Optical Communication Band Research Articles

Ultra-High Performance All-Silicon TM Polarizer Covering O-U Optical Communication Bands

Polarizers are the essential components to purify the polarization in the system demanding for the single polarization propagation. Polarizers with broad working bandwidth, low excess loss (EL) and large polarization extinction ratio (PER) are usually preferred. TE polarizers with ultrahigh performance have been demonstrated. However, the bandwidth of the previously reported transverse magnetic (TM) integrated polarizers is still limited to be < ∼160 nm when the EL < 1 dB and PER > 20 dB are simultaneously satisfied. In this paper, an all-silicon integrated TM polarizer with > 415 nm working bandwidth from 1260 nm to 1675 nm is experimentally demonstrated. The device consists of a double slot structure with 180° Euler bending on silicon waveguides. The effective refractive index (ERI) for TE polarization in the double slot structure is lower than that of the silica buffer/cladding while ERI for TM polarization is much larger than that of the silica buffer/cladding. Thus, TE polarization will leak into the substrate while TM polarization can propagate through the device with low loss. The measured transmission spectra for the fabricated polarizer show low EL < 0.9 dB and large PER > 25 dB over an ultrabroad band beyond the O-, E-, S-, C-, L-and U- bands.

Broadband plasmonic switches based on nanodisc-dimers with progressively increasing diameters on a plasmonic film with a VO2 spacer

In this paper, we propose active broadband plasmonic switches based on a periodic array of metallic dimers of nanodiscs with progressively increasing diameters on a gold coated SiO 2 substrate with a VO2 thin film as a spacer between the nanodiscs and the underlying plasmonic substrate. Periodic arrays based on unit cells with plasmonic nanodisc dimers of varying diameters are employed to couple multiple wavelengths of the incident radiation into the plasmonic modes of the nanostructure, which results in a broadband plasmonic response. We employ a vanadium dioxide (VO2) thin film as a spacer between these periodic sets of plasmonic nanodisc-dimers and the underlying gold coated substrate to achieve broadband switching. On exposure to voltage, infrared light or heat, the VO2 spacer layer changes from its semiconducting state to its metallic state. This transformation leads to significant changes in its optical properties leading to changes in the reflectance spectra of the proposed nanostructure. This results in an efficient switching action over wide range of wavelengths spanning the C, L and U bands of optical communication. We demonstrate a broadband extinction ratio of 5 dB over an operating wavelength range of 650 nm spanning from 1460 nm to 2110 nm, and a broadband extinction ratio of 4 dB over an operating wavelength range of 702 nm spanning from 1432 nm to 2134 nm. In addition, we demonstrate the trade-off between the extinction ratio and the operating wavelength range of these switches. An exhaustive analysis of these switches has been carried out using finite difference time domain (FDTD) modelling to demonstrate that the spectral range over which switching can be achieved by employing these switches can be varied by varying the geometrical parameters of the proposed switches and thus these switches offer the flexibility of being employed for different wavelength regimes. These switches can be potentially employed in optical communication networks or as switching elements in plasmonic circuits.

Wideband improvement for hybrid plasmonic fractal patch nanoantenna

A wideband improvement for hybrid plasmonic fractal patch nanoantenna is presented for use in intra/inter chip optical interconnects. The suggested fractal patch antenna covering a part of U-band (1625-1675), L-band (1565-1625nm), C-band (1530-¬1565nm), S-band (1460-1530nm), E-band (1360-1460nm) and most of O-band (1260-1360nm) optical communication bands. The proposed antenna has a promising future use in inter and intra chip optical communications to eliminate electrical interconnection limitations such as interconnect density, power consumption and also increasing data rate. The performance of this antenna has been evaluated using full wave simulation computer simulation technology (CST) Microwave software. The impedance bandwidth is largely enhanced by applying rectangular fractal cuts to the both sides of patch. The proposed antenna achieves a wider bandwidth from 168 THz to 228 THz (B.W.=60 THz), which is about 8 times greater the bandwidth of reference antenna with a good gain and more than 95% radiation efficiency throughout the operational bandwidth.

Faraday effect in CoPt-CaF2 nanogranular films with hard magnetic property for magnetic-field-free faraday devices

Magneto-optical Faraday devices, such as optical isolators, require a structure to apply a magnetic field to the incorporated magneto-optical material. The application of a magnetic field hinders the miniaturization and integration of these devices. Nanogranular films show Faraday rotation angles up to 40 times larger than Bi-YIG in the optical communication band (1,550 nm). Nanogranular films are submicron-to several-micrometer-thick and contribute to the miniaturization and integration of optical devices. Here, we introduce (Co-Pt)–(CaF2) nanogranular films exhibiting a magnetic-field-free magneto-optical Faraday effect, which are Co3Pt alloy that are hard magnets with residual magnetization. These nanogranular films exhibit the Faraday effect without requiring a magnetic field owing to their residual magnetization.

Design of Compact, Broadband, and Low-Loss Silicon Waveguide Bends with Radius under 500 nm

Waveguide bend is an indispensable component in the on-chip compact photonic integrated circuits (PICs) and the minimum bend size greatly limits the increase of integration density of PICs. Here, we propose broadband and low-loss silicon waveguide bend schemes using air trenches on both sides and embedded germanium arc in the inner side of waveguide bend. Using these ways, the silicon waveguide bending radius can be greatly reduced to less than 500 nm and the obtained insertion loss (IL) can be as low as 0.12 dB compared with IL = 1.73 dB obtained by direct silicon waveguide bend under the same bending radius. Meanwhile, the working bandwidth can be extended over 500 nm covering the whole optical communication band by keeping IL &lt; 0.5 dB. Therefore, the proposed device schemes could push the development of on-chip PICs toward higher integration density.

Discussion on fabrication accuracy of infrared topological photonic structures using hyperspectral Fourier image spectroscopy

Compared to conventional periodic nanostructures such as photonic crystals, topological photonic systems require more precise fabrication techniques due to the need to place multiple dielectrics at appropriate positions within a unit cell, and this becomes particularly remarkable in the optical communication band. In this paper, observing photonic band structures by using hyperspectral Fourier image spectroscopy, we quantitatively discuss the fabrication accuracy of typical topological photonic systems in which triangle nanoholes were arranged in a deformed honeycomb lattice with C 6 v symmetry. Here the size and corner radius of triangle nanoholes were precisely controlled using shape correction electron beam lithography. The results showed that the fabrication error must be controlled to below ∼ 10 n m for the size and ∼ 30 n m for the corner radius, in order to achieve the desired topological edge state that occurs at the interface between two photonic structures different in topology.

Frequency Conversion Interface towards Quantum Network: From Atomic Transition Line to Fiber Optical Communication Band

Quantum repeater is a key component of quantum network, and atomic memory is one of the important candidates for constructing quantum repeater. However, the atomic transition wavelength is not suitable for long-distance transmission in optical fiber. To bridge atomic memory and fiber communication, we demonstrate a frequency conversion interface from rubidium D1 line (795 nm) to the optical communication L-band (1621 nm) based on difference frequency generation. To reduce broadband noise of spontaneous Raman scattering caused by strong pumping light, we use a combination of two cascaded etalons and a Fabry-Perot cavity with low finesse to narrow the noise bandwidth to 11.7 MHz. The filtering system is built by common optical elements and is easy to use; it can be widely applied in frequency conversion process. We show that the signal-noise ratio of the converted field is good enough to reduce the input photon number below 1 under the condition of low external device conversion efficiency (0.51%) and large duration of input pulse (250 ns). The demonstrated frequency conversion interface has important potential application in quantum networks.

High-performance silicon TE-pass polarizer assisted by anisotropic metamaterials.

The polarizer is a key component for integrated photonics to deal with the strong waveguide birefringence, especially for silicon photonics. A high-performance silicon TE-pass polarizer covering all optical communication bands with low insertion loss (IL) and high polarization extinction ratio (PER) is proposed here. This polarizer is based on anisotropic subwavelength grating (SWG) metamaterials, which maintain the fundamental TE mode as a guided mode but make the fundamental TM mode leaky. Furthermore, based on this working mechanism, the proposed polarizer can work well for any upper cladding material, including air and silicon dioxide (SiO2). The numerical results show that our proposed TE-pass polarizer has a remarkable performance with IL < 0.34 dB over 420 nm (PER > 23.5 dB) or 380 nm (PER > 30 dB) for the air cladding, and IL < 0.3 dB over 420 nm (PER > 25 dB) or 320 nm (PER > 30 dB) for the SiO2 cladding. The fabricated polarizer shows IL < 0.8 dB and PER > 23 dB for the bandwidths of 1.26-1.36 µm and 1.52-1.58 µm (other bandwidths were not measured due to the limited instrument in our research center, but it still covers the most important O-band and C-band).

Ultra-Broadband and Low-Loss Silicon-Based Power Splitter Based on Subwavelength Grating-Assisted Multimode Interference Structure

High-performance and compact power splitters are fundamental components in on-chip photonic integrated circuits (PICs). We propose a silicon-based power splitter based on a subwavelength grating (SWG)-assisted multimode interference (MMI) structure. To shorten the device size and enhance the device performance, an inverse-tapered SWG is embedded in the central region of the MMI and two rows of uniform SWG are embedded on both sides, together with two right-angled cutting structures on the input side. According to the results, the MMI length was obviously reduced to 3.2 μm (5.2 μm for conventional MMI structure under the same waveguide width), while the insertion loss (IL) and reflection loss were 0.08 dB and &lt;−35 dB, respectively. Moreover, the allowable working bandwidth could be extended to 560 nm by keeping IL &lt;0.6 dB, covering the whole optical communication band. On the basis of these features, we believe that such a power splitter is very promising for building on-chip large-scale PICs where power splitting is indispensable.

Graphene Metamaterial Embedded within Bundt Optenna for Ultra-Broadband Infrared Enhanced Absorption.

Graphene is well-known for its extraordinary physical properties such as broadband optical absorption, high electron mobility, and electrical conductivity. All of these make it an excellent candidate for several infrared applications such as photodetection, optical modulation, and optical sensing. However, a standalone monolayer graphene still suffers from a weak infrared absorption, which is ≅2.3%. In this work, a novel configuration of graphene metamaterial embedded inside Bundt optical-antenna (optenna) is demonstrated. It can leverage the graphene absorption up to 57.7% over an ultra-wide wavelength range from 1.26 to 1.68 µm (i.e., Bandwidth ≅ 420 nm). This range covers the entire optical communication bands of O, E, S, C, L, and U. The configuration mainly consists of a Bundt-shaped plasmonic antenna with a graphene metamaterial stack embedded within its nano-wide waveguide that has a 1.5 µm length. The gold average plasmonic loss is ≅25%. This configuration can enhance graphene ultra-broadband absorption through multiple mechanisms. It can nano-focus the infrared radiation down to a 50 nm spot on the graphene metamaterial, thus yielding an 11.5 gain in optical intensity (i.e., 10.6 dB). The metamaterial itself has seven concentric cylindrical graphene layers separated by silicon dioxide thin films, thus each layer contributes to the overall absorption. The focused infrared propagates tangential to the graphene metamaterial layers (i.e., grazing propagation), and thus maximizes the light–graphene interaction length. In addition, each graphene layer experiences a double-face exposure to the nano-focused propagating spot, which increases each layer’s absorption. This configuration is compact and polarization-insensitive. The estimated maximum absorption enhancement compared to the standalone monolayer graphene was 25.1 times (i.e., ≅4 dB). The estimated maximum absorption coefficient of the graphene stack was 5700 cm−1, which is considered as one of the record-high reported coefficients up to date.

Enhancing and Broadening the Photoresponse of Monolayer MoS2 Based on Au Nanoslit Array.

Two-dimensional molybdenum disulfide (MoS2), featuring unique optoelectronic properties, has attracted tremendous interest in developing novel photodetection devices. However, the limited light absorption and small carrier transport rate of the monolayer MoS2 result in low photoresponse, and the large band gap limits its detection range in the visible region. In this study, we propose a nanoslit array-MoS2 hybrid device architecture with enhanced and broadened photoresponse. The nanoslit array can localize free-space light to achieve strong interactions with MoS2, and acts as the channel to improve charge transport. As a result, the Au nanoslit array-MoS2 hybrid detector exhibits a nearly 100-fold increase in photocurrent compared to the pure MoS2 device. More importantly, the hybrid device can broaden the photoresponse to the optical communication band of 1550 nm which is lower than the band gap of MoS2, by efficiently utilizing the hot carriers generated by the Au nanoslits. The experimental results are supported by both theoretical analysis and numerical simulation. Since our demonstration leverages the engineering of the hybrid photodetectors with metal nanostructures rather than semiconductor materials, it should be universal and applicable to other devices for broadband, high-efficiency photoelectric conversion.

Modeling of thin-film lithium niobate modulator for visible light

Integrated modulators are essential components for optical sensing and communication applications. However, most efforts have been focused on infrared telecommunications wavelengths, with very little consideration for visible light applications. Here, we design an electro-optic (EO) modulator at 532 nm with the waveguides formed by SU8 polymer structures on the thin-film lithium niobate (LN). The simulation results show a low loss, high modulation efficiency, and large bandwidth simultaneously for the modulator. The predicted low loss is attributed to the LN etchless waveguide and high coupling efficiency, i.e., 93%, with the single-mode fiber theoretically using a SU8 edge coupler. The simulated low voltage-length product of 1.15 V · cm and a 3-dB-bandwidth of >120 GHz at a length of 5 mm are superior to any other modulator in the optical communication band. The modeling of the modulator proposed here shows great potential for visible light communications applications such as energy-efficient and large-capacity underwater wireless optical interconnects.

Design of single-dual channel conversion filter based on one-dimensional photonic crystal

This study proposes a quantum well photonic crystal to realize a single-dual channel conversion filter in the optical communication band, which aggregates dual-channel into single-channel by reducing the coupling strength between the air defect modes. When the photonic crystal structure with defect mode is symmetric, there is a single channel in the forbidden bandgap due to the disappearance of coupling between the two channels and the transmittance is up to 100%. The coupling coefficient α is inversely proportional to the period number M. When M is greater than the threshold-2N, the transmittance of the channel gradually decreases as M increases, correspondingly. The proposed filter can be applied in wavelength division multiplexing to increase the network capacity.

Design of a Nonlinear Plasmonic Coupler with High Coupling Efficiency over All Optical Communication Bands

Design of a Nonlinear Plasmonic Coupler with High Coupling Efficiency over All Optical Communication Bands

Numerical analysis of optical phase modulator operating at 2 μm wavelength using graphene/III–V hybrid metal-oxide-semiconductor capacitor

We propose an optical phase modulator with a hybrid metal-oxide-semiconductor (MOS) capacitor, consisting of single-layer graphene and III–V semiconductor waveguide. The proposed modulator is numerically analyzed in conjunction with the surface conductivity model of graphene. Since the absorption of graphene at a 2 μm wavelength can be suppressed by modulating the chemical potential of graphene with the practical gate bias, the phase modulation efficiency is predicted to be 0.051 V·cm with a total insertion loss of 0.85 dB when an n-InGaAs waveguide is used, showing the feasibility of the low-loss, high-efficiency graphene/III–V hybrid MOS optical phase modulator, which is useful in the future 2 μm optical fiber communication band.

Optical performance monitoring using SOI-based spectral analysis.

A novel optical performance monitoring (OPM) method based on Fourier transform spectrum analysis (FTSA) is designed for optical signal-to-noise ratio (OSNR) monitoring, modulation format and baud rate recognition in the presence of fiber nonlinearities. The interference intensities, which reflect spectral features of signals, are obtained by exploiting the FTSA consisting of two-stage Mach-Zehnder interferometer (MZI) arrays. Then, the mapping between the OPM parameters and modulated interference intensity (MII) is characterized using neural networks without prior knowledge of the configuration of the communication network. Results show that optical performance parameters are monitored simultaneously. Meanwhile, the accuracy of modulation format and baud rate recognition is 94.8% and most (over 86%) OSNR monitoring errors are less than ±1 dB under complex transmission conditions in presence of frequency offset and delay jitter. Besides, the FTSA can be fabricated on a silicon on insulator (SOI) platform with a large fabrication tolerance, and it has broad working bandwidth to support the full optical communication band. Therefore, the proposed OPM method is capable of integration and miniaturization, which can be ubiquitously applied in network intermediate nodes to support the construction of smart optical networks.

Pr3+/Tm3+/Er3+ tri-doped tellurite glass with ultra-broadband luminescence in the optical communication band

Pr3+/Tm3+/Er3+ tri-doped tellurite glass was synthesised using a melt-quenching method, and the broadband luminescent properties were measured in the near-infrared band. With an optimal combination of the three ions, a broadband luminescence ranging from 1280 to 1630 nm, located within the optical communication window of silica transmission fibre, was acquired under 808 nm excitation. The broadband luminescence comprises three bands: 1.34 μm (Pr3+), 1.47 μm (Tm3+), and 1.53 μm (Er3+); and the full width at half-maximum was about 248 nm. This valuable result is of great significance for novel broadband fibre amplifiers and ultra-short pulsed lasers.

Highly birefringent hollow-core anti-resonant terahertz fiber with a thin strut microstructure.

A novel highly birefringent and low transmission loss hollow-core anti-resonant (HC-AR) fiber with a central strut is proposed for terahertz waveguiding. To the best of our knowledge, it is the first time that a design of a highly birefringent terahertz fiber based on the hybrid guidance mechanism of the anti-resonant mechanism and the total internal reflection mechanism is provided. Several HC-AR fibers with both ultra-low transmission loss and ultra-low birefringence have been achieved in the near-infrared optical communication band. We propose a HC-AR fiber design in terahertz band by introducing a microstructure in the fiber core which leads to tremendous improvement in birefringence. Calculated results indicate that the proposed HC-AR fiber achieves a birefringence higher than 10-2 in a wide wavelength range. In addition, low relative absorption loss of 0.8% (8.6%) and negligible confinement loss of 1.69×10-4 dB/cm (9.14×10-3 dB/cm) for x-polarization (y-polarization) mode at 1THz are obtained. Furthermore, the main parameters of the fiber structure are evaluated and discussed, proving that the HC-AR fiber possesses great design and fabrication tolerance. Further investigation of the proposed HC-AR fiber also suggests a good balance between birefringence and transmission loss which can be achieved by changing strut thickness to cater numerous applications ideally.

Significant improvement in optical propagation of lithium niobate written waveguides by a thermal annealing

In this paper, we analysed, studied, and implemented an experimental procedure to improve the guiding performance at the telecommunications band of type II waveguides made in x-cut lithium niobate by femtosecond laser writing. This method relies on conducting a suitable temperature treatment on the samples. In particular, the thermal annealing reached the highest temperature of 450 °C stepped by 25 °C starting from room temperature.A discussion about the involved effects that produces the improvement of waveguides performance has been conducted in this paper. This analysis was supported by considering anisotropy expansion of the waveguiding region and by also a strong reduction of colour centres and waveguide border smoothing after thermal treatment. In addition, a better mode matching for coupling was observed. A relative enhancement from 6 dB to 10 dB (output/input power), with respect to the sample without thermal annealing, has been determined in our experiments. The deep analyses presented in this work suggest that the optical circuits fabricated by femtosecond laser writing, after the proposed thermal treatment, improve their guidance quality for photonics applications at the optical communications band where high performance waveguides systems are required.

A low-crosstalk and high-density multi-core few-mode fiber based on heterogeneous core and trench-assisted air-holes isolation

The capacity of a traditional optical communication system based on single-mode fiber has approached to its theoretical limit. Multi-core few-mode fibers provide an effective way to break through the bottleneck of existing transmission capacity. In this paper, a 5-LP-mode weakly-coupled low-crosstalk 7-core fiber is designed by using a combination of trench assistance and air hole isolation structure. The fiber with a standard outer diameter achieves low crosstalk between cores and modes. The inter-core crosstalk area and the effective mode area of the core are calculated by the finite element method. After design optimization, there are 5 stable transmission LP modes in the C+L band of optical communication in this fiber. The effective refractive index difference between LP&lt;sub&gt;21&lt;/sub&gt; mode and LP&lt;sub&gt;02&lt;/sub&gt; mode is the smallest and is greater than 1.1 × 10&lt;sup&gt;–3&lt;/sup&gt;. The LP&lt;sub&gt;31&lt;/sub&gt; mode in the optical fiber has the largest inter-core crosstalk and the loss is lower than –50 dB/km. The fiber can achieve low crosstalk transmission between modes and cores at the same time. The mode areas of the 5 LP modes in the 7 cores are larger than 86 μm&lt;sup&gt;2&lt;/sup&gt;, and the relative core multiplexing factor is 57.63 at a wavelength of 1550 nm. Therefore, this fiber can be used in a large-capacity high-speed fiber transmission system.