Low Polarization Dependent Loss Research Articles

Free-standing microscale photonic lantern spatial mode (De-)multiplexer fabricated using 3D nanoprinting

Photonic lantern (PL) spatial multiplexers show great promise for a range of applications, such as future high-capacity mode division multiplexing (MDM) optical communication networks and free-space optical communication. They enable efficient conversion between multiple single-mode (SM) sources and a multimode (MM) waveguide of the same dimension. PL multiplexers operate by facilitating adiabatic transitions between the SM arrayed space and the single MM space. However, current fabrication methods are forcing the size of these devices to multi-millimeters, making integration with micro-scale photonic systems quite challenging. The advent of 3D micro and nano printing techniques enables the fabrication of freestanding photonic structures with a high refractive index contrast (photopolymer-air). In this work we present the design, fabrication, and characterization of a 6-mode mixing, 375 µm long PL that enables the conversion between six single-mode inputs and a single six-mode waveguide. The PL was designed using a genetic algorithm based inverse design approach and fabricated directly on a 7-core fiber using a commercial two-photon polymerization-based 3D printer and a photopolymer. Although the waveguides exhibit high index contrast, low insertion loss (−2.6 dB), polarization dependent (−0.2 dB) and mode dependent loss (−4.4 dB) were measured.

Compactly integrated polarization insensitive 24-channel variable optical attenuator arrays using silica-based

A 24-channel variable optical attenuator array with a compact size of 15 × 10 mm2 is demonstrated using silica-based waveguides with 2.0% refractive index difference. Asymmetric Mach-Zehnder structures are well optimized to achieve low polarization-dependent loss (PDL) and wide bandwidth. In addition, the power consumption is reduced by optimizing the trench structures. Experimental results show that the power consumption is 160 mW when the attenuation changes by 10 dB. Within the operating voltage range, the PDL is less than 0.7 dB, and the device can operate in the C+L band with a wavelength-dependent loss (WDL) of less than 0.9 dB.

Power-efficient polarization-insensitive tunable microring filter on a multi-layer Si3N4-on-SOI platform.

We develop and demonstrate a non-duplicate polarization-diversity tunable bandpass optical filter by leveraging the bi-directional transmission of add-drop dual-coupled microring resonators (MRRs) on a multi-layer Si3N4-on-silicon-on-insulator (SOI) platform. By using Euler-bends, we implement compact and low-loss Si3N4 MRRs with an equivalent bending radius of ∼38 µm. Fiber-to-fiber (on-chip) insertion loss of 4.3 dB (1.7 dB) with a low polarization-dependent loss of <0.5 dB and low differential group delay of <2.5 ps is achieved. The extinction ratio is more than 30 dB. The thermo-optic tuning efficiency of the Si3N4 MRRs is improved with a suspended micro-heater design. As a result, a wavelength tuning range of ∼2 nm and a 3-dB bandwidth tuning range of 20 GHz are experimentally demonstrated. The tuning efficiency is 33 pm/mW, which is ∼7.5 times higher than the previous design. This reconfigurable polarization-insensitive filter, with low loss, low cross talk, and high power efficiency, is highly promising for practical applications in optical communication and signal processing.

4×100Gb/s PAM4 Multi-Channel Silicon Photonic Chipset With Hybrid Integration of III-V DFB Lasers and Electro-Absorption Modulators

A silicon photonic based transmitter and receiver chipset for 4x106Gb/s 400GBASE-DR4 data rates is presented. Each channel of the transmitter chip reaches high extinction ratio and optical modulation amplitude (OMA) with a low TDECQ penalty in full compliance with the IEEE standard. The receiver chips possess high responsivity with low polarization dependent loss. The use of discrete III-V arrayed components hybridized onto the silicon platform and passive alignment of single-mode fibers provides a low-cost, compact and scalable solution extendable to even higher aggregate rates and channel count.

Design of a Completely Vertical, Polarization-Independent Two-Dimensional Grating Coupler with High Coupling Efficiency

An efficient optical coupler to transfer the signal between an optical fiber and a silicon waveguide is essential for realizing the applications of silicon photonic integrated circuits such as optical communication and optical sensing. In this paper, we numerically demonstrate a two-dimensional grating coupler based on a silicon-on-insulator platform to obtain completely vertical and polarization-independent couplings, which potentially ease the difficulty of packaging and measurement of photonic integrated circuits. To mitigate the coupling loss induced by the second-order diffraction, two corner mirrors are respectively placed at the two orthogonal ends of the two-dimensional grating coupler to create appropriate interference conditions. Partial single-etch is assumed to form an asymmetric grating to obtain high directionalities without a bottom mirror. The two-dimensional grating coupler is optimized and verified with finite-difference time-domain simulations, achieving a high coupling efficiency of −1.53 dB and a low polarization-dependent loss of 0.015 dB when coupling to a standard single-mode fiber at approximately 1310 nm wavelength.

High-efficiency dual-layer grating coupler for vertical fiber-chip coupling in two polarizations.

Efficient coupling between optical fibers and high-index-contrast silicon waveguides is essential for the development of integrated nanophotonics. Herein, a high-efficiency dual-layer grating coupler is demonstrated for vertical polarization-diversity fiber-chip coupling. The two waveguide layers are orthogonally distributed and designed for y- and x-polarized L P 01 fiber modes, respectively. Each layer consists of two 1D stacked gratings, allowing for both perfectly vertical coupling and high coupling directionality. The gratings are optimized using the particle swarm algorithm with a preset varying trend of parameters to thin out the optimization variables. The interlayer thickness is determined to ensure efficient coupling of both polarizations. The optimized results exhibit record highs of 92% (-0.38d B) and 85% (-0.72d B) 3D finite-difference time-domain simulation efficiencies for y and x polarizations, respectively. The polarization-dependent loss (PDL) is below 2dB in a 160nm spectral bandwidth with cross talk between the two polarizations less than -24d B. Fabrication imperfections are also investigated. Dimensional offsets of ±10n m in etching width and ±8nm in lateral shift are tolerated for a 1dB loss penalty. The proposed structure offers an ultimate solution for polarization diversity coupling schemes in silicon photonics with high directionality, low PDL, and a possibility to vertically couple.

Polarization-insensitive 90° optical hybrid using silica-based PLC.

A 90° optical hybrid (OH) based on a 4×4 multimode interference (MMI) coupler is characterized and analyzed using the silica-based planar light circuit. The OH is designed to be polarization-insensitive with an extremely low polarization-dependent loss (<0.18d B) and phase error (w i t h i n±1.3∘). The fabricated chip exhibits an excess loss of 3.4dB and 3.8dB for TE and TM polarizations at 1550nm. The measured phase error is w i t h i n±7.2∘ over 50nm in the C+L band with a polarization-dependent error w i t h i n±3.5∘. The fabrication tolerance of the MMI width and waveguide sidewall angle is analyzed, which can well explain the deviation from the theoretical calculation.

XPoM-based 8-port photonic interconnection with low polarization sensitivity utilizing S–E–S hybridization

In this paper, we propose a hybrid amplifier-based 8-port Cross-Polarization Modulation (XPoM) interconnection with low polarization sensitivity unlike conventional Semiconductor Optical Amplifier (SOA)-based switching. The three-stage configuration utilizes Erbium-Doped Fiber Amplifier (EDFA) as an intermediate stage to reduce the effects of inherent polarization modulation creating large Transverse Electric – Transverse Magnetic (TE–TM) relative phase shifts. The appreciable error rate of [Formula: see text] and Extinction Ratio (ER) of 14.166 dB have been obtained with 100 GHz of port-spacing at different input powers. The low Polarization-Dependent Loss (PDL) of 1.0045 dB, small variations of azimuth and ellipticity with device angle in S–E–S configuration in contrast to that of SOA, adds to the superiority of the proposed model.

Core Selective Switch With Low Insertion Loss Over Ultra-Wide Wavelength Range for Spatial Channel Networks

The core selective switch (CSS) is an optical spatial switch that has been recently proposed as a key building block to achieve a scalable and low-insertion-loss spatial cross-connects for use in future spatial channel networks. In this paper, we report on a novel CSS design employing a two dimensionally arranged microlens-based multicore fiber (MCF) collimator array and a micro-electromechanical systems (MEMS) mirror array. The former enables precise alignment between MCFs and collimator lenses, and the latter yields polarization-independent high reflection over a wide wavelength range and a large tilt angle. Based on the design, a compact (∼50 mm) five-core <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \times 8$</tex-math></inline-formula> CSS prototype is fabricated. We experimentally show that the CSS prototype exhibits low insertion loss (1.2∼2.7 dB), low polarization dependent loss (< 0.25 dB), and low crosstalk (< <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - 30$</tex-math></inline-formula> dB) characteristics over an ultra-wide wavelength range from 1500 nm to 1630 nm. Bit-error-rate measurements using optical signals in the C-band, S-band, and L-band show that the CSS prototype incurs no optical signal-to-noise ratio penalty in spatial channel routing over an ultra-wide wavelength band.

Polarization-independent silicon photonic grating coupler for large spatial light spots* *Project supported by the National High Technology Research and Development Program of China (Grant No. 18-H863-04-ZD-006-007-01).

We design and demonstrate a one-dimensional grating coupler with a low polarization-dependent loss (PDL) for large spatial light spots. Based on current fabrication conditions, we first utilize genetic algorithms to find the optimal grating structure including the distributions of duty and periods, making the effective refractive index of transverse electric mode the same as that of transverse magnetic mode. The designed grating coupler is fabricated on the common silicon-on-insulator platform and the PDL is measured to be within 0.41 dB covering the C-band.

A compact and polarization-insensitive silicon waveguide crossing based on subwavelength grating MMI couplers.

In this work, we proposed and experimentally demonstrated a compact and low polarization-dependent silicon waveguide crossing based on subwavelength grating multimode interference couplers. The subwavelength grating structure decreases the effective refractive index difference and shrinks the device footprint. Our designed device is fabricated on the 220-nm SOI platform and performs well. The measured crossing is characterized with low insertion loss (< 1 dB), low polarization-dependence loss (< 0.6 dB), and low crosstalk (< -35 dB) for both TE and TM polarizations with a compact footprint of 12.5 μm × 12.5 μm.

Mirror-Based Broadband Silicon-Photonics Vertical I/O With Coupling Efficiency Enhancement for Standard Single-Mode Fiber

The broadband vertical optical coupling of silicon (Si) photonics poses significant challenges regarding the use of wavelength division multiplexing (WDM). The grating coupler, a type of vertical optical coupling device, strongly depends on the wavelength. As an alternative, we developed another vertical optical coupling device using a 45° curved micro-mirror. To experimentally demonstrate the broadband vertical optical coupling, the curved micro-mirror was designed based on a measured optical beam output from a Si inverse taper waveguide and integrated on a Si photonic chip to experimentally demonstrate the broadband vertical optical coupling. Without using the use of additional lenses, the Si waveguides were coupled with standard single-mode fiber (SMF) by applying fabricated mirrors. Owing to its lens function, the coupling efficiency was enhanced by 1.4 and 3.4 dB for TE and TM polarization. Over the entire O-band, low-wavelength dependent losses of 1 dB were also obtained. Misalignment tolerances were evaluated and 1-dB tolerances of approximately plus or minus 2 micrometers were obtained. By optimizing the design of the Si inverse taper waveguide, a low-polarization dependent loss of 0.3 dB on average was also obtained for the O-band. The average loss was 3.3 dB. Compared with grating couplers, a broadband and low polarization-dependent Si-photonics vertical coupling was demonstrated using an integrated curved mirror.

Two-dimensional grating coupler on silicon with a high coupling efficiency and a low polarization-dependent loss.

Two-dimensional grating couplers are important components for silicon photonic circuits to achieve light coupling from/to a fiber for both polarizations. A two-dimensional grating coupler structure with a high coupling efficiency and a low polarization dependent loss is demonstrated. Using two crossing ellipses as the grating scatter and a diamond-like grating lattice, the polarization dependent loss of the grating coupler can be reduced. The coupling loss is further decreased with a metal mirror, which reaches -1.73 dB theoretically at 1310 nm wavelength. Experimentally, -2.37 dB coupling loss is achieved with an 1 dB coupling bandwidth of 29 nm. The corresponding PDL was measured lower than 0.2 dB in a wavelength range of 78 nm. The proposed configuration for the metal mirror also facilitates a robust wafer-scale post-processing as well as an easy fiber alignment.

Femtosecond Laser-inscribed Non-volatile Integrated Optical Switch in Fused Silica based on Microfluidics-controlled Total Internal Reflection

We demonstrate a non-volatile optical power switch, fabricated by femtosecond laser inscription in a fused silica substrate, with switching operation based on microfluidics-controlled total internal reflection. The switch consists of crossed waveguides and a rectangular, high aspect ratio microfluidic channel, located at the waveguide crossing. The switching between total internal reflection and transmission at the channel wall is determined by the refractive index of the medium inside the channel. Femtosecond laser inscription allows for co-integration of low-loss optical waveguides and channels with smooth sidewalls and thus the fabrication of low insertion loss switches that are broadband and show low polarization dependent losses. The measured total internal reflection loss of the fabricated switch is about 1.5 dB at the wavelength 1550 nm. The loss due to transmission through the channel filled with refractive index matching liquid is about 0.5 dB. Detailed finite difference time domain and beam propagation method simulations of the switch's performance indicate that the losses can be further reduced by optimizing its geometry, together with further adjusting the inscription parameters.

Low connector-to-connector loss through silicon photonic chips using ultra-low loss splicing of SMF-28 to high numerical aperture fibers.

Here we present extremely low connector-to-connector loss (≤3 dB) through silicon photonic chips using ultra-low loss (≤0.15 dB) splicing between SMF-28 and ultra-high numerical aperture (UHNA) fibers. The small MFD from the UHNA fibers enables strong coupling to hybrid TE/TM edge couplers achieving TM (TE) losses of 1.25 (2.35) dB per coupler and low polarization-dependent loss. Mode coupling simulations and tolerance are investigated to understand performance.

Two-dimensional silicon photonic grating coupler with low polarization-dependent loss and high tolerance.

We design and demonstrate a two-dimensional grating coupler with an elliptical-like etching pattern on the silicon-on-insulator platform. The polarization-dependent loss of the fabricated device is measured to be 0.2 dB at the C-band, lower than that of most conventional 2D GCs. The etching unit can be easily fabricated with a large tolerance of ± 10 nm and a large feature size of 310nm, which is beneficial to its manufacturing process and wide application.

A CMOS-compatible and polarization-insensitive graphene optical modulator

Graphene is a suitable material for high efficient optical modulator. However, the graphene-based modulator has a strong polarization dependence. Here a high performance polarization-insensitive graphene modulator is proposed by utilizing a mode conversion structure. Our modulator is not only CMOS-compatible but also improves the modulation performance of the TE mode that has weak light–graphene interaction in graphene-on-Si structure. The theoretical analysis of the modulator was performed. The device can operate at the spectrum of 1.5 μm to 1.6 μm with the extinction ratio of >17dB, the insertion loss of <5.7dB for both modes, and the polarization-dependent loss of <2.3dB. When the modulator operates at the center wavelength of 1.550 μm, a low polarization-dependent loss of 0.6 dB is obtained. The 3dB modulation bandwidth is about 14.3 GHz based on theoretical calculation at the cost of 270 μm device length.

Design and fabrication of ring resonator spectral response through-drop wavelengths selective

In this paper, we discuss the basic characteristics of the SOI based micro-ring resonator, and understand the influence of the high refractive index difference and the small size on the spectral characteristics of the ring resonator. The novelty and importance of the spectral response selectivity of ring resonators is driven by the grating couple technique. This advantage is the periodic variation of the refractive index in the coupled grating, which will result in the coupling of the conduction wave modes. By means of measurement and simulation, observe the changes and trends of the spectrum in steady state and transient state, and finally analyze the performance of the ring resonator in the application of signal processing. It achieves polarization-independent grating couplers with high efficiency, wide bandwidth and low polarization-dependent loss at around the 1560 nm optical communication band. Drop end set laser wavelength correction 1591.9 nm.

Highly uniform and polarization-independent Y-branch with mode filter based on deeply etched silica-on-silicon waveguide

A slight misalignment between an optical fiber and a Y-branch would excite high-order modes and subsequently induce a snaking behavior of the input light, which would dramatically deteriorate the device performance. Here, a highly uniform and polarization-independent Y-branch based on deeply etched silica-on-silicon (SoS) waveguide is proposed, optimized, fabricated, and characterized, in which a mode filter with the quadratic-spline boundary is inserted to suppress the high-order modes. The proposed Y-branch is optimized by using three-dimensional full-vectorial finite difference beam propagation method. The experimental results show that the proposed Y-branch is with an ultra-low nonlinearity over a broad bandwidth from 1250 nm to 1620 nm, a low polarization dependent loss of less than 0.2 dB, and an ultra-low relative power difference of less than 0.04.

High-speed waveguide photodetector for 64 Gbaud coherent receiver.

A high-speed waveguide photodetector has been successfully fabricated for an integrated coherent receiver. Dual laterally tapered structures are introduced for a spot-size converter. We optimize the responsivity and the polarization-dependent loss of the spot-size converter-integrated waveguide photodetector through the beam propagation method simulation. The waveguide photodetector is designed with electrical as well as optical optimizations. The photodetector provides sufficient alignment tolerance, high responsivity of 0.73A/W, and low polarization-dependent loss of 0.27dB, which is in good agreement with the simulation results. By increasing the thickness of the matching layer and the n-doped upper taper, the electrical properties of the photodetector are enhanced. The photodetector has a 3dB bandwidth of 45GHz, providing high-speed operation. Through the electrical and optical optimizations, we successfully obtain the high-speed waveguide photodetector for a 64Gbaud integrated coherent receiver.