Broadband Optical Communication Research Articles

Emerging near‐infrared luminescent materials for next‐generation broadband optical communications

AbstractThe rapid development of emerging technologies observed in recent years, such as artificial intelligence, machine learning, mobile internet, big data, cloud computing, and the Internet of Everything, are generating escalating demands for expanding the capacity density, and speed in next‐generation optical communications. This poses a significant challenge to existing communication techniques. Within this context, the integration of near‐infrared broadband, tunable, and high‐gain luminescent materials into silicon optical circuits or fiber architectures to transmit and modulate light shows enormous potential for advancing next‐generation communication techniques. Here, this review provides an overview of the recent breakthroughs in near‐infrared luminescent epitaxial/colloidal quantum dots, and metal‐active‐center‐doped materials for broadband optical amplifiers and tunable lasers. We also expound on efforts to enhance the bandwidth and gain of these materials‐based amplifiers and lasers, exploring the challenges associate with developing ultra‐broadband and high‐speed optical communication systems. Additionally, the potential applications in Fifth Generation Fixed Networks, integration with 5G and 6G wireless networks, compensation for current Si electronic based CMOS for high computing capability, and the prospects of these light sources for next‐generation optoelectronic devices are discussed.image

Improving Photoelectric Conversion with Broadband Perovskite Metasurface.

The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 μs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Broadband Optical Amplification of PbS Quantum‐Dot‐Doped Glass Fibers

The rapid development of global telecommunication, cloud computing, big data, consumer electronics create increasing demands to expand the capacity and rate of data transmission for next generation broadband optical communication. It raises a great challenge for current rare‐earth ions doped fiber amplifiers due to the narrow emission bandwidth of rare‐earth ions. In this context, broadband near‐infrared (NIR) emitter based optical amplifiers are in urgent demand. Here, broadband optical amplification in S + C + L bands is achieved for the first time in PbS quantum dot (QD)‐doped low‐melting‐point glass fiber. With the pump of a 976 nm laser, the on‐off gain of PbS QD‐doped fiber (PQDF) ranges from 1.4 to 8.7 dB in 1500–1630 nm, while that of PbS QD‐doped glass ranges from 6.9 to 8.4 dB in 1530–1630 nm. Both QD‐doped glass and optical fiber show tunable luminescence covering the entire optical communication windows (O + E + S + C + L + U). The much higher quantum yield of PbS QDs in glass than that of colloidal QDs, the ultra‐broad and controllable emission bands and bandwidth make PbS QDs doped glass and fiber promising for applications in next generation broadband optical fiber amplifiers and tunable fiber lasers.

Low-Noise Phase-sensitive Parametric Amplifiers Based on Integrated Silicon-Nitride-Waveguides for Optical Signal Processing

Low-noise optical amplification has been a long-lasting research topic in various fields including communication, metrology and quantum optics. Here we report optical signal processing at different wavelengths enabled by a low-noise optical phase-sensitive amplifier (PSA) using a single compact integrated silicon nitride photonic waveguide. For 10 Gbit/s non-return-to-zero optical signals with central wavelengths tuned in the telecommunication band, the waveguide-based PSA exhibits a gain of more than 10 dB and a noise figure of about 1.2 dB when the coupling losses are neglected. For a bit-error-rate level of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> , penalties of less than 0.8 dB for all channels are observed after the signals propagating through the chip-based PSA, which may be due to the distortions in the few-mode waveguide. These experimental results generally agree with theoretical prediction and indicate that silicon-nitride-based PSAs are promising for the next generation of broadband optical signal processing and communication systems.

Tunable Broadband Mode Converter Based on Long-Period Fiber Gratings at 2-μm Waveband

We demonstrate the fabrication of a broadband mode converter (MC) operating at 2 μm waveband based on long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. The proposed MC offers a 10-dB bandwidth of 58 nm, making it suitable for high-speed broad-band optical communication system at 2 μm waveband. The maximum conversion efficiency reaches 98.89% at wavelength of 2 μm. The torsion, strain, bending, and polarization characteristics of the broadband MC were measured experimentally. The torsion sensitivity of the LPFGs was measured to be 1.32 nm/(rad/m) at 2 μm waveband, which was almost three times higher than that of the grating with the same order mode coupling at 1.55 μm waveband. The proposed broadband MC can function as not only a promotion in fiber communication system but also a high sensitivity torsion sensor in fiber sensing system.

1 × 2 Switchable Dual-Mode Optical 90° Hybrid Device Based on Thermo-Optic Phase Shifters and 2 × 2 MMI Couplers on SOI Platform

This paper designs a 1 × 2 switchable dual-mode optical 90° hybrid device on the silicon-on-insulator (SOI) platform for the mode division multiplexing hybridized coherent transmission systems in optical communication. Our proposed hybrid uses various 2 × 2 multimode interference (MMI) couplers, symmetric Y-junctions, and thermo-optic phase shifters. The suggested hybrid device working principle is proved on theoretical analysis about the multimode interference and the transfer matrix relation. The optimization of geometrical parameters and metallic micro-heaters is executed through the numerical simulation method. The proposed hybrid coupler expresses lots of optical performance advantages, with an insertion loss lower than 2.5 dB, a common-mode rejection ratio (CMRR) better than −24 dB, and phase error smaller than 4° in the 50-nm wavelength bandwidth of the third telecom window for both two TE modes. Besides, the device is proven large geometrical tolerances in terms of width and height tolerances larger than ±50 nm and ±6 nm in the 1.5-dB variation limit of transmission, respectively. In addition, the proposed device attains relatively low power consumption under 75 mW and an ultrafast switching time below 10 μs. Such advantages of good performances make the device a promising potential for broadband and intra-chip MDM-hybridized coherent optical communication systems and photonic integrated circuits.

High-order mode conversion in a few-mode fiber via laser-inscribed long-period gratings at 1.55 µm and 2 µm wavebands.

We demonstrate high-order mode conversion in a few-mode fiber (FMF) via CO2 laser inscribed long-period fiber gratings (LPFGs) at both the 1.55 µm and 2 µm wavebands. At the 1.55 µm waveband, five high-order core modes (the LP11, LP21, LP02, LP31, and LP12 modes) can be coupled from the LP01 mode with high efficiency by the FMF-LPFGs. The orbital angular momentum beams with different topological charges (±1,±2,±3) are experimentally generated by adjusting the polarization controllers. At the 2 µm waveband, three high-order modes (the LP11, LP21, and LP02 mode) can be coupled by the FMF-LPFGs with different grating periods. The proposed LPFG-based mode converters could have a potential prospects in mode-division multiplexing and multiwindow broadband optical communication applications.

Integrated Plasmonics: Broadband Dirac Plasmons in Borophene.

The past decade has witnessed numerous discoveries of two-dimensional (2D) semimetals and insulators, whereas 2D metals were rarely identified. Borophene, a monolayer boron sheet, has recently emerged as a perfect 2D metal with unique electronic properties. Here we study collective excitations in borophene, which exhibit two major plasmon modes with low damping rates extending from the infrared to ultraviolet regime. The anisotropic 1D plasmon originates from electronic transitions of tilted Dirac cones in borophene, analogous to that in extreme doped graphene. These features enable borophene as an integrated platform of 1D, 2D, and Dirac plasmons, promising for directional polariton transport and broadband optical communication in next-generation optoelectronic devices.

Efficient Mid-Infrared Electro-Optical Waveguide Modulators Using Ferroelectric Barium Titanate

Mid-Infrared (mid-IR) electro-optical (E-O) modulators are critical components in developing broadband optical communication since present E-O modulators were mainly operated in the near Infrared (NIR) range. Meanwhile, the conventional ferroelectric E-O material - lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> or LN) has lower Pockels effect comparing to barium titanate (BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> or BTO). This work demonstrated an E-O modulator in the mid-IR region applying a BTO substrate with low-loss titanium dioxide (TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) waveguides. The TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> waveguide device was first fabricated on a Si wafer by the complementary metal-oxide-semiconductor (CMOS) fabrication process. A fundamental waveguide mode was observed at λ = 2.5-02.7 μm, and an optical loss of 2.33 dB/cm was obtained. To achieve an efficient E-O modulation, the TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> waveguide was bonded with a <;001> BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystalline wafer that has a strong Pockels effect. The measured E-O coefficient γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> at λ = 2.6 μm was 119.7 pm/V, which is five times higher than the conventional LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> modulators, and a large modulation depth of 8.45 dB was accomplished.

Ultra-Low Material Loss Quasi Pattern Based Photonic Crystal Fiber for Long Distance THz Wave Propagation

In this study, photonic crystal fiber in quasi pattern (Q-PCF) has been proposed at the terahertz (THz) regime. In Q-PCF, the core area is designed using the two elliptical air holes for the best performance of optical communications to obtain the ultra-low effective material loss (EML). In Q-PCF, perfectly matched layer (PML) and the finite element method (FEM) have been applied to simulate the desired outcomes. The mentioned Q-PCF is optimized by shifting the core diameter, cladding air holes pitch and diameter, porosity of the model, and background materials. The numerically controlled outcomes ensure an EML of 0.01542 cm−1 with high power fraction response at 1 THz frequency. In addition, other loss profiles and operating modes have also been well described. Moreover, operating mode profile indicates single mode propagation up to 2.2 THz operating frequency. The noted outcomes ensure that the designed structure will be perfectly suitable for long distance broadband optical communication applications in THz technology.

Circular polarization shift-keying modulation based on orbital angular momentum division multiplexing in free space optical communication

Circular polarization shift-keying (CPolSK) modulation is crucial for optical communication in free space optical communication. In this paper, we propose a new optical communication system with multiple orbital angular momentum (OAM) modes for division multiplexing CPolSK modulation signals. A well-designed reference beam is employed to interfere with the CPolSK-OAM-division multiplexing (CPolSK-OAM-DM) signals for coherent detection. Then, the complex modulation signal is converted into intensity change directly detected by photo-detector. The communication performance can be further improved by subtracting the two interference beams detected. In numerical simulation, the demodulated bit error rate (BER) is 2.25 × 10−4 with the signal noise ratio (SNR) of 12-dB, and it can be reduced to 1.25 × 10−4 after difference operation. Through mapping the signals to both the initial phases and circular polarization states of the multiplexing channels, we construct the phase-CPolSK-OAM-DM system, the demodulated BER is 1.07 × 10−4 with the SNR of 15-dB. The results demonstrate that the CPolSK-OAM-DM modulation can effectively expand the communication capacity of the CPolSK system. The CPolSK-OAM-DM system has great potential in the field of high-speed broadband optical communications.

Utilization of a cascoded-inverter in an RGC structure as a low-power, broadband TIA

A low-power circuit structure is reported in this paper as a broadband TIA, in which the DC current and hence the power dissipation is lessened by applying cascoded structure to an inverter as an active-feedback network to a conventional Regulated Cascode (RGC) circuit. The improved RGC structure also benefits from an added active-inductor at the output node, which resonates with the output capacitance and provides extra bandwidth without the requirement of excessive current. Results for the proposed RGC-based TIA, which operates at 10 ​Gb/s, show only 1.6 ​mW power dissipation for obtaining 40 ​dBΩ transimpedance gain, 6.4 ​GHz frequency bandwidth and 25pA/√Hz input referred noise. Analysis indicate that the proposed TIA is suitable to work as a transimpedance amplifier in low-power, broadband optical communication receivers.

Surface plasmon induced broadband single-polarization filter based on high birefringence photonic crystal fiber

A broadband and single-polarization filter based on high birefringence photonic crystal fiber (HB-PCF) with selectively infiltrated gold wires and liquid crystal is proposed, and its filtering characteristics are studied by the full-vector finite element method (FV-FEM). Two gold wires are selectively filled on both sides of the liquid crystal core to act as two defect cores. The results show that the confinement loss of x-polarized core mode is much larger than that of y-polarized core mode in the wavelength range of 0.85–1.75 μm. The confinement losses of x- and y-polarized core modes are 99.87 dB/cm and 0.001 dB/cm at 0.88 um, 726.50 dB/cm and 0.004 dB/cm at 1.02 um, and 474.60 dB/cm and 0.046 dB/cm at 1.30 um, respectively. The bandwidth with extinction ratio less than 20 dB is 900 nm. Moreover, compared with the traditional structures, we find that our structure has higher birefringence. High birefringence helps to separate the resonant wavelengths of x- and y-polarized core modes. Therefore, the excellent properties of the proposed filter can make it widely used in sensing systems and broadband optical communication systems.

Broadband and fast photodetectors based on multilayer p-MoTe2/n-WS2 heterojunction with graphene electrodes

Two-dimensional (2D) atomic crystals, such as graphene, black phosphorus and transition metal dichalcogenides (TMDCs) are attractive for use in optoelectronic devices, due to their unique optical absorption properties and van der Waals (vdWs) force between layers. Heterostructures based on layered semiconductors provide a new platform for broadband high-performance photodetectors. In this work, graphene-MoTe2-WS2-graphene vdWs heterojunctions are fabricated for photodetection. The fundamental electric properties and the band structures of the heterojunctions are investigated and discussed. The devices show a high responsivity (≈ 140 mA W -1 at 825 nm), stable and broadband photodetection from UV to NIR wavelength range (300 - 1350 nm), fast response time of 186 µs and self-driven photodetectors. The scanning photocurrent microscopy maps are also employed to study the mechanism of photocurrent generation in the heterojunction. Our results reveal that the vdWs heterojunctions with graphene electrodes offer a new route to broadband detection, optical communication and energy harvesting applications.

Design of low-loss and near-zero ultraflattened dispersion PCF for broadband optical communication

A 2D finite difference time domain method is used to design a single-mode photonic crystal fiber (PCF) with engineered parameters that is studied with the aim of simultaneous achievement of low loss (below 10−14dB/km) and low chromatic dispersion over a broad band for future optical communication. The superiority of the proposed structure over recently reported structures is shown by the near-zero ultraflattened chromatic dispersion of 1.69±0.08 ps/(km nm) obtained over a broad wavelength range from 1 to 2 μm, covering the entire optical communication bands (O, E, S, C, L, and U). Also, the large effective area of more than 130 μm2 and as a result the low nonlinearity in addition to the near-zero ultraflattened chromatic dispersion and ultralow loss achieved for the proposed PCF make the structure a good candidate for high-capacity optical communication with a high bitrate over long distances for future ultradense wavelength-division multiplexing.

A MEMS Variable Optical Attenuator with Ultra-Low Wavelength-Dependent Loss and Polarization-Dependent Loss.

Applications in broadband optical fiber communication system need variable optical attenuators (VOAs) with low wavelength-dependent loss (WDL). Based on analysis on the dispersion of the optical system of a MEMS-based VOA, we provide a method to reduce the WDL significantly with minor revision on the end-face angle of the collimating lens. Two samples are assembled, and the measured WDL is <0.4 dB over the C-band (1.53–1.57 μm) at a 0–20 dB attenuation range. Meanwhile, the new structure helps to reduce the polarization-dependent loss (PDL) to <0.15 dB, which is only half that of conventional devices.

Design and analysis of 1xN symmetrical optical splitters for photonic integrated circuits

Optical power splitters play a momentous role because of its capability to completely eliminate active devices in the Passive Optical Network (PON) link which progressively improves the broadband optical communication. Communication link between the service provider and the user premises of PON networks depends on the splitter. Even though various types of splitters based on optical fibre are available, we report the design and simulation results of 1 × 2, 1 × 4 and 1 × 8 symmetrical splitters based on photonic crystal ring resonators. Photonic crystal optical splitters will provide technological compatibility for integration in Photonic Integrated Circuits (PIC). The designed layout operates at 1490 nm wavelength, which is the recommended ITU-T standard for down streaming in PON architecture. Finite difference time domain (FDTD) method and Plane wave expansion (PWE) method is used for simulation. The upshot of this simulation exhibits good splitting of optical power in terms of its overall efficiency, splitting ratio and propagation delay. Its miniaturised size and capacity to integrate as PIC, the reported layout makes itself as a best and suitable candidate for PON architecture.

Luminescent Properties and Optical Amplification of Erbium‐Doped Nano‐Engineered Scandium‐Phospho‐Yttria‐Alumina‐Silica Glass Based Optical Fiber

In this work, a new erbium (Er) doped nano‐engineered scandium‐phospho‐yttria‐alumina‐silica (SPYAS) glass‐based optical fiber is reported, fabricated through the MCVD process, and solution doping technique, followed by a suitable thermal annealing of pristine preform. The fabrication process comprises the incorporation of erbium doped phase‐separated scandium‐yttria rich crystalline nanoparticles into the core region through in situ process based on phase‐separation and crystal growth phenomena. The material characterization results, obtained from transmission electron microscopy, electron diffraction pattern, energy dispersive X‐ray, electron probe micro analysis, and X‐ray fluorescence, confirm the formation of Er2O3 doped crystalline phase‐separated scandium‐yttria rich nanoparticles in the core region. The formation of scandium‐ultra rich nano‐crystalline environment, possessing low photon energy around the erbium ions, enhanced the fluorescence intensity. Such kind of nano‐engineered glass reduces the noise figure around 4.35 dB, and provides broadband optical flat gain with an average value of 38.675 dB, varied by less than ±0. 7 dB spanning over a broad wavelength region of 1530–1590 nm compared to the pristine and Sc free Er‐doped fibers. Such kind of nano‐engineered glass based Er doped fiber will be useful for making highly efficient optical amplifiers, suitable for present broadband optical communication systems.

New functionalities of potassium tantalate niobate deflectors enabled by the coexistence of pre-injected space charge and composition gradient

In most beam steering applications such as 3D printing and in vivo imaging, one of the essential challenges has been high-resolution high-speed multi-dimensional optical beam scanning. Although the pre-injected space charge controlled potassium tantalate niobate (KTN) deflectors can achieve speeds in the nanosecond regime, they deflect in only one dimension. In order to develop a high-resolution high-speed multi-dimensional KTN deflector, we studied the deflection behavior of KTN deflectors in the case of coexisting pre-injected space charge and composition gradient. We find that such coexistence can enable new functionalities of KTN crystal based electro-optic deflectors. When the direction of the composition gradient is parallel to the direction of the external electric field, the zero-deflection position can be shifted, which can reduce the internal electric field induced beam distortion, and thus enhance the resolution. When the direction of the composition gradient is perpendicular to the direction of the external electric field, two-dimensional beam scanning can be achieved by harnessing only one single piece of KTN crystal, which can result in a compact, high-speed two-dimensional deflector. Both theoretical analyses and experiments are conducted, which are consistent with each other. These new functionalities can expedite the usage of KTN deflection in many applications such as high-speed 3D printing, high-speed, high-resolution imaging, and free space broadband optical communication.

Three order increase in scanning speed of space charge-controlled KTN deflector by eliminating electric field induced phase transition in nanodisordered KTN.

In this paper, we report a three orders-of-magnitude increase in the speed of a space-charge-controlled KTN beam deflector achieved by eliminating the electric field-induced phase transition (EFIPT) in a nanodisordered KTN crystal. Previously, to maximize the electro-optic effect, a KTN beam deflector was operated at a temperature slightly above the Curie temperature. The electric field could cause the KTN to undergo a phase transition from the paraelectric phase to the ferroelectric phase at this temperature, which causes the deflector to operate in the linear electro-optic regime. Since the deflection angle of the deflector is proportional to the space charge distribution but not the magnitude of the applied electric field, the scanning speed of the beam deflector is limited by the electron mobility within the KTN crystal. To overcome this speed limitation caused by the EFIPT, we propose to operate the deflector at a temperature above the critical end point. This results in a significant increase in the scanning speed from the microsecond to nanosecond regime, which represents a major technological advance in the field of fast speed beam scanners. This can be highly beneficial for many applications including high-speed imaging, broadband optical communications, and ultrafast laser display and printing.