Polarization Splitter And Rotator Research Articles

Low-loss and broadband polarization-diversity edge coupler on a thin-film lithium niobate platform.

Fiber-to-chip coupling is an essential issue for taking high-performance integrated photonic devices into practical applications. On a thin-film lithium niobate platform, such a high-performance coupler featuring low loss, large bandwidth, and polarization independence is highly desired. However, the mode hybridization induced by the birefringence of lithium niobate seriously restricts a polarization-independent coupling. Here, we propose and experimentally demonstrate a high-performance and polarization-diversity cantilever edge coupler (EC) with the assistance of a two-stage polarization splitter and rotator (PSR). The fabricated cantilever EC shows a minimal coupling loss of 1.06 dB/facet, and the fully etched PSR structure shows a low insertion loss (IL) of -0.62 dB. The whole polarization-diversity cantilever EC exhibits a low IL of -2.17 dB and -1.68 dB for TE0 and TM0 mode, respectively, as well as a small cross talk of <-15 dB covering the wavelength band from 1.5 µm to 1.6 µm. A polarization-dependent loss <0.5 dB over the same wavelength band is also obtained. The proposed fiber-to-waveguide coupler, compatible with the fabrication process of popular thin-film lithium niobate photonic devices, can work as a coupling scheme for on-chip polarization-diversity applications.

A Monolithic Polarization Controller and Tracking Loop for Optical Interconnect Demonstrated on a 90 nm Silicon CMOS-Photonic Platform

We report, to the best of our knowledge, the first monolithically-integrated silicon CMOS-photonic co-designed polarization tracking circuit implemented in a 90 nm CMOS SOI process. The Polarization Splitter and Rotator (PSR) decompose the optical input into TE and TM component and rotate the TM component to TE mode. The directional couplers combine the two TE components from PSR into single optical output. The thermal phase shifters on the optical path maximize the final optical output. A feedback loop implemented with monitoring photodetector, current mode ADC and Finite State Machine (FSM), control the current mode DACs to adjust the current input of thermal phase shifter. The circuit tracks randomized polarization rotation across the Poincaré sphere, maximizes TE-mode optical power, and experimentally achieves a BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-11}$</tex-math></inline-formula> with 10 Gb/s PAM-2 modulation.

Ultracompact silicon-based polarization splitter and rotator based on asymmetric directional couplers with subwavelength gratings

An ultrasmall polarization splitter and rotator (PSR) on a silicon-on-insulator (SOI) platform is proposed based on an asymmetric directional coupler (ADC) consisting of a bridged subwavelength grating (BSWG) and a strip silicon waveguide with another SWG embedded in the middle slot. The inputting TE mode is tightly confined in BSWG and directly passes through a bar port with little coupling to the adjacent waveguide because of the phase mismatch, while the phase-matching condition allows the injected TM mode to be evanescently decoupled from BSWG and simultaneously converted into the TE mode supported by a SWG slot waveguide before propagating out through the cross port. Both polarization-dependent behaviors within the proposed PSR can be readily realized and efficiently manipulated by independently controlling the attributes of the ADC waveguides, which are distinctly engineered by different SWG segments. We have verified what we believe, to the best of our knowledge, is the first combination of BSWG and a SWG slot waveguide that provides an additional degree of freedom in the design and a stronger modal hybridization due to the doubled effects from jointly used SWGs. Further, the increasing number of inserted SWGs in the present device drastically reduces the device footprint, leading to a coupling length of only 5.5 µm. From the results, the conversion loss of − 0.28 d B (polarization conversion ratio of 93.7%) has been obtained for a TM-to-TE conversion at 1.55 µm. The TM mode of the resulting PSR shows its cross talk is less than − 14.3 d B and the conversion loss is better than − 0.63 d B in a 95 nm bandwidth that has covered the entire C and L bands. The cross talk and insertion loss of the TE mode are lower than − 29 d B and − 0.42 d B , respectively, as the operating wavelength varies.

Demonstration of a silicon polarization splitter and rotator based on a bow-tie structure

We propose and demonstrate a polarization splitter and rotator (PSR) operating in the range of 1530–1590 nm, built on the SOI platform and based on a new bow tie converter TM0-to-TE1. Comparing with other compact (<100 µm length) state-of-the-art photonic integrated devices, the proposed device exhibits simultaneously the lowest crosstalk and penalty to fabrication errors, paving the way for high-volume photonic integration.

Fabrication-tolerant and CMOS-compatible polarization splitter and rotator based on a compact bent-tapered directional coupler

In this paper, we demonstrate a broadband, low-loss, compact, and fabrication-tolerant polarization splitter and rotator (PSR) on a silicon-on-insulator platform. The PSR is based on an asymmetric directional coupler (ADC), which is covered with SiO2 from the top to make it compatible with the standard metal back end of line (BEOL) process. Conventional ADC-based PSRs suffer from stringent fabrication requirements and relatively low bandwidth, while the proposed bent-tapered design is highly insensitive to the fabrication errors (＞70 nm tolerance on the coupling gap) with an enlarged bandwidth and a compact footprint of 53 µm × 7 µm. It yields a polarization conversion loss less than 0.7 dB, a transverse electric (TE) insertion loss better than 0.3 dB, an ultra-low crosstalk with the TE extinction better than 30 dB, and the transverse magnetic extinction better than 25 dB, over a 200 nm wavelength range (1.5 µm–1.7 µm), in both ports. At the 1.55 µm wavelength, the calculated ultra-low polarization conversion loss and TE insertion loss are 0.27 dB and 0.08 dB, respectively.

Function-expansion-based topology optimization of three-dimensional optical waveguide devices with multi-layered structure considering layer thickness

So far, as an optimal design method for 2D and 3D design of optical waveguide devices, topology optimization based on a function expansion method has been reported. In this paper, we improve the conventional function-expansion-based topology optimization method to design three-dimensional photonic devices with structural variation in the depth direction. In order to design realistic 3D optical waveguide devices, we consider a layered structure and design each layer by using function-expansion-based topology optimization method. In this approach, undesired floating structures are avoided and layer thickness can be optimized. The validity of this method is verified through design examples of a polarization splitter and rotator (PSR) and a crossing waveguide which have two-layer structure.

Demonstration of a 120° hybrid based simplified coherent receiver on SOI for high speed PON applications.

We demonstrate the first simplified coherent receiver using a 120° hybrid on silicon-on-insulator (SOI) for high speed PON applications. This coherent receiver integrates an inverse taper edge coupler for the received signal, a vertical grating coupler for the local oscillator input, a polarization splitter and rotator (PSR), a 120° hybrid based on a 3×3 multimode interference (MMI) coupler, and three germanium photodetectors. We achieved 25 Gbit/s two-level pulse amplitude modulation (PAM-2) transmission over 30 km standard single mode fiber (SMF) in the C-band without any digital signal processing (DSP) (e.g., pre-emphasis, pulse shaping, equalization, nonlinearity compensation) and dispersion compensation (e.g., optical or digital) either at the transmitter or at the receiver. The requirements for frequency and phase locking of the local oscillator (LO) were avoided due to the use of intensity modulated signals. Receiver sensitivities of -23.70 dBm, -20.30 dBm, and -15.10 dBm are achieved at a bit error rate (BER) below the hard-decision forward error correction (HD-FEC) threshold (i.e., 3.8 × 10-3) in back-to-back (B2B), after 21 km and 30 km, respectively. We also demonstrate 25 Gbit/s PAM-4 transmission achieving receiver sensitivities of -15.30 dBm, -13.90 dBm, and -9.50 dBm below the HD-FEC threshold in B2B, after 10.5 km and 21 km, respectively.

Demonstration of an on-chip broadband polarization splitter and rotator using counter-tapered coupler

Abstract A broadband and fabrication-tolerant polarization splitter and rotator (PSR) using a counter-tapered coupler is proposed and demonstrated on silicon-on-insulator (SOI) platform. Compared to previously reported PSRs based on a conventional directional coupler, which suffer from relatively low bandwidth and stringent fabrication requirements, the introduced counter-tapered structure can be used to enlarge the bandwidth and fabrication tolerance. The proposed PSR can be fabricated through a single step of lithography and etching, with a total length of 170 μ m . The measured results show that the fundamental transverse magnetic (TM0) to fundamental transverse electric (TE0) polarization conversion loss is less than 0.8 dB, while the TE0-TE0 insertion loss is less than 0.5 dB, over the wavelength range from 1520 to 1580 nm. The polarization crosstalk is lower than -20/-18 dB for TE0/TM0 mode, respectively.

Fabrication-Tolerant Polarization Splitter and Rotator Based on Slanted Silicon Waveguides

A fabrication-tolerant and highly efficient polarization splitter and rotator (PSR) based on the slanted silicon waveguides is proposed and optimized by using the full-vectorial finite element method and three-dimensional full-vectorial finite difference time domain method. The numerically simulated results show that the proposed PSR is with ultra-low losses and with the high PERs of 19.92 and 40 dB at the cross- and through-ports, respectively. The fabrication tolerances are also studied for the proposed PSR.

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.

Experimental realization of an O-band compact polarization splitter and rotator

We experimentally realize a compact wideband polarization splitter and rotator (PSR) with CMOS compatibility. The fabricated PSR is then tested by utilizing a fabrication-tolerant TE-pass on-chip polarizer we propose to practically solve the issue of accurately aligning the polarizations in fiber and modes on chip. Both of these polarization handling devices take the advantage of bend structure that confines TE mode better than TM mode. The fabricated PSR has a high TM-TE and TE-TE mode conversion efficiency of -0.4 dB and -0.2 dB at 1310 nm, while the extinction ratio is better than 18 dB and the broad bandwidth exceeds 100 nm.

Compact highly-efficient polarization splitter and rotator based on 90° bends.

We propose a compact highly-efficient CMOS-compatible polarization splitter and rotator (PSR) with a wide bandwidth covering the whole O-band. It benefits from the different confinement capability of TE and TM modes in bend structure. This bend structure helps shorten the PSR and maintain high efficiency, achieving the bending, polarization splitting, rotating of light beam at the same time. Numerical simulations utilizing Lumerical 3-D FDTD solutions demonstrate that the present PSR has a high TM-TE conversion efficiency of -0.11 dB and high TE-TE conversion efficiency of -0.09 dB at 1310 nm, while the extinction ratio is 27.36 dB and 30.61 dB respectively.

A Polarization Splitter and Rotator Based on a Partially Etched Grating-Assisted Coupler

A fabrication-tolerant mid-infrared silicon polarization splitter and rotator (PSR) based on a partially etched grating-assisted coupler is proposed. The design of the partially etched structure allows to use different cladding layers, such as SiO2, to make the device compatible with the metal back-end of line process. Moreover, by using the grating-assisted coupler, the device is no longer limited by the precise requirement of the coupling length and strength as those in its counterparts based on directional couplers. The simulation results show that the PSR can work over a wide spectral range of 50 nm around the mid-infrared wavelength of $2.5~\mu \text{m}$ with the typical transverse electric (TE) to transverse magnetic (TM) polarization conversion efficiency of 96.83%, the conversion loss of −0.97 dB, and the polarization crosstalk of −21.48 dB. The TM-to-TM through insertion loss is around −0.76 dB. The effects of the fabrication errors are analyzed. The numerical simulation results demonstrate that the device has a good fabrication tolerance larger than 45 nm.

Polarization splitter and rotator with subwavelength grating for enhanced fabrication tolerance.

We propose a novel method to implement a compact and fabrication-tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform. The PSR consists of a silicon wire waveguide coupled to a subwavelength grating (SWG) waveguide in an asymmetrical directional coupler. The SWG effect allows an additional degree of design freedom to engineer the equivalent material refractive index. This is advantageously used to effectively compensate for fabrication inaccuracies in PSRs. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.13 dB (a conversion efficiency of 97%) at the wavelength of 1550nm, and better than -0.4 dB conversion loss over the entire C-band wavelength range. Compared to the PSRs made of conventional wire waveguides, the use of SWG index engineering improves the waveguide width fabrication tolerance substantially, from ±3 nm to ±40 nm. A compact device size with a coupling length of 25μm is achieved.

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.

High-Efficiency Low-Crosstalk 1310-nm Polarization Splitter and Rotator

We demonstrate the first example of a polarization splitter and rotator (PSR) at 1310 nm built on a silicon-on-insulator platform using 248-nm deep-ultraviolet lithography. The PSR is constructed with a directional coupler, a bilevel taper-based TM0-to-TE1 mode converter, and an asymmetric Mach-Zehnder-based TE1-to-TE0 mode converter. A worst-case TM0-to-TE0 mode-conversion loss of 2 dB, with polarization crosstalk lower than -20 dB over a wide bandwidth of 40 nm is experimentally demonstrated. The worst-case polarization-dependent loss is 0.76 dB.

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

We present a highly efficient polarization splitter and rotator (PSR), fabricated using 248 nm deep ultraviolet lithography on a silicon-on-insulator substrate. The PSR is based on a double-etched directional coupler with a length of 27 µm. The fabricated PSR yields a TM-to-TE conversion loss better than 0.5 dB and TE insertion loss better than 0.3 dB, with an ultra-low crosstalk (-20 dB) in the wavelength regime 1540-1570 nm.

Wideband polarization splitter and rotator with large fabrication tolerance and simple fabrication process

We propose and demonstrate a polarization splitter and rotator (PSR) built on a silicon-on-insulator platform. The PSR is constructed with a tapered waveguide followed by a 2×2 multimode interferometer and can be simply fabricated in a single lithography and etching step. A low insertion loss (<2.5 dB with minimum insertion loss of 0.6 dB) and a low polarization crosstalk (<-12 dB) over a wide operation bandwidth (~100 nm) with a large fabrication tolerance (>50 nm) are experimentally demonstrated.

Fabrication tolerant polarization splitter and rotator based on a tapered directional coupler

A polarization splitter and rotator (PSR) based on a tapered directional coupler with relaxed fabrication tolerance is proposed and demonstrated on the silicon-on-insulator platform. The device is simply constructed by parallel-coupling a narrow silicon waveguide with a linearly tapered wider waveguide. Compared to previously reported PSRs based on a normal directional coupler, which suffer from stringent requirements on the accuracy of the narrow waveguide width, the introduced tapered structure of the wide waveguide can be used to compensate the fabrication errors of the narrow waveguide. In addition, only a single step of exposure and etching is needed for the fabrication of the device. Similar high conversion efficiencies are experimentally demonstrated for a narrow waveguide width deviation of 14 nm with large tolerance to the coupler length.