Communication Window Research Articles

Artificial Intelligence-Assisted Accurate Spectrum Prediction in Design of Terahertz Fiber Operating in 6G Communication Window

Artificial Intelligence-Assisted Accurate Spectrum Prediction in Design of Terahertz Fiber Operating in 6G Communication Window

112 Gbps CMOS-compatible waveguide germanium photodetector for the 2 μm wavelength band with a 3.64 A/W response

The 2 μm wavelength band emerges as a promising candidate for the next communication window to enhance the transmission capacity of data. A high-responsivity and high-speed photodetector operating at 2 μm is crucial for the 2-μm-wavelength-band communication system. Here, we present an on-chip waveguide-coupled germanium photodetector with remarkably high responsivity and data-receiving rate, employing subbandgap light absorption and avalanche multiplication. The device is designed with an ingenious and simple asymmetric lateral p-i-n junction structure and fabricated through a standard CMOS process by a commercial factory. It has a responsivity of 3.64 A/W and a maximum bandwidth of 50 GHz at 2 μm wavelength. For the first time, to the best of our knowledge, an optical receiving rate of up to 112 Gbps is demonstrated at 2 μm, verifying its feasibility in a high-speed 2-μm-band communication system. To the best of our knowledge, the proposed device stands out as the fastest photodiode with the highest responsivity among all group III-V and group IV photodetectors working in the 2 μm wavelength band.

Tunable Fano resonances at 2 μm waveband based on a single microring resonator

Abstract Development of silicon photonic devices for the 2 μm waveband is urgent since the 2 μm waveband has been considered as a potential communication window for next-generation optical communications. Asymmetric Fano resonances are very promising in the development of devices such as optical switches, sensors, and modulators for the 2 μm waveband. In this paper, a Fano resonance at the 2 μm waveband is theoretically realized by using a silicon-based microring resonator and the thermo-optic effect is employed to tune it. Utilizing this single microring resonator structure instead of the present coupled microring resonator structures, we achieve the Fano resonance with an extinction ratio of 30.5 dB and a slope ratio of 69.3 dB nm−1 at the 2 μm waveband. The metal heater and graphene heater are designed and optimized to achieve the Fano tuning. The thermal-optical tuning efficiencies are 0.46 nm mW−1, 0.61 nm mW−1 and 1 nm mW−1 for the metal heater, the metal heater with air adiabatic slots, and the graphene heater, respectively. This work provides an effective scheme for the formation and tuning of the Fano resonance at the 2 μm waveband, which is conducive to the development of devices and optical communication systems at the 2 μm waveband.

Er3+ -doped oxyfluoroborosilicate glass ceramics with embedded CaF2 nanoparticles for 1.53 μm broad band applications

The CaF2 based oxyfluoroborosilicate glasses and glass ceramics doped with Er3+ ions were prepared via melt quench process followed by heat treatment and characterized for 1.53 μm broadband applications. The optimized glass ceramic sample was obtained at 450oC/1h heat treatment. The Er3+ concentration was optimized as 1.0 mol% for efficient emission at 460 nm excitation through concentration dependent luminescence analysis. The spectroscopic parameters such as Ωλ = 2,4,6 parameters and the radiative parameters such as spontaneous transition probability rates (AR), branching ratios (βR) and decay times (τR) were calculated applying the standard Judd-Ofelt theory. The effective bandwidth (Δλeff), stimulated emission cross-section (σe), gain bandwidth (σe × Δλeff), quantum efficiency (η) and figure of merit (σe × τR) were calculated as 25.78 nm, 13.42 × 10−21 cm2, 3.46 × 10−26 cm3, 82.83% and 5.32 × 10−23 cm2s, respectively for the optimized glass ceramic sample. The exchange type of energy transfer among the excited Er3+ ions results the quenching in luminescence and the non-exponentiality in decay curves. The systematic investigations carried out indicate that the glass ceramic obtained at 450oC/1h heat treatment was proficient for 1.53 μm broadband fiber lasers and optical amplifiers in S and C band communication window.

Er3+/Tm3+ codoped CaF2 based oxyfluoroborosilicate glass-ceramics for fiber laser applications

Transparent Er3+/Tm3+ codoped CaF2 based oxyfluoroborosilicate glass-ceramics (BSEr1TmxGCs) with variable Tm3+ concentration were prepared through melt quench process followed by reheat treatment at 450 °C/1h. They were characterized through differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) Raman spectroscopy, near infrared (NIR) emission and luminescence decay. The formation of CaF2 nanocrystallites against oxyfluoroborosilicate glassy phase was confirmed by scanning electron microscopic (SEM) and hi-resolution transmission electron microscopic (HRTEM) studies. The NIR emission properties were investigated at 460 nm diode laser pumping. The applicability of BSEr1TmxGCs were examined by evaluating effective bandwidth (Δλeff), stimulated emission cross-section (σe), gain bandwidth (σe × Δλeff), figure of merit (σe × τR) and quantum efficiency (ηQE). The energy transfer efficiency (ηET), rate of energy transfer (WET) between Er3+ and Tm3+ and the rate of non-radiative transitions (WNR) were also calculated. The comparative NIR emission performance suggests that the BSEr1Tm1GC has proficiency for 1530 nm broadband fiber lasers and optical amplifiers in short wavelength and conventional wavelength (S + C) band communication window.

Characteristics study of a dual-wavelength polarization filter based on gold-filled square lattice photonic crystal fiber

This study proposes a polarization filter based on gold-filled photonic crystal fiber (PCF) with a square lattice structure. Utilizing the finite element method (FEM), the filtering characteristics of the model were numerically simulated. The dispersion relation and loss characteristics of this structure were analyzed, and the optimal structural parameters were obtained through structural adjustments. Furthermore, the performance of the filter was evaluated under a 2% variation in manufacturing tolerances for gold film thickness (), air hole spacing (), and air hole diameters () and (). Results indicated that at the 1310 nm and 1550 nm communication windows, the confinement losses of the filter were 1354.6 dB/cm and 869.04 dB/cm, respectively, while the losses for the -polarization were merely 16.97 dB/cm and 0.98 dB/cm. When the filter length was set to 0.5 mm, the maximum extinction ratios (ERs) for the two windows reached 588.2 dB and 370.6 dB, and the filter bandwidth extended to 640 nm. Moreover, the characteristics of the filter under manufacturing tolerances were computed, revealing that the filter’s performance remained stable and feasible despite manufacturing errors. The proposed rectangular gold-coated PCF is anticipated to have broad applications in optical fiber communication and optical information processing.

Understanding of efficient optical circuit using silicon waveguide at third communication window

Understanding of efficient optical circuit using silicon waveguide at third communication window

Carbon and Silicon Impurity Defects in GaN: Simulating Single-Photon Emitters by First Principles.

Defect single-photon emitters (SPE) in gallium nitride (GaN) have garnered great attentions in recent years due to the advantages they offer, including the ability to operate at room temperature, narrow emission linewidths, and high brightness. Nevertheless, the precise nature of the single-photon emission mechanism remains uncertain due to the multitude of potential defects that can form in GaN. In this work, our systematical investigation with the ab initio calculation indicates that carbon and silicon, as common dopants in gallium nitride, can interact with intrinsic defects in GaN and form new high-speed defect single-photon sources. Our findings identify a ternary defect NGaVNCN that possesses a short lifetime of less than 1 ns and a small zero-photon line (ZPL) of 864 nm. In other words, this defect can serve as a high-speed single photon source in the short wavelength window for fiber communication. In sharp contrast, the Si-supported defect NGaVNSiN has a higher unoccupied defect energy level which enters the conduction band and is therefore unsuitable for single photon emission. A systematic investigation has been conducted into the potential defects, thermal stability, and single-photon emission properties. The relaxation calculation and self-consistent calculations employed the Perdew-Burke-Ernzerhof exchange-correlation functional and Heyd-Scuseria-Ernzerhof exchange-correlation functional, respectively. These findings indicate the potential for high-performance single-photon sources through carbon or silicon doping of GaN.

Numerical simulation of an in-fiber broadband plasmonic polarization filter using photonic crystal fiber with dual elliptical gold layers

The demand for advanced and compact photonic filters is increasing in today's optical communication systems. This work introduces a new all-fiber broadband polarization filter using photonic crystal fiber with dual elliptical gold layers. The filtering performance is evaluated and analyzed by the finite element method. The simulation results demonstrate that the two elliptical gold layers can effectively induce a significant change in the energy of x-polarized light, leading to a great difference between the two polarized lights. By selecting reasonable structural parameters of the proposed PCF, the 1 mm-long in-fiber filter has a maximum extinction ratio of -108.90 dB, a low insertion loss of 0.64 dB around 1.31 µm and an ultra-broad bandwidth of 1020 nm, covering two common communication windows of 1.31 µm and 1.55 µm. Additionally, the filtering experimental scheme is discussed. This filter is expected to drive the advancement of optical integrated chips and all-optical communication networks.

Polarization independent perfect absorption of borophene metamaterials operating in the communication band

Two-dimensional (2D) materials, such as graphene and black phosphorus, support deeply confined and tunable plasmons, making them suitable for designing absorbers with ultra-compact size and flexible manipulation. However, the operating frequency of such plasmonic absorbers is difficult to control to the communication band. Here, we propose a metamaterials composed of a borophene array, a dielectric layer and a metal reflector to achieve an optical perfect absorber near the communication wavelength of 1550 nm. In order to overcome the polarization sensitivity caused by anisotropic borophene materials, another borophene layer is introduced to achieve a polarization independent absorber, which can be attributed to the fact that the energy of the electromagnetic field is transferred between two borophene arrays as the polarization angle changes. In addition, through the modulation of carrier density, it is feasible to fine-tune the resonance wavelength of the absorber to 1330 nm, which corresponds precisely to the second communication window. This work may provide a theoretical foundation for the development of polarization independent devices, potentially broadening the scope of their applications.

Proposal and investigation of a distributed learning strategy in Orbital Edge Computing-endowed satellite networks for Earth Observation applications

One of the key enabling solutions to in-orbit extract information from Earth Observation images is given by deep learning techniques. However, the accuracy of these algorithms is strictly related to the availability of large datasets of satellite images for training purposes. Limitations on the available transmission bandwidth in the orbital context may prevent the possibility to downlink all acquired images to a node where centralized training happens. Instead, Federated Learning (FL) could be fruitfully leveraged in this scenario, since it provides for each satellite to train a local model only with its own dataset, and then to share its trained model with a central server, which receives models trained by the different satellites and aggregates them into a new global model being eventually shared with all the satellites, and this repeats until convergence is reached. However, because communication with a node acting as a central parameter server may be still limited by short visibility time, the described process may need a long time because of limited communication windows, negatively impacting the time needed to reach model convergence. For this reason, we propose a communication strategy to support a completely distributed learning technique to train a deep learning model in-orbit, by leveraging the fact that satellites may form a network thanks to the potential availability of Inter-Satellite Links (ISLs) within and between orbital planes. Our proposal is different from a FL approach since we provide for each satellite to receive all the information needed to calculate an updated global model by itself, without leaning on a central parameter server. Numerical results show that distributed learning outperforms FL in number of learning rounds completed in the unit time, allowing for reaching validation accuracy convergence in a shorter time, as it has been verified on a land coverage classification task based on the EuroSAT dataset.

Sustainable photonics for energy harvesting and beyond, from solar cells to OLEDs, efficient communication, and smart windows

Sustainable photonics for energy harvesting and beyond, from solar cells to OLEDs, efficient communication, and smart windows

MONET: The Minor Body Generator Tool at DART Lab.

Minor bodies exhibit considerable variability in shape and surface morphology, posing challenges for spacecraft operations, which are further compounded by highly non-linear dynamics and limited communication windows with Earth. Additionally, uncertainties persist in the shape and surface morphology of minor bodies due to errors in ground-based estimation techniques. The growing need for autonomy underscores the importance of robust image processing and visual-based navigation methods. To address this demand, it is essential to conduct tests on a variety of body shapes and with different surface morphological features. This work introduces the procedural Minor bOdy geNErator Tool (MONET), implemented using an open-source 3D computer graphics software. The starting point of MONET is the three-dimensional mesh of a generic minor body, which is procedurally modified by introducing craters, boulders, and surface roughness, resulting in a photorealistic model. MONET offers the flexibility to generate a diverse range of shapes and surface morphological features, aiding in the recreation of various minor bodies. Users can fine-tune relevant parameters to create the desired conditions based on the specific application requirements. The tool offers the capability to generate two default families of models: rubble-pile, characterized by numerous different-sized boulders, and comet-like, reflecting the typical morphology of comets. MONET serves as a valuable resource for researchers and engineers involved in minor body exploration missions and related projects, providing insights into the adaptability and effectiveness of guidance and navigation techniques across a wide range of morphological scenarios.

Design of L+Band Bismuth-doped Fiber Laser

At present, the development of new communication bands to meet the needs of large communication transmission capacity in the current era is one of the key research topics. The conventional erbium-doped laser is limited by its structure which has the problem of short luminous bandwidth and can not cover a large communication band. Therefore, there is growing interest in designing bismuth-doped glass and fiber with ultra-wide gain bandwidth for fiber laser applications. By utilizing its ultra-wide gain bandwidth characteristics, important communication devices such as broadband fiber amplifiers and broadband tunable fiber lasers can be designed to operate in the L+band, thereby solving the problem that the ultra-wide bandwidth of the optical communication window cannot be achieved in the future high-speed communication systems. In this study, the 1550nm pump source is used to excite the bismuth-doped fiber, and the MATLAB numerical simulation modeling is used to design a bismuth-doped fiber laser with a central wavelength of 1650nm and a threshold power of 10W. To a certain extent, this design addresses the research gap in L+ band bismuth-doped fiber lasers.

Design and simulation of a compact plasmonic photonic crystal fiber filter with wide band and high extinction ratio

In order to realize the performance optimization and function expansion of modern all-optical systems, this work presents a compact plasmonic photonic crystal fiber (PCF) filter, using finite element tool. The deposited gold and graphene layers within the elliptical pores interact with the transmitted light, creating surface plasmon resonance (SPR) effect that amplifies the energy difference between x- and y-polarized light greatly. The simulation results indicate that the structural parameters are configured with the cladding holes' diameter of 0.6 μm, the large-holes’ diameter of 1.2 μm, the inner small-holes’ diameter of 0.2 μm, the lattice constant of 2.0 μm, the elliptical holes' minor axis length of 0.45 μm, the elliptical holes' major axis length of 1.30 μm, the gold layer thickness of 50 nm, and the graphene layer thickness of 20 nm, the central wavelength of the proposed PCF filter is 1.56 μm. When the length of this PCF filter is 1 mm, a maximum extinction ratio (ER) of 133 dB, an operating bandwidth exceeding 800 nm, covering two common communication windows of 1.31 μm and 1.55 μm, as well as a low insertion loss (IL) of 0.59 dB can be achieved. What's more, the feasibility of fabricating the device is also examined. This filter with compactness, broadband and high-extinction demonstrates promising applications in optical communication, optical sensing, optical computing, and various other fields.

Optical amplification across the whole communication windows in PbS quantum-dot-doped fiber

The surge of big data, cloud computing, and AI bring in explosive requirement of data transmission capacities. However, the transmission capacity of traditional single-mode-fiber already reaches its limit due to the narrow emission bandwidth of rare-earth-ion. Here, we achieved ultra-broadband optical amplification in O + E + S + C + L bands for the first time in PbS quantum-dot-doped glass fiber with the maximum gain of 5 dB at 1370 nm under 100 mW pump. The mechanism of the broadband optical amplification, the advantages of quantum dot-doped fiber amplifier (QDFA), and a new measurement scheme for QDFA are systematically discussed. It is believed that in the future, the further optimization of the composition and architectures of the glass fiber, and the preparation method of quantum dot-doped fiber may lead to important applications in optical interconnection, artificial intelligence, Internet of things, mobile electronics and so on.

Structural, optical properties, and laser spectroscopy of erbium-doped low-melting borate glasses

Glasses that contain Erbium oxide as a dopant exhibit garnered considerable interest for its use in photonic applications, primarily because of the intra-4f sublevel transitions of Erbium ions, which enable emission in the technologically significant 1.5 μm communications window. In this work, a series of Erbium oxide doped sodium lithium borate glasses were synthesized via melt quenching with compositions xEr2O3-(55-x)B2O3–25Li2O–10Na2O–5CaO–3SrO–2Al2O3 (x = 0, 1, 2, 3, and 5 mol%). An investigation was conducted to examine the consequences of Er3+ on the crystal structure and laser spectroscopy. Furthermore, the optical properties of the samples that were created. XRD analysis showed degradation of the glass network with increasing Er2O3 content due to disruption of boroxol rings. FTIR spectra indicated variations in BO3/BO4 ratios and Er–O vibrations with doping. Absorption spectra revealed characteristic Er3+ f-f transitions and a redshift in the UV edge. The bright field imaging proved the effect of Er3+ on the homogeneity of the surfaces. Meanwhile, 3D laser Raman mapping chemically proves the good distribution of Er2O3 on the glass surface. With an excitation under 980 nm, visible emissions at 550 and 660 nm, along with NIR emission at 1550 nm arising from Er3+ 4f levels, were observed. The results demonstrate that Er3+ doping effectively modifies the glass structure and optical properties. The borate glass host provides an excellent platform for customized erbium-doped photonic devices operating in the telecom C-band.

Origami metamaterials for ultra-wideband and large-depth reflection modulation

The dynamic control of electromagnetic waves is a persistent pursuit in modern industrial development. The state-of-the-art dynamic devices suffer from limitations such as narrow bandwidth, limited modulation range, and expensive features. To address these issues, we fuse origami techniques with metamaterial design to achieve ultra-wideband and large-depth reflection modulation. Through a folding process, our proposed metamaterial achieves over 10-dB modulation depth over 4.96 – 38.8 GHz, with a fractional bandwidth of 155% and tolerance to incident angles and polarizations. Its ultra-wideband and large-depth reflection modulation performance is verified through experiments and analyzed through multipole decomposition theory. To enhance its practical applicability, transparent conductive films are introduced to the metamaterial, achieving high optical transparency (>87%) from visible to near-infrared light while maintaining cost-effectiveness. Benefiting from lightweight, foldability, and low-cost properties, our design shows promise for extensive satellite communication and optical window mobile communication management.

A four-band coherent perfect absorber of borophene metamaterial operating in the communication band

We propose a coherent perfect absorber based on a borophene metamaterial grating structure. Based on the theoretical analysis of the temporal coupled mode theory and the numerical results of the finite-difference time-domain simulations, the transmission characteristics of the system are investigated. The dynamic tunability of the absorption intensity and resonance wavelength is realized by adjusting the phase difference of the coherent incident beams and carrier density of borophene. Moreover, by optimizing the parameters, the coherent perfect absorption peak (99.99%) is obtained near the wavelength of 1550 nm, that is, the third communication window. Meanwhile, the structure can be operated in ultra-thin gratings as low as 6 nm and angle tolerances as large as 45°, which provides the possibility for applications in highly integrated and wide-angle tolerated devices.

P–i–n photodetector with active GePb layer grown by sputtering epitaxy

In this paper, single-crystal GePb films were obtained by magnetron sputtering with high substrate temperature and rapid deposition rate. The GePb films have high crystalline qualities and smooth surface. The Pb content reached 1.29% and no segregation was observed. Based on this, a GePb based p–i–n photodetector was successfully prepared. The device showed a RT dark current density of 5.83 mA cm−2 at −1.0 V and a cutoff wavelength of 1990 nm, which covers all communication windows. At the wavelength of 1625 nm, responsivity of the photodetector reached 0.132 A W−1 at −1.0 V. The device demonstrates potential application in optical communications.