator polarization diversity scheme in the mid-infrared

We propose a silicon-on-insulator (SOI) polarization diversity scheme in the mid-infrared wavelength range. In consideration of absorption loss in silicon dioxide (SiO₂), the polarization splitter-rotator (PSR) is designed and optimized with silicon nitride (SiN) upper-cladding and SiO₂ lower-cladding. This asymmetry allows the PSR, which consists of mode-conversion tapers and subsequent mode-sorting asymmetric Y-junctions, to be fabricated with a simple one-step etching process. Simulation shows that our PSR has good performance with low mode conversion loss (< 0.25 dB) and low crosstalk (< -18 dB) in a very large wavelength range from 4.0 μm to 4.4 μm. The PSR also exhibits large fabrication tolerance with respect to the size deviations in waveguide width, height and refractive index of the upper-cladding. Additionally, PSR devices based on Y-junctions with SiO₂ upper-cladding, and SiN upper- and lower-claddings are designed for potential applications at shorter and longer wavelengths, respectively. These PSR devices could facilitate the development of silicon photonic devices in the mid-infrared.

Efficient silicon polarization rotator based on mode-hybridization in a double-stair waveguide

We present a compact silicon polarization rotator (PR) based on mode-hybridization by breaking the cross-sectional symmetry of a double-stair waveguide. The device fabrication is fully compatible with the commonly used silicon photonics processes with no extra masks required. The dependence of device performance on the double-stair waveguide dimensions is investigated using FDTD simulations. Characterizations of the fabricated devices reveal that the 23-μm-long PR exhibits a polarization extinction ratio (PER) of >17 dB in the wavelength range of 1500-1540 nm. The maximum PER exceeds 30 dB at 1518 nm.

Proposal for fabrication-tolerant SOI polarization splitter-rotator based on cascaded MMI couplers and an assisted bi-level taper

A novel silicon-on-insulator (SOI) polarization splitter-rotator (PSR) with a large fabrication tolerance is proposed based on cascaded multimode interference (MMI) couplers and an assisted mode-evolution taper. The tapers are designed to adiabatically convert the input TM(0) mode into the TE(1) mode, which will output as the TE(0) mode after processed by the subsequent MMI mode converter, 90-degree phase shifter (PS) and MMI 3 dB coupler. The numerical simulation results show that the proposed device has a < 0.5 dB insertion loss with < -17 dB crosstalk in C optical communication band. Fabrication tolerance analysis is also performed with respect to the deviations of MMI coupler width, PS width, slab height and upper-cladding refractive index, showing that this device could work well even when affected by considerable fabrication errors. With such a robust performance with a large bandwidth, this device offers potential applications for CMOS-compatible polarization diversity, especially in the booming 100 Gb/s coherent optical communications based on silicon photonics technology.

Ultracompact silicon-on-insulator polarization rotator for polarization-diversified circuits

We present an ultracompact (15.3 μm long) and high-efficiency silicon-on-insulator polarization rotator designed for polarization-diversified circuits. The rotator is comprised of a bilevel-tapered TM0-to-TE1 mode converter and a novel bent-tapered TE1-to-TE0 mode converter. The rotator has a simulated polarization conversion loss lower than 0.2 dB and a polarization-extinction ratio larger than 25 dB over a wavelength range of 80 nm around 1550 nm. The rotator has a SiO2 top-cladding and can be fabricated in a CMOS-compatible process.

Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler

We propose a fabrication tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform based on the mode-coupling mechanism. The PSR consists of a silicon wire waveguide coupled to a taper-etched waveguide. Compared to previously reported PSRs based on directional couplers which are sensitive to fabrication variations, the partially etched taper structure can compensate for fabrication inaccuracies. In addition, the taper-etched geometry breaks both the horizontal and vertical symmetries of the waveguide, introducing an additional degree of design freedom to accommodate different upper cladding layers. The proposed PSR can be readily integrated in a planar waveguide circuit using e.g. SiO(2) cladding, making it compatible with typical metal back-end-of-line processes. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.09 dB in the C-band (or a conversion efficiency of 98%). A low TE-to-TE through insertion loss (-0.07 dB) and a very low polarization crosstalk (-30 dB) over a wide wavelength range exceeding 160 nm with a large fabrication tolerance (>50 nm) are numerically demonstrated.

Novel ultra-broadband polarization splitter-rotator based on mode-evolution tapers and a mode-sorting asymmetric Y-junction

A novel silicon-on-insulator (SOI) polarization splitter-rotator is proposed based on mode-evolution tapers and a mode-sorting asymmetric Y-junction. The tapers are designed to adiabatically convert the input TM0 mode into the TE1 mode, which will evolve into the TE0 mode in the wide output arm while the input TE0 mode excites the TE0 mode in the narrow arm. The numerical simulation results show that the mode conversion efficiency increases with the lengths of the tapers and the Y-junction for the output waveguide widths in a large range. This proposed device has < 0.4 dB insertion loss with > 12 dB extinction ratio in an ultra-broad wavelength range from 1350 nm to 1750 nm. With such a broad operating bandwidth, this device offers potential applications for polarization diversity operating across every communication bands. Fabrication tolerance analysis is also performed in terms of the device width variation, the slab height variation and the variation of the upper-cladding refractive index.