Copper nanorod array assisted silicon waveguide polarization beam splitter

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.

Polarization beam splitter based on the asymmetric directional coupler of lithium niobate film

A polarization beam splitter (PBS) based on a lithium niobate film asymmetric directional coupler is proposed. The PBS is located on a lithium niobate platform on an insulator consisting of a silicon nitride-lithium niobate waveguide (SLW) and a lithium niobate waveguide (LNW). By rationally designing the SLW and LNW sizes, TE polarization satisfies the phase matching condition and TM polarization phase mismatch. The numerical simulation results show that the extinction ratio (ER) and insertion loss (IL) of PBS for TE mode are 30.57 and 0.66 dB, respectively, and the ER and IL of PBS for TM mode are 28.15 and 0.11 dB, respectively, at an operating wavelength of 1.55 µm.

Ultracompact and ultrabroadband mode division multiplexer based on an Au nanocube array assisted directional coupler

An ultracompact and ultrabroadband two-mode (de)multiplexer based on an asymmetric directional coupler for mode division multiplexing is proposed. The device structure consists of a pair of silicon waveguides with an array of plasmonic Au nanocubes sandwiched in the coupling region. The coupling region length of the directional coupler is decreased to 1 µm for coupling of the fundamental transverse magnetic (TM) mode to the first order mode by excitation of the surface plasmon polaritons. This is the shortest length reported for multiplexing of the TM modes until now, to the best of our knowledge. The proposed mode (de)multiplexer has a low loss of 0.72 dB and low crosstalk of -28.3dB at the communication wavelength of 1.55 µm. Also, the 3D finite-difference time-domain simulation results show that a broad bandwidth of 190 nm is realized with crosstalk less than -10dB and the insertion loss lower than 1.29 dB. Furthermore, impact of the fabrication tolerances on the performance of the proposed (de)multiplexer is studied in detail.

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.

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.

Ultra-broadband high-performance polarizing beam splitter on silicon

A high-performance silicon-based polarizing beam splitter (PBS) is proposed and demonstrated experimentally by using an improved structure with cascaded bent directional couplers. The measured extinction ratio (ER) is >35dB and the excess losses (EL) is <0.35dB around the central wavelength for both polarizations. The present PBS has a compact footprint of ~6.9 × 20μm². The measured bandwidths for an ER of >20dB, >25dB and >30dB are ~135nm, ~95nm and ~70nm, respectively, while the measured EL is <1dB and <0.5dB in a bandwidth of ~140nm and ~85nm, respectively. The fabrication tolerance of the core-width variation is as large as ± 40nm, which makes the fabrication very easy.

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.

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.

Polarization rotation and coupling between silicon waveguide and hybrid plasmonic waveguide

We present a polarization rotation and coupling scheme that rotates a TE(0) mode in a silicon waveguide and simultaneously couples the rotated mode to a hybrid plasmonic (HP(0)) waveguide mode. Such a polarization rotation can be realized with a partially etched asymmetric hybrid plasmonic waveguide consisting of a silicon strip waveguide, a thin oxide spacer, and a metal cap made from copper, gold, silver or aluminum. Two implementations, one with and one without the tapering of the metal cap are presented, and different taper shapes (linear and exponential) are also analyzed. The devices have large 3 dB conversion bandwidths (over 200 nm at near infrared) and short length (< 5 μm), and achieve a maximum coupling factor of ∼ 78% with a linearly tapered silver metal cap.

Low-loss ultracompact transverse-magnetic-pass polarizer with a silicon subwavelength grating waveguide

An ultracompact and low-loss TM-pass polarizer on silicon is proposed and demonstrated experimentally with a subwavelength-grating (SWG) waveguide. The SWG waveguide is designed to support Bloch mode for TM polarization so that the incident TM-polarized light goes through the SWG waveguide with very low excess loss. On the other hand, for TE polarization, the SWG waveguide works as a Bragg reflector, and consequently the incident TE-polarized light is reflected. For a fabricated ∼9  μm long polarizer (with the period number N=20), the measured extinction ratio is ∼27  dB and the excess loss is ∼0.5  dB at the central wavelength 1550 nm. The bandwidth to achieve an extinction ratio of 20 dB is about 60 nm (from 1520 to 1580 nm). When increasing the period number to N=40, the measured extinction ratio is up to 40 dB (which is not as high as the expected theoretical value 65 dB due to the limit of the measurement system).