Highly efficient silicon optical polarization rotators based on mode order conversions

Fabrication error tolerant broadband mode converters and their working principles

Computational inverse design techniques have shown potential to become reliable means for designing compact nanophotonic devices without compromising the performance. Much effort has been made to reduce the computation cost involved in the optimization process and obtain final designs that are robust to fabrication imperfections. In this work, we experimentally demonstrate TE0-TE1 and TE1-TE3 mode converters (MCs) on the silicon-on-insulator platform designed using the computationally efficient shape optimization method. These MCs have mode conversion efficiencies above 95%, and the insertion loss ranges from 0.3 dB to 1 dB over a wavelength span of 80 nm ranging from 1.5 µm to 1.58 µm. Maximum modal crosstalk found experimentally in the C-band is -19 dB. The conversion efficiency drops at most by 2.2% at 1.55 µm for 10 nm over/under etch, implying good robustness to dimensional variations. We present the mode conversion mechanism of these MCs by studying the simulated electromagnetic field patterns and validate with supportive data. We also demonstrate their performance in the time domain with a 28 Gbps OOK and a 20 GBaud PAM-4 payload transmissions, which supports their utility for high throughput data communications. The open eye diagrams exhibit Q-factors of 8 dB.

Demonstration of polarization-insensitive optical filters on silicon photonics platform

We experimentally demonstrate a polarization-insensitive optical filter (PIOF) using polarization rotator-splitters (PRSs) and microring resonators (MRRs) on the silicon-on-insulator (SOI) platform with complementary metal-oxide-semiconductor (CMOS) compatible fabrication process. The PRS consists of a tapered-rib waveguide and an asymmetrical directional coupler (ADC), which realize the polarization rotation and splitting, to ensure the connected MRRs-based optical filter operating at one desired polarization when light with different polarizations are launched into the device. The measured results show that the optical transmission spectra of the device are identical for TE and TM polarization input. The box-like filtering spectra are also achieved with a 3-dB bandwidth of ∼0.15 nm and a high extinction ratio (ER) over 30 dB.

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.

Integrated silicon reconfigurable optical transmitter

We demonstrate an integrated silicon reconfigurable optical transmitter based on the reconfigurability of the Mach-Zehnder interferometer (MZI). By incorporating modulators into the tunable MZI structure and manipulating the operation states, different modulation formats, including amplitude/phase modulated binary/quaternary signals, as well as polarization multiplexed signals, can be generated as required, to accommodate different transmission links. For a proof-of-concept demonstration, the microring modulators are adopted, and we experimentally generate a 10 GBaud on-off keying (OOK) signal, four-level pulse amplitude signal, and polarization division multiplexing OOK signal using the same transmitter. The device is promising for a next-generation intelligent optical link.

Mode hybridization analysis in thin film lithium niobate strip multimode waveguides

Mode hybridization phenomenon in air-cladded X-cut Y-propagating and Z-propagating thin film lithium niobate strip multimode waveguides is numerically studied and a mathematical relation between structural parameters leading to hybrid modes is formulated. Dependence of hybrid modes on waveguide dimensions, sidewall angles and wavelength is also analyzed. The results obtained are used to design lithium niobate on insulator (LNOI) taper for converting fundamental TM mode to higher order TE mode, and an optimum length for achieving a high conversion efficiency of 99.5% is evaluated. Birefringent Y-propagating LN and isotropic Z-propagating LN tapers are compared in terms of length, figures of merit, and fabrication tolerance. Tapers exhibit a broad bandwidth of 200 nm with an extinction ratio less than − 18 dB. The results of mode hybridization analysis are useful in design optimization of adiabatic tapers, tunable time delays, optical interconnects, mode converters and demultiplexers for mode division multiplexing (MDM) applications.

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.

Framework for tunable polarization state generation using Berry's phase in silicon waveguides

We present a framework for an arbitrary polarization state generator exploiting Berry's phase through a cascade of in-plane and out-of-plane silicon strip waveguides. We establish two criteria required for a passive device to achieve 90° polarization rotation, and derive explicit equations to satisfy the criteria. The results define regions within the parameter space where active tuning of the polarization state is possible over the entire Poincaré sphere. We use numerical modeling to show ±30 dB tuning of the polarization extinction ratio between the quasi-transverse electric and magnetic modes for a range of devices with deflection angles ranging from 5° to 45°, and modal birefringence from 0 to 0.05. We envision control of optical polarization on the chip-scale in integrated waveguides for communications, sensing, and computing applications.

Ultra-compact reflective mode converter based on a silicon subwavelength structure

Mode converters play an essential role in mode-division multiplexing systems. A reflective mode converter (RMC), which is utilized to accomplish the mode conversion in the contra-propagation process, can fold the optical path and realize the mode exchange in an optical network. In this paper, we propose and experimentally demonstrate an RMC based on a silicon subwavelength structure. It can convert the input fundamental mode (${\text{TE}_0}$TE₀) into the first-order mode (${\text{TE}_1}$TE₁) in a ${2.0}\;\unicode{x00B5} \text{m} \times {2.0}\;\unicode{x00B5} \text{m}$2.0µm×2.0µm footprint. The simulated insertion loss and cross talk are lower than 0.6 dB and $ - {20.3}\;\text{dB}$-20.3dB in 1525-1565 nm. Experimental results verify the functionality of the device. The measured insertion loss and cross talk are lower than 2.2 dB and $ - {16.2}\;\text{dB}$-16.2dB. To further prove the generality of the methodology, we design another two RMCs realizing the mode conversion functions of ${\text{TE}_0}$TE₀ to ${\text{TE}_2}$TE₂ and ${\text{TE}_0}$TE₀ to ${\text{TE}_3}$TE₃ modes. The simulated insertion losses are lower than 1.1 dB and 1.8 dB.

Fundamental mode hybridization in a thin film lithium niobate ridge waveguide

The high performance of thin film lithium niobate on insulator (LNOI) platform shows potential for electro-optical signal processing and nonlinear optics systems. To realize precise polarization management for sub-wavelength devices, we theoretically and experimentally investigate fundamental transverse electric (TE) and transverse magnetic (TM) mode hybridization in an x-cut LNOI ridge waveguide. Sudden jumps in the free-spectrum-range (FSR) of these modes in a fabricated microring resonator demonstrate the mode hybridization. The measured Q-factor of the lithium niobate (LN) microring is 1.78 million near the critical coupling condition. The hybridization wavelength was designed at 1562 nm and observed at 1537 nm. Potential applications include fundamental mode conversion, polarization rotation, polarization splitter, and polarization insensitive waveguides in optical receiver module.

Ultra-wideband Ge-rich silicon germanium mid-infrared polarization rotator with mode hybridization flattening

In this work we investigate the implementation of ultra-wideband polarization rotator in the mid-infrared spectral region. A new design method of the rotation section is proposed, yielding a polarization rotator with an extinction ratio of at least 15 dB in a wavelength range of 2 µm. For a spectral range wider than 3.8 µm, an extinction ratio of at least 10 dB is achieved for this design. The device is 1660 µm long and the associated insertion loss is below 1.2 dB on the full operational wavelength range. The influence of geometrical parameters with respect to the design method to obtain such a broadband behavior is discussed. Finally, to increase the tolerance to fabrication errors, a tapered rotator design is proposed. Such a device can support up to ± 100 nm fabrication errors and still guarantees remarkable broadband behavior. To the best of our knowledge, this is the first time an integrated polarization rotator is designed to operate for the wavelength range of 4 to 9 µm with a bandwidth exceeding 2 µm.

Rotating polarization using Berry's phase in asymmetric silicon strip waveguides

Light propagating in an out-of-plane curvilinear waveguide acquires a Berry's phase, which rotates optical polarization. The effect is promising for realizing waveguide polarization controllers. In high index contrast platforms, such as silicon-on-insulator, however, small waveguide cross-sectional asymmetries reduce the amount of polarization rotation. To overcome this, we present a method based on the periodic spatial modulation of Berry's phase. Ninety degree polarization rotation is achieved, even in the presence of waveguide asymmetry. Using a numerical model based on Jones calculus, we demonstrate the approach with 303×300  nm² asymmetric silicon waveguides. We convert polarization from transverse electric to transverse magnetic with a polarization extinction ratio (PER) greater than 20 dB PER over a 100 nm bandwidth in a 110×240  μm²footprint.

Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

Ultra-compact and highly efficient polarization rotator utilizing multi-mode waveguides

An ultra-compact and highly efficient polarization rotator is proposed and experimentally realized on the silicon-on-insulator (SOI) platform. The polarization rotation (TM₀-TE₁) process is obtained by utilizing a straight multi-mode waveguide, while the mode conversion (TE₁-TE₀) process is realized by a bent multi-mode waveguide. For the proposed structure, only one etching step is required for the fabrication. The measured extinction ratio and insertion loss at the central wavelength are 19.8 and 0.86 dB, which agree well with the simulations.