Integrated mode-evolution-based polarization splitter

Integrated visible-light polarization rotators and splitters for atomic quantum systems

In this work, we design and experimentally demonstrate the first, to the best of our knowledge, integrated polarization splitters and rotators at blue wavelengths. We develop compact and efficient designs for both a polarization splitter and rotator at a 422-nm wavelength, an important laser-cooling transition for 88Sr+ ions. These devices are fabricated in a 200-mm wafer-scale process and experimentally demonstrated, resulting in a measured polarization-splitter transverse-electric thru-port coupling of 98.0% and transverse-magnetic tap-port coupling of 77.6% for a compact 16-µm-long device and a polarization-rotator conversion efficiency of 92.2% for a separate compact 111-µm-long device. This work paves the way for more sophisticated integrated control of trapped-ion and neutral-atom quantum systems.

High performance on-chip polarization beam splitter at visible wavelengths based on a silicon nitride small-sized ridge waveguide

Due to sensitive scaling of the wavelength and the visible-light absorption properties with the device dimension, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at the visible wavelengths. In this work, a simple and small polarization beam splitter (PBS) for a broad visible-light band, using a tailored silicon nitride (Si3N4) ridge waveguide, is presented, which is based on the distinct optical distribution of two fundamental orthogonal polarized modes in the ridge waveguide. The bending loss for different bending radii and the optical coupling properties of the fundamental modes for different Si3N4 ridge waveguide configurations are analyzed. A PBS composed of a bending ridge waveguide structure and a triple-waveguide directional coupler was fabricated on the Si3N4 thin film. The TM excitation of the device based on a bending ridge waveguide structure shows a polarization extinction ratio (PER) of ≥ 20 dB with 33 nm bandwidth (624-657 nm) and insertion loss (IL) ≤ 1 dB at the through port. The TE excitation of the device, based on a triple-waveguide directional coupler with coupling efficiency distinction between the TE0 and TM0 modes, shows a PER of ≥ 18 dB with 50 nm bandwidth (580-630 nm) and insertion loss (IL) ≤ 1 dB at the cross port. The on-chip Si3N4 PBS device is found to possess the highest known PER at a visible broadband range and small (43 µm) footprint. It should be useful for novel photonic circuit designs and further exploration of Si3N4 PBSs.

Polarization Splitting at Visible Wavelengths with the Rutile TiO2 Ridge Waveguide

On-chip polarization control is in high demand for novel integrated photonic applications such as polarization division multiplexing and quantum communications. However, due to the sensitive scaling of the device dimension with wavelength and the visible-light absorption properties, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at visible wavelengths. In this paper, a new polarization-splitting mechanism based on energy distributions of the fundamental polarized modes in the r-TiO₂ ridge waveguide is investigated. The bending loss for different bending radii and the optical coupling properties of the fundamental modes in different r-TiO₂ ridge waveguide configurations are analyzed. In particular, a polarization splitter with a high extinction ratio operating at visible wavelengths based on directional couplers (DCs) in the r-TiO₂ ridge waveguide is proposed. Polarization-selective filters based on micro-ring resonators (MRRs) with resonances of only TE or TM polarizations are designed and operated. Our results show that polarization-splitters for visible wavelengths with a high extinction ratio in DC or MRR configurations can be achieved with a simple r-TiO₂ ridge waveguide structure.

Shortcut to adiabaticity in bent waveguide couplers with a sign flip of the phase mismatch

Bending the axis of a waveguide coupler can result in phase mismatch between the evanescently coupled waveguides. Using a bent waveguide coupler, we realize a shortcut to adiabatic light transfer between waveguides with a sign flip of the phase mismatch. Counterdiabatic driving with unitary transformation cancels non-adiabatic coupling in the waveguide coupler so that light evolution follows the adiabatic modes at short lengths. The counterdiabatic driving protocol is implemented in the waveguide coupler by engineering the waveguide spacing and axis bending profile. The bent waveguide coupler is compact and robust against parameter variations.

Shortcut to adiabaticity in a silicon polarization splitter rotator using multi-wavelength adiabaticity engineering

We introduce adiabaticity engineering in coupled waveguide devices to achieve shortcuts to adiabaticity in multi-wavelength systems. By engineering the adiabaticity distribution using a single control parameter, we obtain large operating bandwidth in a compact device. Multi-wavelength adiabaticity engineering is applied to the design of silicon polarization splitter-rotators. The total length of the designed polarization splitter-rotator is 141 μm, and simulations show that the device exhibits extinction ratios above 28 dB and 16 dB for the TE₀ and TM₀ modes, respectively, with a bandwidth of 300 nm (from 1.4 μm to 1.7 μm). The fabrication tolerance of the designed device is also simulated.

Mode-evolution-based ultra-broadband polarization beam splitter using adiabatically tapered extreme skin-depth waveguide

We present an ultra-broadband silicon photonic polarization beam splitter (PBS) using adiabatically tapered extreme skin-depth (eskid) waveguides. Highly anisotropic metamaterial claddings of the eskid waveguides suppress the crosstalk of transverse-electric (TE) mode, while the large birefringence of the eskid waveguide efficiently cross-couples the transverse-magnetic (TM) mode. Two eskid waveguides are adiabatically tapered to smoothly translate TM mode to the coupled port via mode evolution while keeping the TE mode in the through port. The tapered cross-section of the eskid PBS was designed numerically, achieving a large bandwidth at 1400-1650 nm with extinction ratios >20dB. We experimentally demonstrated the tapered-eskid PBS and confirmed its broad bandwidth at 1490-1640 nm, limited by laser bandwidth. With its mode evolution, the tapered-eskid PBS is tolerant to fabrication imperfections and should be crucial for controlling polarizations in photonic circuits.

Mid-infrared polarization rotator based on a Si3N4-CaF2 hybrid plasmonic waveguide with asymmetric metal claddings

In this paper, a Si₃N₄-CaF₂ hybrid plasmonic waveguide (HPW) with an asymmetric metal cladding is designed for the mid-infrared polarization rotator (PR). The mode characteristics and polarization rotation performances of the Si₃N₄-CaF₂ HPW-based PR are simulated by using the finite element method. Operating at the wavelength of 3.5 µm, the polarization conversion efficiency between two polarization modes (PM 1 and PM 2) is larger than 99% at a Si₃N₄-CaF₂ HPW length of 104 µm. The Si₃N₄-CaF₂ HPW-based PR maintains good polarization rotation performances within fabrication tolerances from -10 to 10 nm. The polarization rotator based on the Si₃N₄-CaF₂ HPW paves the way to achieve integrated waveplates, driving many important optical functions from free space onto a chip.

Adiabaticity engineering in optical waveguides

The fast quasi-adiabatic dynamics (FAQUAD) protocol has proven to be an effective approach to provide shortcuts to adiabatic light evolution in optical waveguides, resulting in short and robust devices. However, the FAQUAD approach of homogeneously distributing device adiabaticity only works for a single mode (polarization, wavelength, or spatial mode group) system. We propose an adiabaticity engineering approach to redistribute the adiabaticity of optical waveguides in multi-mode systems. By engineering the adiabaticity distribution using a single control parameter, we obtain shortcuts to adiabaticity in optical waveguides for multi-mode systems. The concept is applied to the design of a compact polarization-independent adiabatic 3-dB coupler on silicon.

Ultra-compact and high-performance polarization beam splitter assisted by slotted waveguide subwavelength gratings

We propose an ultra-short polarization beam splitter (PBS) consisting of two slot waveguides assisted by slotted waveguide subwavelength gratings (SWSWGs), located between the two slotted waveguides. By controlling the optical momentum of evanescent waves with the anisotropic characteristics of the SWSWGs, we considerably suppress and enhance the couplings of transverse-electric (TE) and transverse-magnetic (TM) modes, respectively, concurrently improving performances and reducing length of the proposed PBS, compared with conventional slotted waveguide couplers (CSWCs). Exceptionally, a transition point is found to show almost zero crosstalk between waveguides for the TE mode, i.e., infinite coupling length. Differing from conventional single-material SWGs, the SWSWGs not only simplify the fabrication process but improve polarization extinction ratio (PER). Numerical results demonstrate the improvement in PER_TM (PER_TE) from approximately 13 (23) dB for the CSWCs to 26 (24) dB for the present structure, with a > 70% reduction in device length, operating at the wavelength of λ = 1,550 nm. Our design achieves performance of PER_TM > 25 dB and PER_TE > 20 dB, and insertion loss (IL) < 0.05 dB for TE and < 0.3 dB for TM modes within a bandwidth width (BW) of ~ 50 nm from λ = 1,530 to 1,580 nm. Additionally, geometry deviation is also investigated to assess experimental tolerance. The present idea provides an approach for improving PER, device length, and operating BW of PBSs composed of various waveguide couplers.

Polarization splitting directional coupler using tilted subwavelength gratings

On-chip polarization splitters are key elements for coherent optical communication systems and polarization diversity circuits. These devices are often implemented with directional couplers that are symmetric for one polarization and strongly asymmetric for the other polarization. To achieve this asymmetry, highly dissimilar waveguides are used in each coupler arm, often requiring additional material layers or etch steps. Here we demonstrate polarization splitting with a directional coupler composed of two fully etched subwavelength waveguides, differing only in the tilt angle of the silicon segments. Our device exhibits deep-UV compatible feature sizes, is 14 µm long, and covers a 72 nm bandwidth with insertion losses below 1 dB and an extinction ratio in excess of 15 dB.

Numerical design of a high-performance polarization beam splitter assisted by composite subwavelength gratings

We report a compact polarization beam splitter (PBS) consisting of slotted waveguides assisted by composite subwavelength gratings (CSWGs) on a silicon-on-insulator platform. By tailoring the material anisotropy of the CSWGs, coupling strengths of transverse-electric (TE) and transverse-magnetic (TM) polarization coupling strengths are respectively suppressed and enhanced significantly, achieving concurrent improvements in polarization extinction ratio (PER), device footprint, and working bandwidth (BW) compared with purely slotted waveguides. Differing in construction from mono-material SWGs, the CSWGs comprise silicon strips covered with a silicon dioxide (SiO₂) layer of the same thickness as the slot layer of the slotted waveguides, simplifying the fabrication process and further reducing device length. Numerical simulations show significant improvement in PER_TM from about 15 dB for the purely slotted waveguides to 28 dB for the proposed design, with a 40% reduction in device length at a wavelength of λ = 1550 nm. Within a BW of ∼60 nm, the proposed PBS achieves PER_TM ∼25 dB, PER_TE >15 dB, and insertion loss (ILs) <0.1 dB for TE and TM modes. Fabrication tolerance investigations are also described and discussed. The proposed idea paves the way for simultaneous improvements in PER, footprint, and working BW for PBSs comprising a variety of coupled-waveguide systems.

Numerical investigations of an ultra-compact polarization beam splitter based on augmented low-index guiding and subwavelength grating structures

We report the design of an ultra-compact polarization beam splitter with high performance that is based on augmented low-index guiding and subwavelength grating (SWG) structures. The transverse-electric (TE) and transverse-magnetic (TM) modes are confined in high-index silicon (Si) and low-index silicon nitride (Si₃N₄), respectively. They are separated by using, respectively, a gradually curved Si waveguide and a Si₃N₄ SWG structure with optimal grating-element. The footprint of the proposed polarization beam splitters (PBS) is 2.9 × 2.25 μm². The device offers high polarization extinction ratios (PERs) of ~18 dB for the two polarizations, with low insertion losses of ~0.22 dB (~0.71 dB) for the TE (TM) mode at the wavelength of 1550 nm. Over the broad band from λ = 1500-1650 nm, the PERs of the TE and TM modes are above 17 and 16 dB, respectively. By narrowing the operating band to the range from λ = 1500 to 1600 nm, the proposed PBS provides PERs of >17 dB for both polarizations. Finally, the fabrication tolerance of the designed PBS is also addressed and discussed in detail.

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.

Submicron-scale broadband polarization beam splitter using CMOS-compatible materials

We propose a polarization beam splitter (PBS) with a footprint of only 600 × 790 nm² operating at a wavelength of λ = 1550 nm, which is the smallest PBS ever demonstrated. This device uses CMOS-compatible materials, namely, silicon and silica. The present PBS comprises two Si waveguides with different geometrical aspect ratios adjoined side-by-side, which separates the transverse-electric (TE) and transverse-magnetic (TM) modes without relying on an additional coupling region. The designed PBS achieves a polarization extinction ratio of approximately 25 dB for both modes and insertion losses of approximately 0.87 and 1.09 dB for the TE and TM polarizations, respectively. Over a wide bandwidth of 150 nm (from λ = 1475–1625 nm), a high polarization extinction ratio (greater than 20 dB) and a low inversion loss (lower than 1.3 dB) can be obtained. The proposed PBS allows for geometrical errors of ±15 nm while maintaining a polarization extinction ratio of >20 dB and inversion losses of >1.1 and 1.3 dB for the TE and TM modes, respectively. With the submicron footprint, the reported PBS may be able to be used in high-density photonic integrated circuits and nanophotonic devices.

Realization of an ultra-compact polarization beam splitter using asymmetric MMI based on silicon nitride / silicon-on-insulator platform

We have experimentally demonstrated a compact polarization beam splitter (PBS) based on the silicon nitride/silicon-on-insulator platform using the recently proposed augmented-low-index-guiding (ALIG) waveguide structure. The two orthogonal polarizations are split in an asymmetric multimode interference (MMI) section, which was 1.6 μm wide and 4.8 μm long. The device works well over the entire C-band wavelength range and has a measured low insertion loss of less than 1 dB. The polarization extinction ratio at the Bar Port is approximately 17 dB and at the Cross Port is approximately 25 dB. The design of the device is robust and has a good fabrication tolerance.

Mode-evolution-based coupler for high saturation power Ge-on-Si photodetectors

We propose a mode-evolution-based coupler for high saturation power germanium-on-silicon photodetectors. This coupler uniformly illuminates the intrinsic germanium region of the detector, decreasing saturation effects, such as carrier screening, observed at high input powers. We demonstrate 70% more photocurrent generation (9.1-15.5 mA) and more than 40 times higher opto-electrical bandwidth (0.7-31 GHz) than conventional butt-coupled detectors under high-power illumination. The high-power and high-speed performance of the device, combined with the compactness of the coupling method, will enable new applications for integrated silicon photonics systems.

Shortcuts to adiabaticity in optical waveguides using fast quasiadiabatic dynamics

We propose a fast quasiadiabatic approach to the design of optical waveguide devices. This approach distributes the system adiabaticity homogeneously over the device length, thus providing a shortcut to adiabaticity at a shorter device length. A mode sorting asymmetric Y junction is designed by redistributing the adiabaticity of a conventional linearly separating Y junction. Simple procedures for the design of fast quasiadiabatic devices are outlined, and the designed Y junction features large bandwidth at a shorter length than the conventional linearly separating Y junction. The proposed device is verified with beam propagation simulations. A mode conversion efficiency of larger than 99% is observed for the designed Y junction over a 220 nm range.

Ultrashort broadband polarization beam splitter based on a combined hybrid plasmonic waveguide

We propose an ultracompact broadband polarization beam splitter (PBS) based on a combined hybrid plasmonic waveguide (HPW). The proposed PBS separates transverse-electric (TE) and transverse-magnetic (TM) modes using a bent lower HPW with vertical nanoscale gaps and a straight upper HPW with a horizontal nanoscale gap, respectively, without relying on an additional coupling region. This design considerably reduces the length of the PBS to the submicron scale (920 nm, the shortest PBS reported to date) while offering polarization extinction ratios (PERs) of ~19 dB (~18 dB) and insertion losses (ILs) of ~0.6 dB (~0.3 dB) for the TE (TM) mode over an extremely broad band of 400 nm (from λ = 1300 nm to 1700 nm, covering entirely second and third telecom windows). The length of the designed PBS can be reduced further to 620 nm while still offering PERs of 15 dB, realizing a densely photonic integrated circuit. Considering the fabrication tolerance, the designed PBS allows for large geometrical deviations of ±20 nm while restricting PER variations to within 1 dB, except for those in the nanoscale gaps smaller than 10nm. Additionally, we also address the input and ouput coupling efficiencies of the proposed PBS.

Ultra-compact polarization beam splitter utilizing a graphene-based asymmetrical directional coupler

A novel ultra-compact polarization beam splitter (PBS) utilizing an asymmetrical directional coupler with a combination of a silicon waveguide (SW) and a graphene multilayer embedded silicon waveguide (GMESW) has been proposed and investigated. The modal characteristics of the GMESW for the TM mode varies significantly, whereas that for the TE mode changes slightly with respect to the SW, inducing the launched TM mode to directly pass through the SW with little influence from the GMESW, while the TE mode undergoes a strong coupling and is transferred to the GMESW. A designed PBS with an 8.3 μm-long coupler and 200 nm-wide gap separation offers high extinction ratios (18.2 and 21.2 dB) and low insertion losses (0.16 and 0.36 dB) for the thru and cross ports, respectively. The presented PBS also presents the ability to work with variable splitting ratio power for the TM mode by varying the chemical potential of graphene, implying various applications in signal processing on a chip.

Compact and broadband polarization beam splitter based on a silicon nitride augmented low-index guiding structure

We propose a compact polarization beam splitter (PBS) based on a silicon nitride enhanced silicon-on-insulator platform using an augmented low-index guiding (ALIG) waveguide structure. In the ALIG structure, the TM mode is mostly confined in the low-index silicon nitride, while the TE mode is confined in the high-index silicon. Since the two modes are confined in two separate layers, their properties can be controlled independently. The PBS is formed using an asymmetric multimode interference (MMI) section. The TM mode is directed to an output port by the ALIG waveguide, while the TE mode is coupled to the other output port via the multimode interferometer. Such a PBS has a very small footprint, low insertion loss, high polarization extinction ratio, and broadband response.

Mode-evolution-based polarization rotation and coupling between silicon and hybrid plasmonic waveguides

Hybrid plasmonic (HP) modes allow strong optical field confinement and simultaneously low propagation loss, offering a potentially compact and efficient platform for on-chip photonic applications. However, their implementation is hampered by the low coupling efficiency between dielectric guided modes and HP modes, caused by mode mismatch and polarization difference. In this work, we present a mode-evolution-based polarization rotation and coupling structure that adiabatically rotates the TE mode in a silicon waveguide and couples it to the HP mode in a strip silicon-dielectric-metal waveguide. Simulation shows that high coupling factors of 92%, 78%, 75%, and 73% are achievable using Ag, Au, Al, and Cu as the metal cap, respectively, at a conversion length of about 5 μm. For an extremely broad wavelength range of 1300–1800 nm, the coupling factor is >64% with a Ag metal cap, and the total back-reflection power, including all the mode reflections and backscattering, is below −40 dB, due to the adiabatic mode transition. Our device does not require high-resolution lithography and is tolerant to fabrication variations and imperfections. These attributes together make our device suitable for optical transport systems spanning all telecommunication bands.

Wideband silicon photonic polarization beamsplitter based on point-symmetric cascaded broadband couplers

We design and demonstrate a wideband silicon photonic polarization beamsplitter on a silicon-on-insulator platform. The device consists of two 3 dB broadband couplers cascaded in a point-symmetric network. The transverse electric (TE) and transverse magnetic (TM) modes are coupled to different output ports due to a large difference between their coupling strengths. The device exhibits large isolation at both the two output ports, of more than 20 dB over a large bandwidth of 125 nm, and a small excess loss, of less than 0.5 dB for the entire C-band.

Optimization of adiabaticity in coupled-waveguide devices using shortcuts to adiabaticity

Conventional strategies to design adiabatic coupled-waveguide devices focus on optimizing the system adiabaticity but can only guarantee 100% efficiency at specific lengths. We establish a simple technique allowing the optimization of device adiabaticity and ensuring 100% coupling/conversion efficiency at any physically realizable length. Specifically, we use the shortcuts-to-adiabaticity technique to represent the system state precisely and engineer the system evolution to be as close to the adiabatic state as possible. Smooth parameters are derived for coupled-waveguide devices, which feature good robustness against wavelength and fabrication variations at the same time. The proposed device is verified with beam propagation simulations.

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.

Polarization rotation Bragg grating using Si wire waveguide with non-vertical sidewall

We report polarization independent Bragg grating wavelength filter using polarization rotation. A non-vertical waveguide sidewall and antisymmetric grating structure can be used to generate the polarization rotation of the fundamental modes. The diffraction efficiencies and peaks becomes the same for two orthogonal input polarizations. The concept was verified by simulation and experiment.

Robust coupled-waveguide devices using shortcuts to adiabaticity

Shortcuts to adiabaticity, originally developed in the context of quantum control, are powerful tools for the design of high coupling efficiency, robust, and short coupled-waveguide devices. The counterdiabatic protocol cancels the unwanted coupling in system evolution by the addition of a counterdiabatic term. The invariant-based inverse-engineering approach designs system evolution using the decoupled eigenstates of the Lewis-Riesenfeld invariant. The single-shot shaped-pulse technique directly parameterizes the solution of the coupled-mode equation. Starting from the counterdiabatic protocol, we show that these seemingly very different shortcuts to adiabaticity techniques are equivalent in the framework of coupled-waveguide systems. When combined with perturbation treatment of the coupled-mode equation, robust coupled-waveguide devices against errors can be obtained.

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.

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.

Polarization rotator-splitters in standard active silicon photonics platforms

We demonstrate various silicon-on-insulator polarization management structures based on a polarization rotator-splitter that uses a bi-level taper TM0-TE1 mode converter. The designs are fully compatible with standard active silicon photonics platforms with no new levels required and were implemented in the IME baseline and IME-OpSIS silicon photonics processes. We demonstrate a polarization rotator-splitter with polarization crosstalk < -13 dB over a bandwidth of 50 nm. Then, we improve the crosstalk to < -22 dB over a bandwidth of 80 nm by integrating the polarization rotator-splitter with directional coupler polarization filters. Finally, we demonstrate a polarization controller by integrating the polarization rotator-splitters with directional couplers, thermal tuners, and PIN diode phase shifters.

Design of a SiO₂ top-cladding and compact polarization splitter-rotator based on a rib directional coupler

A compact polarization splitter-rotator based on a silicon-on-insulator rib asymmetrical directional coupler with SiO2 top-cladding is proposed. Unlike previously reported PSRs which specifically required the top-cladding material to be different from the bottom cladding in order to break the symmetry of the waveguide cross-section, our proposed PSR has no such limitation on the top-cladding due to the horizontal asymmetry of the rib waveguide. In addition, the device is highly compact and has a total length as short as 24 μm. Numerical simulation shows that a high conversion efficiency of ~97% is obtained at the wavelength of 1550 nm. With the width variation of ± 15 nm and the gap variation of ± 50 nm, the PSR still has high ER of 12 dB at the cross-port, showing large fabrication tolerance. This device can be cascaded to improve the performance at the through port and an example of a two-stage PSR is presented. The mode conversion between the strip waveguide and the rib waveguide is also discussed.

A plasmonic based ultracompact polarization beam splitter on silicon-on-insulator waveguides

An ultracompact polarization beam splitter (PBS) is designed on silicon-on-insulator (SOI) platform based on the localized surface plasmons (LSPs) excited by particular polarization light. The device uses nanoscale silver cylinders as the polarization selection between two silicon waveguides of a directional coupler. The transverse-magnetic (TM) polarization light excites localized surface plasmons and is coupled into the cross port of the directional coupler with a low insert loss, while the transverse-electric (TE) polarization light is under restriction. The PBS has a coupling layer with 50 nm width and 1.1 μm length supporting broadband operation. The simulation calculations show that 22.06dB and 23.06dB of extinction ratios for the TE and TM polarizations were obtained, together with insertion losses of 0.09dB and 0.40dB.

Four-port integrated polarizing beam splitter

In this Letter, we report on the first integrated four-port polarizing beam splitter. The device operates on the principle of mode evolution and was implemented in a silicon-on-insulator silicon photonics platform and fabricated on a 300 mm CMOS line using 193 nm optical immersion lithography. The adiabatic transition forming of the structure enabled over a 150 nm bandwidth from λ~1350 to λ~1500  nm, achieving a cross-talk level below -10  dB over the entire band.

Counterdiabatic mode-evolution based coupled-waveguide devices

The goal in designing mode-evolution based devices is to realise short and high-fidelity components. The counterdiabatic protocol in coherent quantum state control can be used to cancel unwanted coupling between adiabatic modes in mode evolution but is not directly realisable in the coupled-waveguide system. By finding alternative coupled-mode equations that links to the same interaction picture dynamical equation as the counterdiabatic protocol via unitary transformations, we have derived a universal formalism for the design of short and high-fidelity mode-evolution based coupled-waveguide devices. Starting from a traditional adiabatic device design, the counterdiabatic protocol leads to a high-fidelity device, with its evolution following the adiabatic modes exactly even when the adiabatic condition is violated. Tolerance analysis shows that the countera-diabatic devices combine the advantages of adiabatic and resonant devices. The formalism is used to design asymmetric waveguide couplers.

Polarization splitter using horizontal slot waveguide

In this paper a compact and efficient polarization splitter using horizontal slotted waveguides is presented. The splitter is designed by finite-difference-time-domain simulation and realized experimentally. The splitter is built by using a direction coupler consisting of two horizontal slotted waveguides and achieves a high extinction ratio of 14.1 and 16.8 dB for cross and through ports. The optimal coupling length is found to be 15 µm. The device exhibits a good response of extinction ratio across C + L broadband. The splitter obtained is readily used for a polarization diversity circuit, particularly for platforms with horizontal slot waveguides.

Adiabatic microring modulators

In this work, we demonstrate and experimentally characterize a new class of high-performance silicon photonic modulators-the adiabatic microring modulator. The adiabatic microring modulator utilizes a vertical PN junction and interior electrical contacts, leveraging all the advantages of previously-demonstrated microdisk modulators. However, this device also incorporates an adiabatic transition from the wide, multimode contact region, to a narrow, single-mode coupling region, eliminating unwanted spatial modes common to microdisks. As a result, the adiabatic microring modulator demonstrated in this work is the smallest microring modulator demonstrated to date, with a diameter of only 4 μm, yielding a 6.92-THz uncorrupted free spectral range. Here, we perform an experimental comparative analysis between silicon adiabatic microring modulators, silicon microdisk modulators, and a commercial lithium-niobate Mach-Zehnder modulator. We show that the silicon adiabatic microring modulator using partial doping is capable of operating at 12.5-Gb/s data rates and beyond. This device combines the best of all modulator designs, leveraging the depletion-based method to maximize the speed, utilizing the vertical-junction configuration to minimize the power consumption, employing a unique adiabatic design to eliminate higher-order modes, and using partial doping to reduce resistance, further enhancing the speed of the device.

Ultracompact polarization splitter-rotator based on an asymmetric directional coupler

An ultracompact polarization splitter-rotator based on an asymmetric directional coupler has been designed and analyzed. The device consists of one channel waveguide and one vertical slot waveguide, which couple to each other to form the asymmetric directional coupler. The coupling length is only 17.4 μm, and the polarization extinction ratio, with its largest value of 18 dB, is more than 15 dB in the operating wavelength range from 1515 to 1560 nm. The TM-to-TE conversion efficiency can reach up to 95% (0.02 dB) at 1550 nm wavelength.

Mode-evolution-based polarization rotator-splitter design via simple fabrication process

A mode-evolution-based polarization rotator-splitter built on InP substrate is proposed by combining a mode converter and an adiabatic asymmetric Y-coupler. The mode converter, consisting of a bi-level taper and a width taper, effectively converts the fundamental TM mode into the second order TE mode without changing the polarization of the fundamental TE mode. The following adiabatic asymmetric Y-coupler splits the fundamental and the second order TE modes and also converts the second order TE mode into the fundamental TE mode. A shallow etched structure is proposed for the width taper to enhance the polarization conversion efficiency. The device has a total length of 1350 µm, a polarization extinction ratio over 25 dB and an insertion loss below 0.5 dB both for TE and TM modes, over the wavelength range from 1528 to 1612 nm covering all C + L band. Because the device is designed based on mode evolution principle, it has a large fabrication tolerance. The insertion loss remains below 1 dB and the polarization extinction ratio remains over 17 dB with respect to a width variation of +/- 0.12 µm at the wavelength of 1570 nm, or +/- 0.08 µm over the entire C + L band.

Broadband integrated polarization beam splitter with surface plasmon

A broadband integrated waveguide polarization beam splitter consisting of a metal nanoribbon and two dielectric waveguides is proposed and numerically investigated. This surface plasmon based device provides a unique approach for polarization sensitive manipulation of light in an integrated circuit and will be essential for future classical and quantum information processes.

Silicon polarization independent microring resonator-based optical tunable filter circuit with fiber assembly

We report the design, fabrication, photonic packaging and the characterization of a silicon polarization independent optical tunable filter circuit with fiber assembly. We demonstrate the polarization transparent filter characteristics with an insertion loss of ~13.4 dB, an extinction ratio of ~20 dB, and a 3 dB bandwidth of 0.2 nm. The tuning range is of ~11.72 nm, along with the tuning speed of less than 400 μs. The tuning efficiency is ~0.23 nm/mW. The use of polarization diversity scheme and the silicon photonic packaging bridges the gap between the silicon photonic circuits and the real applications.

A tunable polarization diversity silicon photonics filter

A tunable polarization diversity silicon waveguide based optical filter was demonstrated. With the polarization diversity scheme, less than 0.5dB polarization dependent loss of the silicon optical filter was achieved in the wavelength range from 1525nm to 1600nm. The insertion loss of the whole polarization diversity circuits is 6.3dB. The extinction ratio of the optical filter is more than 27dB.

Silicon waveguide based TE mode converter

A silicon waveguide based TE mode converter was designed for the mode conversion between a horizontal waveguide and vertical waveguide in the two-layer structure waveguide based polarization diversity circuit. The TE mode converter's performance was studied. The polarization mode converter with minimum length of 5 μm was demonstrated to provide the TE mode conversion while maintaining the polarization status. The insertion loss at the transition region was less than 2 dB.

Adiabatic microring resonators

A class of whispering-gallery-mode resonators, herein referred to as adiabatic microring resonators, is proposed and numerically demonstrated. Adiabatic microrings enable electrical and mechanical contact to be made to the resonator without inducing radiation, while supporting only a single radial mode and therein achieving an uncorrupted free spectral range (FSR). Rigorous finite-difference time-domain simulations indicate that adiabatic microrings with outer diameters as small as 4 μm can achieve resonator quality factors (Qs) as high as Q = 88,000 and an FSR of 8.2 THz, despite large internal contacts.

Control of optical mode properties in cross-slot waveguides

A cross-slot waveguide geometry provides high confinement of the mode field of both fundamental quasi-TE and quasi-TM modes in geometrically perpendicular slots. A unique possibility to tailor optical mode characteristics such as effective index and confinement factor quite independently for the two polarizations with geometric and material parameters is shown. Nonbirefringent cross-slot geometries are presented. Fabrication related tolerances of the cross-slot geometry for low birefringence operation are studied. Means to externally tune the birefringence by a thermo-optic effect is also analyzed. Fabrication of a cross-slot waveguide test structure is demonstrated.

Study of asymmetric silicon cross-slot waveguides for polarization diversity schemes

We study cross-slot waveguides for polarization diversity schemes that simultaneously offer strong confinement in the slot region for both TE and TM polarizations. A symmetric configuration is initially presented to demonstrate that the same strong confinement and propagation constants can be obtained for both polarizations and slot thicknesses down to 50 nm. To make easier further realization of these waveguides, an asymmetric waveguide configuration with a vertical slot height of up to 120 nm is proposed. The waveguide parameters are then optimized taking into account a 220 nm silicon thickness. Our simulation results show that no beating issues between TE and TM modes is observed for beating lengths under 6.68 microm, thus opening the way to efficient implementations of polarization diversity approaches.

On-chip silicon photonic wavelength control of optical fiber lasers

Tunable silicon microring filters are used to demonstrate CMOS-compatible on-chip wavelength control of Er(+) doped fiber-lasers. An onchip Ni-Cr micro-heater consuming up to 20 mW is capable of tuning the Si microring filter by 1.3 nm with a lasing linewidths narrower than 0.02 nm. This approach enables arbitrary multiple wavelength generation on a silicon chip. Possible applications include on-chip and chip-to-chip dense-wavelength division multiplexed communications and sensor interrogation.

Polarization-independent optical directional coupler based on slot waveguides

A polarization-independent optical directional coupler based on slot waveguides is proposed and analyzed by using rigorous full-vectorial analysis methods based on a finite-element scheme. Properly choosing materials and structural parameters makes the coupling length for quasi-TE modes become equal to that for quasi-TM modes. Tolerances to operating wavelength and structural parameters are also discussed. The proposed coupler can be used for highly integrated optical circuits without polarization diversity schemes.