Communication Wavelength Research Articles

Coherent self-control of free-space optical beams with integrated silicon photonic meshes

In technologies operating at light wavelengths for wireless communication, sensor networks, positioning, and ranging, a dynamic coherent control and manipulation of light fields is an enabling element for properly generating and correctly receiving free-space optical (FSO) beams even in the presence of unpredictable objects and turbulence in the light path. In this work, we use a programmable mesh of Mach–Zehnder (MZI) interferometers to automatically control the complex field radiated and captured by an array of optical antennas. The implementation of local feedback control loops in each MZI stage, without global multivariable optimization techniques, enables an unlimited scalability. Several functionalities are demonstrated, including the generation of perfectly shaped beams with nonperfect optical antennas, the imaging of a desired field pattern through an obstacle or a diffusive medium, and the identification of an unknown obstacle inserted in the FSO path. Compared to conventional devices used for the manipulation of FSO beams, such as spatial light modulators, our programmable device can self-configure through automated control strategies and can be integrated with other functionalities implemented onto the same photonic chip.

Design of a hybrid photonic-plasmonic crystal refractive index sensor for highly sensitive and high-resolution sensing applications

In this paper, an optical refractive index sensor based on a hybrid photonic-plasmonic crystal (PhPlC) topology, in which some metal nano-rods are embedded in a rod-type GaAs photonic crystal (PhC) lattice, is proposed. The FDTD method is used for numerical simulation of the sensor. The simulation results illustrate a fairly high sensitivity of 1350 nm/RIU, a high figure of merit (FoM) of 488 RIU−1, and a detection of limit of 2.04×10−4 RIU. The sensitivity of the PhC sensor is dramatically enhanced by employing the excitations of plasmonic mods of the metal nano-rods in the sensor structure. The sensitivity and FoM of the proposed sensor are significantly improved compared to the results published in the literature. The higher sensitivity and the larger FoM are the two important features of the proposed sensor which make it an incredibly suitable candidate for sensing applications in optic communication wavelengths.

Optimization of Epsilon-Near-Zero Multilayers for Near-Perfect Light Absorption Using an Enhanced Genetic Algorithm

Using epsilon-near-zero (ENZ) subwavelength optical multilayer materials with simple structure and thin total thickness to achieve target characteristics is extremely important and beneficial for the realization of on-chip integration and large-scale application of optical devices. Combining with the enhanced genetic algorithm (EGA), this work breaks the limitation of the periodicity of traditional ENZ multilayer structures, and investigates the aperiodic ENZ transparent conducting oxide (TCO)-dielectric multilayer structures. It is realized that under the given conditions, an optimal structure can possess a maximum peak absorption and the broadest absorption bandwidth near the communication wavelength. In the 6-layer structure with a total thickness of 600 nm studied in this work, EGA can optimize the peak absorption from 0.91 to 0.95. Additionally, the absorption bandwidth is optimized from 120 nm to 227 nm, which is enhanced more than 180%. The absorption performance of this optimized structure is comparable to that of a more complex structure with the same total thickness but more layers, or a structure with the same number of layers but a larger total thickness. Conclusively, the proposed EGA optimization method can simplify the structure of the multilayer system, reduce the total thickness of the required ENZ material, and thus greatly simplify the production process and reduce the production cost.

(Invited) Selective Epitaxy of Submicron Ge Wire Structures for Photodetectors and Optical Modulators in Si Photonics

A selective-area epitaxial growth of Ge on the submicron scale is studied using chemical vapor deposition (CVD) on Si in terms of the fabrication of integrated photonic devices operating at the optical communication wavelengths of around 1.55 μm. A mesa structure of Ge on the micron scale, having a vertical pin junction, is effective for fabricating a waveguide-integrated photodetector in the optical receiver. A 3-dB cutoff frequency is obtained up to approximately 50 GHz. For a higher frequency operation, a narrower structure of submicron-wide Ge wire, having a lateral pin junction, is required. Such a wire structure is also important for the application to an electro-absorption optical intensity modulator in the optical transmitter. Structural and optical properties are investigated for a submicron-wide Ge wire on Si selectively grown by CVD.

A Study on Material Dispersion around Zero Material Dispersion Wavelength of Different Material Composition based Optical Fiber

The Optical Fiber-based communication system has established its proficiency and inevitability towards regular progress and advancement worldwide. The most attractive wavelength for optical fiber communication is 1.55 μm, as it represents the lowest loss. The other challenging parameter ‘Material Dispersion’ gets reduced to ‘Zero’ at 1.27 μm wavelengths for conventional pure silica-based Optical Fiber. To improve the system towards a better unification between the loss and dispersion, the Dispersion Shifted Fiber (DSF) has been introduced. The Dispersion Flattened Fiber has introduced the concept of flat dispersion over a wide range of wavelengths. But the effective combination of the mechanisms to compensate for all the challenges is yet to be established properly. The said mechanisms are complex to design and implement. So, there is an immense scope to search for an alternative to get control over the loss and dispersion. At present, a fair number of material compositions of optical fiber are reported with different specifications. Our study on some of these fiber compositions has produced some interesting data towards the broader flatness and the minimum dispersion effect over a considerable range of wavelengths around the Zero Material Dispersion Wavelength (ZMDW). It helps to have more effective wavelength division Multiplexing (WDM). In this paper, we have studied different prospective options of optical fiber doping profiles to explore and propose an effective and optimized alternative among the available fiber profiles. We have studied the samples of pure SiO2 fibers along with B2O3 and GeO2 doped fibers and samples of Fluoride-based ABCY and ZBLAN glass Fibers to propose an effective combination of materials among the available options to get the optimized conjugation of loss and dispersion. Our report on the comparative study of different fiber materials has produced some effective results to have minimum material dispersion at the lowest loss wavelength to invite worldwide attention from system designers.

Maple-Leaf Shaped Broadband Optical Nano-Antenna with Hybrid Plasmonic Feed for Nano-Photonic Applications

This article presents a broadband optical nano-antenna, which covers a broader range of optical communication wavelengths (666 to 6000 nm), used in nano-photonic applications. The proposed design is modeled and analyzed to obtain a satisfactory gain of up to 11.4 dBi for a single element-based antenna. The unique feature of the proposed antenna is the hybrid plasmonic waveguide-based feed, which receives the optical signal from the planar waveguide and redirects the signal out of the plane. The proposed antenna provides highly directional radiation properties, which makes it a suitable candidate for inter- and intra-chip optical communications and sensing applications. Moreover, an extension of the work is performed for an array configuration of the order 2 × 1 and 64 × 1, to increase the gain and directionality. Therefore, this shows that it can be equally useful for optical energy harvesting applications with a significant gain up to 26.8 dBi.

Photostability of organic electro-optic polymer under practical high intensity continuous-wave 1550 nm laser irradiation

Information on the photochemical stabilities of organic electro-optic (EO) polymers under laser irradiation at 1550 nm, in the optical communication wavelength band (C-band), is crucial for understanding the long-term stability of various optical devices that employ EO polymers and providing guidelines for extending the devices’ lifetime. Thus, in this study, we constructed a pump–probe photostability evaluation system with an intensity of as high as 1.49 MW cm−2 and intensively investigated the photochemical stability of an EO polymer at 85 °C under irradiation by using a continuous-wave (CW) 1550 nm laser. We found that the photobleaching rate was proportional to the square of the pump intensity, suggesting that the dominant photodegradation pathway was associated with two-photon absorption. Our findings provide new insights into a photobleaching mechanism under non-resonant CW laser excitation conditions.

Realization of 3D reflectors by using metal-air and semiconductor-air based photonic structures at three communication windows

This study is based on analysis of 3D photonic crystal structure (PCS) for realization of photonic reflector pertaining to suitable optical communication wavelengths of 850, 1310 and 1550 nm. The said photonic reflector application is envisaged separately by two 3D PCSs, which comprise semiconductor (germanium) and metal (iron) based circular rods respectively, arranged on a square lattice having air as the background material. The plane wave expansion (PWE) technique is employed to investigate the photonic band gap (PBG) vis-à-vis all the aforementioned wavelengths. PBG is meticulously controlled by suitably selected various structure parameters, such as lattice spacing, diameter of the circular rods and nature of their material. Simulation outcomes explored that semiconductor based PCS reflects wavelengths of 850, 1310 and 1550 nm, when selecting the diameter of the circular rods as 282, 608 and 771 nm, respectively, whereas metal based PCS reflects the aforementioned wavelengths for diameters of the circular rods close to 335, 1070 and 871 nm, respectively. Further, we assayed the variation in reflected wavelength with respect to different diameters of circular rods for both proposed structures. Thus, the proposed optical reflectors can find a wide range of applications vis-à-vis three communication windows.

Ultra-broadband 3 dB power splitter from 1.55 to 2 µm wave band.

Extending the optical communication wavelengths to 2 µm can significantly increase data capacity. Silicon photonics, which is a proven device integration technology, has made rapid progress at 2 µm recently. As a fundamental functional element in the photonic design kit, the 3 dB power splitter has been extensively studied in both the 1.55 µm and 2 µm regime. While the device is highly desirable to operate over both wave bands, the large waveguide dispersion in silicon makes it challenging. In this work, we demonstrate an ultra-broadband power splitter on silicon, which has a 0.2 dB bandwidth exceeding 520 nm from 1500 to 2020 nm according to simulations. The beam splitter is realized by a triple tapered Y-junction, and its operational bandwidth is greatly increased by subwavelength grating structure. The device has an ultra-compact footprint of only 3µm×2µm. Due to the limitations on the setup and coupling technique, we measure the device bandwidth in 1.55 µm and 2 µm wave bands. The device insertion loss is measured to be below 0.4 dB from 1500 to 1620 nm and from 1960 to 2020 nm, respectively. According to these results, the proposed device is believed to be capable of operating over a broadband from 1.55 µm and 2 µm wavelengths.

Operating regimes of cavity solitons by virtue of a graphene flake saturable absorber.

Exploiting the. broadband operating frequency regimes of a graphene flake saturable absorber (GFSA), a cavity soliton (CS) is excited in heretofore unexplored ultraviolet and visible regions. A broad-area device, namely, a vertical cavity surface emitting laser (VCSEL) is taken as a host of CSs; a two-dimensional (2D) transverse soliton, which is quite different from the conventional propagating one. The VCSEL with an embedded 2D homogeneous transverse layer of a GFSA is coupled with a frequency selective feedback. A CS is also generated in the infrared region, especially at the optical communication wavelength. Spontaneous dynamics and interaction behavior of CSs as well as generation of CS molecules and the push-broom effect are reported in this broad cross-sectional device. In comparison with other existing and potential models, the proposed VCSEL with GFSA model shows greater efficiency.

A novel connected structure of all-optical high speed and ultra-compact photonic crystal OR logic gate

Abstract A new connected structure of an all-optical “OR” logic gate realized with photonic crystals is proposed in this study. The structure is based on coupling the input guides with two microcavities; the unit cell of the structure is designed to achieve a band gap around the communication wavelength (i.e., 1.55 µm). The performance of the structure results in transmission efficiency and low losses. This compact size logic gate is considered an important element in the integration of a nanoscale photonic device.

On-Chip Cascaded Bandpass Filter and Wavelength Router Using an Intelligent Algorithm

Cascaded nanophotonic devices play a vital role in all-optical connection, all-optical computation and all-optical network. However, there is almost no effective method for the direct design of on-chip cascaded nanophotonic devices, since current study of nanophotonic devices mostly focuses on single device. Here, on-chip cascaded nanophotonic devices are designed based on an intelligent algorithm by combining genetic algorithm, simulated annealing algorithm and finite element method for the first time, and verified experimentally by using silicon-based planar structures. The cascaded devices consist of a bandpass filter and a wavelength router operating in optical communication range. The operation bandwidth of the bandpass filter is 408 nm with transmission more than 80%, within which the communication wavelengths of 1,300 nm and 1,550 nm are routed into different output ports through the wavelength router component. The footprint is only 3.62 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the bandpass filter and only 2.56 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the wavelength router, which are easy for integration with planar structures and ultrasmall size. This work provides a highly-efficient scheme for the realization of on-chip cascaded nanophotonic devices on the same chip, and lays a foundation for the realization of photonic chip based on intelligent algorithm.

Ultracompact Energy Transfer in Anapole-based Metachains.

Realization of electromagnetic energy confinement beyond the diffraction limit is crucial for high-performance on-chip devices. Herein we construct an array of nonradiative anapoles that originate from the destructive far-field interference of electric and toroidal dipole modes to achieve ultracompact and high-efficiency electromagnetic energy transfer without the coupler. We experimentally investigate the proposed metachain at mid-infrared frequencies and give the first near-field experimental evidence of anapole-based energy transfer, in which the spatial profile of the anapole mode is also unambiguously identified on the nanoscale. We further demonstrate that the metachain is intrinsically lossless and scalable at infrared wavelengths, realizing a 90° bending loss down to 0.32 dB at the optical communication wavelength. The present scheme bridges the gap between the energy confinement and the transfer of anapoles and opens a new gate for more compactly integrated photonic and energy devices, which can operate in a broad spectral range.

Performance characteristics of low threshold current 1.25 μm type-II GaInAs/GaAsSb ‘W’-lasers for optical communications

Type-II ‘W’-lasers have made an important contribution to the development of mid-infrared laser diodes. In this paper, we show that a similar approach can yield high performance lasers in the optical communications wavelength range. (GaIn)As/Ga(AsSb) type-II ‘W’ structures emitting at 1255 nm have been realised on a GaAs substrate and exhibit low room temperature threshold current densities of 200–300 A cm−2, pulsed output powers exceeding 1 W for 100 µm wide stripes, and a characteristic temperature T 0 ≈ 90 K around room temperature. Optical gain studies indicate a high modal gain around 15–23 cm−1 at 200–300 A cm−2 and low optical losses of 8 ± 3 cm−1. Analysis of the spontaneous emission indicates that at room temperature, up to 24% of the threshold current is due to radiative recombination, with the remaining current due to other thermally activated non-radiative processes. The observed decrease in differential quantum efficiency with increasing temperature suggests that this is primarily due to a carrier leakage process. The impact of these processes is discussed in terms of the potential for further device optimisation. Our results present strong figures of merit for near-infrared type-II laser diodes and indicate significant potential for their applications in optical communications.

Study on the mechanical and optical properties of SiSn alloy by first principles

Group-IV semiconductor alloys are important materials for in-chip and inter-chip optoelectronic applications in silicon-based high-performance large-scale integrated circuits. As photovoltaic materials, silicon–tin alloys can improve the performance of solar cells by expanding the spectral range and increasing absorption. Silicon–tin alloys have broad application prospects within the communication wavelength range of 1.0–1.6 µm. In this work, the lattice constants, mechanical and optical properties of Si32−xSnx alloy are calculated. Generalized gradient approximation with Hubbard correction parameter U was used for exchange-correlation potential. The calculation results demonstrate that the lattice constant and formation energy increase with increasing number of tin atoms. In terms of mechanical properties, Si32−xSnx alloy shows mechanical stability and strong resistance to resist external forces, and it is not prone to volume deformation. It shows metallicity and tends to be ductile. In terms of optical properties, the width of the bandgap of Si32−xSnx alloy decreases with increasing tin concentration. The absorption of visible and infrared lights of Si32−xSnx alloy increases with increasing tin concentration. The dielectric properties of Si32−xSnx alloy are significantly changed by the addition of tin atoms. Doping prolongs the carrier lifetime of Si32−xSnx alloy, and its lifetime reaches maximum when x equals 6.

Efficient Er/O Doped Silicon Photodiodes at Communication Wavelengths by Deep Cooling

AbstractWide band infrared photodetectors have found a wide range of applications in sensing, communication, and spectral analysis. However, the commonly used infrared photodetectors are based on Ge and III‐V semiconductors which are not complementary metal‐oxide‐semiconductor (CMOS) compatible and therefore have limited applications. There is a huge demand for silicon‐based infrared photodetectors due to its low‐cost and compatibility with CMOS processes. Nevertheless, the spectral bandwidth of Si photodetectors is limited to wavelengths below 1.1 µm. Several approaches are developed to extend Si photodetection bandwidth to communication wavelengths. Er/O doped Si is a promising approach which, however, suffers from low infrared responsivities at room temperature when the samples are treated with the standard rapid thermal annealing (RTA). In this work, a novel deep cooling process to treat Er/O doped silicon waveguide photodiodes is applied. In comparison with RTA process, the deep cooling process reduces the defect concentration in silicon by two orders of magnitude, resulting in a two‐orders‐of‐magnitude reduction in leakage current density and an enhanced photoresponsivity to 100 mA W−1 at 1510 nm. The 3dB bandwidth of the silicon waveguide photodiode reaches 30 kHz. The device performance can be further improved by optimizing the deep cooling condition and Er/O doping concentration.

Demonstration of high-speed thin-film lithium-niobate-on-insulator optical modulators at the 2-µm wavelength.

Optical communication wavelength is being extended from the near-infrared band of 1.31/1.55 µm to the mid-infrared band of 2 µm or beyond for satisfying the increasing demands for high-capacity long-distance data transmissions. An efficient electro-optic (EO) modulator working at 2 µm is highly desired as one of the indispensable elements for optical systems. Lithium niobate (LiNbO3) with a large second-order nonlinear coefficient is widely used in various EO modulators. Here, we experimentally demonstrate the first Mach-Zehnder EO modulator working at 2 µm based on the emerging thin-film LiNbO3 platform. The demonstrated device exhibits a voltage-length product of 3.67 V·cm and a 3-dB-bandwidth of >22 GHz which is limited by the 18 GHz response bandwidth of the photodetector available in the lab. Open eye-diagrams of the 25 Gb/s on-off keying (OOK) signals modulated by the fabricated Mach-Zehnder EO modulator is also measured experimentally with a SNR of about 14 dB.

Effect of pitch on gain-noise figure and crosstalk performance of different core structure Multicore Erbium-Doped Fiber Amplifier: A comparative analysis

A detailed analysis of 7-cores Erbium-Doped Fiber Amplifier has been done to assess the effect of different core and pitch arrangement on the fiber amplifier's performance. The investigation is performed within the C-band communication wavelengths to compare the performance of Y-structure, T-structure, circular and linear structure of the core. The comparison is also extended further by including the performance of a widely used hexagonal structure. A thorough analysis is conducted by examining the profiles of gain, noise figure, and mode of transmission. The inter-core crosstalk (IC-XT) is also calculated for all the core structures and pitch, making this research effort conclusive. Maximum gain has been observed for the Y-shape/structure core arrangement, whereas minimum IC-XT has been observed for the hexagonal structure. However, an encouraging observation is that the percentage decrease in crosstalk due to an increase in pitch is significantly higher for Y-structure than hexagonal. Therefore, the authors believe that this detailed investigation gives significant insight to design efficient multicore EDFA in current research and development.

Behavior of strain-assisted self-trapped holes in pure-silica optical fibers upon pulsed-X-ray irradiation

Radiation-induced absorption (RIA) in a pure-silica fiber (PSF) was investigated at four wavelengths λ = 659, 828, 1310, and 1550 nm after seven X-ray pulses on the scale 1 μs–2 s. The behavior of RIA due to strain-assisted self-trapped holes (STHs) and of RIA at 1310 and 1550 nm was analyzed. The 1.63- and 1.88-eV strain-assisted STH bands were found to decay nearly completely after seven pulses with the total dose of just 440 Gy. Contrary to the previous assumptions, the 1.63- and 1.88-eV STH bands proved not to be responsible for RIA at the communication wavelengths (1.31 and 1.55 μm) upon pulsed-X-ray irradiation. The real origin of RIA at the communication wavelengths was identified, namely, the recently discovered STH bands centered near 1 eV. RIA due to STHs of other classes and types was found to emerge at the expense of the decayed strain-assisted STHs.

Gallium Arsenide‐Based Active Antennas for Optical Communication Photodetection with Robustness to Voltage and Temperature

AbstractAlthough gallium arsenide (GaAs) is one of the most commonly used semiconductor substrate materials, its intrinsic bandgap of 1.42 eV hinders the use of GaAs photodetectors for optical communication. In this study, hot‐electron‐based GaAs active antenna devices are demonstrated, displaying photoresponses well below the bandgap at the telecommunication wavelengths. Using a deep‐trench/thin metal (DTTM) active antenna, a metallic plasmonic structure, high photoresponsivities are achieved under zero bias at wavelengths of 1310 and 1550 nm. Even though the resistance of the semi‐insulating GaAs substrate is approximately 106 times larger than that of the n‐type silicon (Si) substrate, the photoresponsivities are commensurate with most previously reported hot‐electron n‐Si‐based photodetectors operating at communication wavelengths. Furthermore, the devices can be operated under a reverse‐biased voltage with significant enhancements in the photoresponsivities; the highest photoresponsivity (19.96 mA W−1 at 1310 nm) is greater than those reported in all previous studies. Moreover, these GaAs‐based devices are sufficiently robust to be operated over a wide range of operating temperatures (from −193 to +200 °C) while displaying a relatively large bandgap, low dark leakage currents, and high electron mobilities at low temperature. Because these devices can operate at high and low temperatures and at large voltage biases, they are suitable for use under harsh environmental conditions.