Optical Data Communication Research Articles

(Invited) Epitaxial III-V Nanomaterials on the Si Platform for Quantum and Data Communications

As Si-based integrated photonic components begin to replace older technologies for optical data communications, the need for better integration of semiconductor lasers on the Si platform is greater than ever. In addition, emerging interest in integrated-photonic-based quantum computing and quantum sensing presents a further need for quantum light emitters—entangled and single-photon—on Si. III-V quantum dots represent a promising solution, both for the active region of telecommunications-wavelength lasers and for quantum emitters. However, epitaxial quantum dots grown on conventional (100) substrates typically lack the high symmetry required for entangled-photon emission.This presentation will discuss our recent work to develop a novel approach for III-V direct growth on Si via an ultrathin (<10 nm) single crystal strain relieving buffer layer, realized by the selective oxidation and condensation of an amorphous SiGe surface layer. The fabrication and properties of this novel Ge-rich buffer layer, as well as its use for subsequent III-V organometallic vapor-phase epitaxy will be presented.The second part of the talk will present our recent work on the epitaxial growth and characterization of (In,Ga)As quantum dots for the above-mentioned applications. Specifically, I will show that surface-energy-modifying surfactants (Sb, Bi) present the possibility to externally direct quantum dot formation “on-demand” in strained InAs layers on GaAs(111) substrates, where quantum dot formation by the Stranski–Krastanov process does not naturally occur. These nanostructures are prospective for high-symmetry entangled-photon emitters. The three-dimensional (3D) composition profile of InAs quantum dot layers is characterized by atom-probe microscopy (APT), elucidating the real-world quantum dot matrix. This 3D composition data is input into a finite element solver to determine the quantum dot energies and wavefunctions for electrons and holes. The modeling reveals that in high-density quantum dot layers, the electron and hole states are hybridized between multiple quantum dots in the matrix. This has important consequences for design of quantum dot light emitters. The predicted transition energies are in agreement with low-temperature photoluminescence. This work constitutes a promising approach for integrating telecommunications-wavelength quantum dot lasers and entangled photon emitters on the Si integrated photonics platform.

AC-FPSK modulation for low-energy and low data rate optical wireless communications

This article proposes an amalgam of phase-shift keying and asymmetrically clipped frequency-shift keying (i.e., AC-FPSK) for low-energy/low-data rate optical wireless communications. We design a near-optimal, low-complexity harmonic receiver by analyzing the AC-FPSK time-domain (TD) and frequency-domain (FD) waveforms. The Euclidean distance analysis between AC-FPSK waveform pairs yields lower and upper bounds for theoretical bit error probability. Using numerical simulations, we present an exhaustive evaluation of the optimal cardinality of PSK, which illustrates the superiority of AC-FPSK. We show that AC-FPSK provides better energy efficiency versus spectral efficiency trade-off than state-of-the-art AC-FSK, direct current-FPSK, pulse amplitude modulation, and asymmetrically clipped optical orthogonal frequency division multiplexing. We also demonstrate that by making a suitable choice for the cardinality of PSK, the proposed 2-tap harmonic receiver for AC-FPSK can reach a similar complexity as the AC-FSK harmonic receiver.

Ultra-broadband magneto-optical isolators and circulators on a silicon nitride photonics platform

Broadband optical isolators and circulators are highly desirable for wavelength-division multiplexing, light detection, and ranging systems. However, the silicon-integrated optical isolators and circulators reported so far have a limited isolation bandwidth of only several nanometers, due to waveguide and material dispersion. In this paper, we report the development of broadband magneto-optical isolators on silicon nitride waveguides. We proposed a general method of dispersion compensation to achieve a constant phase difference between reciprocal and nonreciprocal phase shifts in a Mach–Zehnder interferometer over a wide frequency range. This method enabled a theoretical 30 dB isolation/circulation bandwidth of more than 240 nm, which covers the S, C, L, and U bands. The fabricated devices showed a maximum isolation ratio of 28 dB, crosstalk of −28dB, high 20-dB isolation bandwidth of 29 nm (3.48 THz), and a relatively low loss of 2.7 dB in the wavelength range of 1520–1610 nm. By further heating the reciprocal phase shifter based on the thermo-optic effect, the experimental 20 dB isolation bandwidth of the device increased to 90 nm (11.03 THz). This method has also been applied to the design of broadband, low-loss isolators, and O/C dual-band isolators/circulators. Our work experimentally demonstrated broadband-integrated optical isolators and circulators on silicon, paving the way for their use in optical communication, data communication, and LiDAR applications.

Low Surface Recombination in Hexagonal SiGe Alloy Nanowires: Implications for SiGe-Based Nanolasers.

Monolithic integration of silicon-based electronics and photonics could open the door toward many opportunities including on-chip optical data communication and large-scale application of light-based sensing devices in healthcare and automotive; by some, it is considered the Holy Grail of silicon photonics. The monolithic integration is, however, severely hampered by the inability of Si to efficiently emit light. Recently, important progress has been made by the demonstration of efficient light emission from direct-bandgap hexagonal SiGe (hex-SiGe) alloy nanowires. For this promising material, realized by employing a nanowire structure, many challenges and open questions remain before a large-scale application can be realized. Considering that for other direct-bandgap materials like GaAs, surface recombination can be a true bottleneck, one of the open questions is the importance of surface recombination for the photoluminescence efficiency of this new material. In this work, temperature-dependent photoluminescence measurements were performed on both hex-Ge and hex-SiGe nanowires with and without surface passivation schemes that have been well documented and proven effective on cubic silicon and germanium to elucidate whether and to what extent the internal quantum efficiency (IQE) of the wires can be improved. Additionally, time-resolved photoluminescence (TRPL) measurements were performed on unpassivated hex-SiGe nanowires as a function of their diameter. The dependence of the surface recombination on the SiGe composition could, however, not be yet addressed given the sample-to-sample variations of the state-of-the-art hex-SiGe nanowires. With the aforementioned experiments, we demonstrate that at room temperature, under high excitation conditions (a few kW cm-2), the hex-(Si)Ge surface is most likely not a bottleneck for efficient radiative emission under relatively high excitation conditions. This is an important asset for future hex(Si)Ge optoelectronic devices, specifically for nanolasers.

Direct photoluminescence manipulation in Er-doped AgNbO3 lead-free antiferroelectrics via reversible electric-field-induced symmetry modulations

The electric field-modulated photoluminescent (E-PL) effect has received much attention because of its potential applications in optical data storage and communication. Based on the E-induced antiferroelectric-ferroelectric (AFE-FE) phase transition, a large and reversible modulation of the PL intensity was previously realized in lanthanum-doped Pb(Zr0.9Ti0.1)O3 (PLZT) ceramics. However, such systems contain abundant leads and are thus toxic to humans and the environment. In this work, Er-doped AgNbO3 (ANO) lead-free ceramics were prepared, and their E-PL effect was investigated. From the perspective of the crystallographic symmetry, the reversibly E-induced orthorhombic Pbcm to orthorhombic Pmc21 transition results in a negative PL intensity change of 25%, which is different from the positive 30% PL modulation in the PLZT system triggered by the AFE-FE orthorhombic-rhombohedral phase transition. This symmetry-dependent negative PL modulation in ANO ceramics enriches the understanding of the E-PL effect in AFEs and provides an environmentally friendly alternative for multifunctional optoelectronics.

Integrated silicon photonic MEMS

Silicon photonics has emerged as a mature technology that is expected to play a key role in critical emerging applications, including very high data rate optical communications, distance sensing for autonomous vehicles, photonic-accelerated computing, and quantum information processing. The success of silicon photonics has been enabled by the unique combination of performance, high yield, and high-volume capacity that can only be achieved by standardizing manufacturing technology. Today, standardized silicon photonics technology platforms implemented by foundries provide access to optimized library components, including low-loss optical routing, fast modulation, continuous tuning, high-speed germanium photodiodes, and high-efficiency optical and electrical interfaces. However, silicon’s relatively weak electro-optic effects result in modulators with a significant footprint and thermo-optic tuning devices that require high power consumption, which are substantial impediments for very large-scale integration in silicon photonics. Microelectromechanical systems (MEMS) technology can enhance silicon photonics with building blocks that are compact, low-loss, broadband, fast and require very low power consumption. Here, we introduce a silicon photonic MEMS platform consisting of high-performance nano-opto-electromechanical devices fully integrated alongside standard silicon photonics foundry components, with wafer-level sealing for long-term reliability, flip-chip bonding to redistribution interposers, and fibre-array attachment for high port count optical and electrical interfacing. Our experimental demonstration of fundamental silicon photonic MEMS circuit elements, including power couplers, phase shifters and wavelength-division multiplexing devices using standardized technology lifts previous impediments to enable scaling to very large photonic integrated circuits for applications in telecommunications, neuromorphic computing, sensing, programmable photonics, and quantum computing.

Phase Stability and Dual-Mode Photoluminescence Modulation in Er-Doped Lead Zirconate Titanate Antiferroelectrics.

The electric-field-modulation (E-modulation) of photoluminescence (PL) properties in bulk ceramics has attracted tremendous interest due to its potential application in optical data storage and communication devices. One promising approach of reversibly and largely modulating the PL intensity has been proposed in rare-earth Er3+-doped Pb0.96La0.04Zr0.9Ti0.1O3 (PLZT) antiferroelectrics (AFEs) based on the unique E-dependent antiferroelectric-ferroelectric (AFE-FE) phase transition. However, the AFE phase stability of PLZT doped with various Er contents and their E-modulated PL properties have not been systematically investigated. In this paper, the intrinsic AFE phase of PLZT-Er is found to be stabilized in the high-temperature and high-E regions with increasing Er3+ content. The enhanced AFE nature caused by increasing Er doping leads to a larger E-dependent PL tunability (∼35%). Moreover, the ceramics exhibit the characteristics of both upconversion and downconversion PL (UCPL and DCPL) effects. Based on the excellent E-dependent dual-mode PL tunability, an optoelectronic device named the optical latch is demonstrated, where an electric signal can be used to trigger a notable intensity change in both the UCPL and DCPL modes. This reversible E-dependent dual-mode capability in PLZT-Er sheds light on a feasible approach to optoelectronic applications.

Simulation of ultra long reach and high speed date rate optical wireless multiplexing communication systems based on various modulation codes

Abstract This article has clarified ultralong reach and high data rate optical wireless multiplexing communication systems based on various modulation codes. Optical wireless communications can provide communication at high speed and achieve farther long distance compared to RF links. It is assured that the low bit rates can be used for long distance error-free transmission. The Q-factor decreases as the distance increases. The error at the receiving station increases with increase in transmission distance. Higher antenna aperture can achieve long distance transmission. The simulation results presented ultralong reach high speed date rates by utilizing VCSEL light sources and APD receivers. The optical signals with low bit rates can be used for long transmission reach. The performance of OWC link can be improved by using VCSELs and APDs. Therefore, the distance reach can be extended to 5000 km. The Q reduces and BER increases as the data rate increases for both NRZ and RZ schemes over various transmission reach on different temperature and various aperture diameters.

Wavelength-division multiplexing communications using integrated soliton microcomb laser source.

In this Letter, we report an investigation of the feasibility and performance of wavelength-division multiplexed (WDM) optical communications using an integrated perfect soliton crystal as the multi-channel laser source. First, we confirm that perfect soliton crystals pumped directly by a distributed-feedback (DFB) laser self-injection locked to the host microcavity has sufficiently low frequency and amplitude noise to encode advanced data formats. Second, perfect soliton crystals are exploited to boost the power level of each microcomb line, so that it can be directly used for data modulation, excluding preamplification. Third, in a proof-of-concept experiment, we demonstrate seven-channel 16-quadrature amplitude modulation (16-QAM) and 4-level pulse amplitude modulation (PAM4) data transmissions using an integrated perfect soliton crystal as the laser carrier; excellent data receiving performance is obtained for various fiber link distances and amplifier configurations. Our study reveals that fully integrated Kerr soliton microcombs are viable and advantageous for optical data communications.

Establishment of line-of-sight optical links between autonomous underwater vehicles: Field experiment and performance validation

Establishing a line-of-sight link between autonomous underwater vehicles (AUVs) is an unavoidable challenge for realizing high data rate optical communication in ocean exploration. We propose a method for link establishment by maintaining the relative position and orientation between AUVs. Using a reinforcement learning algorithm, we search for the policy that can suppress external disturbances and optimize the link establishment efficiency. To evaluate the performance of the proposed method, we prepared a hovering AUV to conduct the link establishment experiments. The reinforcement learning policy trained in a simulation environment was deployed on the AUV in real environments. In field experiments, our approach successfully performed the link establishment from the hovering AUV to an autonomous surface vehicle. Based on the experimental results, we evaluate the performance of the AUV in executing the link establishment policy. Comparisons with existing optical search-based link establishment methods are presented.

Customized Vectorial Optical Fields in Homogeneous and Inhomogeneous Media

The ability to generate structured light in different media is important for various applications, such as optical trapping, imaging, and data communication, but is technically difficult. This study uses ideas from the calculus of variations to develop a simplified, consistent framework that can be used to generate the required customized optical fields. The proposed approach is easily generalized, extending its applicability beyond optics to other wave-related phenomena and subjects, such as acoustics.

Novel low-cost high-speed optic–electric laser diode pigtail module packaging technology

• At present, the research and development of high-speed laser modules for communication are designed with ceramic components globally. • In module packaging without the ceramic part, the misalignment caused by the deformation results in coupling efficiency loss. • A new type of high-speed laser diode pigtail module is designed to change the structure of a laser module and remove ceramic and process parts. • A laser diode pigtail module package achieves the best coupling efficiency. A high-speed laser diode pigtail for wide-band fiber-optic communications is a key component in optical fiber user loop systems, optical fiber data communication systems, and cable television systems. With the advent of a new type of optic–electric high-speed laser, wide-band communication has emerged. These three systems constitute future mainstreams of optical fiber communications. However, high-speed laser diode pigtails used in module components and process assembly account for extremely high production costs. The proposed pigtail module eliminates ceramic parts and facilitates mass production of the components. An optic fiber (including a jacket) was placed into a ferrule sleeve. The optic fiber did not undergo laser damage, and its stability in the reliability test was ensured by the plastic jacket. The optic fiber terminal surfaces grind the 7° angle to enhance the surface quality of the connector, thereby avoiding reflection and incurring a power loss of 4%. Moreover, achieving alignment in a conventional laser module package without a ceramic part and fiber ferrule is a considerable challenge, and apparently, there are still some issues that need to be resolved. In conventional optical fiber module package, laser welding technique provides a superior joint strength. However, in module packaging without the ceramic part, the misalignment caused by the deformation results in coupling efficiency loss. Therefore, a novel design of laser diode pigtail module packaging is necessary. A transmitter optical subassembly device, receiver optical subassembly device, and transceiver pigtail module can be manufactured in a unified process package. By applying the low-cost packaging technique, the cost and fabrication time of the module packaging were reduced, and the fabrication yield was increased.

Unraveling hole interlayer-dependent interfacial energetics of LEDs

To fundamentally understand the underlying interfacial physics of hole transporting layer is a key to rotationally engineer high-performance light-emitting diodes (LEDs). Here, we unravel the hole interlayer-dependent interfacial energetics of LEDs via combining imaged contact potential/polarization difference, and multi-time scale capacitance/voltage response. A universally existing but undesired stagnation of current density with voltages, under the direct contact of light-emitting Poly-TPD with electrodes, was proved to be rooted from the alternant dominance of capacitance/resistance behaviors with alternating strong/weak polarizations of Poly-TPD. The introduction of hole interlayer (PEDOT:PSS) with more disordered polarization direction and greater intensity can build plentiful carrier hopping channels and ensure the rapidly dynamic charge transfer from electrodes to Poly-TPD, conducing to the increased capacitive reactance contribution at low voltages and inductive effects at high voltages. Also, we clarify its frequency-response difference for emerging high-speed modulation application. This discovery might enable us to explore the next-generation high-efficiency display and optical data-communication.

Silicon photonic flat-top WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band.

The 2 µm wavelength band has proven to be a promising candidate for the next communication window. Wavelength-division multiplexing (WDM) transmission at 2 µm can greatly increase the capacity of optical communication systems. Here, we experimentally demonstrate a high-performance silicon photonic flat-top 8-channel WDM (de)multiplexer based on cascaded Mach-Zehnder interferometers for the 2 µm wavelength band. A three-stage-coupler scheme is utilized to provide passbands and reduce channel crosstalk, and 11 thermo-optic phase shifters have allowed active compensation of waveguide phase errors. The fabricated device shows low insertion loss (< 0.9 dB), channel crosstalk (< 20.6 dB) and 1-dB bandwidth of 2.3 nm for operating wavelength ranging from 1955nm to 1985nm. The demonstrated (de)multiplexer could potentially be used for WDM optical data communication in the 2 µm spectral band.

Top-emitting 940-nm thin-film VCSELs transferred onto aluminum heatsinks

Thin-film vertical cavity surface emitting lasers (VCSELs) mounted onto heatsinks open up the way toward low-power consumption and high-power operation, enabling them to be widely used for energy saving high-speed optical data communication and three-dimensional sensor applications. There are two conventional VCSEL polarity structures: p-on-n and n-on-p polarity. The former is more preferably used owing to the reduced series resistance of n-type bottom distributed Bragg reflection (DBR) as well as the lower defect densities of n-type GaAs substrates. In this study, the p-on-n structures of thin-film VCSELs, including an etch stop layer and a highly n-doped GaAs ohmic layer, were epitaxially grown in upright order by using low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The p-on-n structures of thin-film VCSELs were transferred onto an aluminum heatsink via a double-transfer technique, allowing the top-emitting thin-film VCSELs to keep the p-on-n polarity with the removal of the GaAs substrate. The threshold current (Ith) and voltage (Vth) of the fabricated top-emitting thin-film VCSELs were 1 mA and 2.8 V, respectively. The optical power was 7.7 mW at a rollover point of 16.1 mA.

A Conversion Protocol for 2W Telephone Signal over Ethernet in a Private PSTN

In this paper, we proposed a protocol to convert 2W telephone analog signals to Ethernet data in a private PSTN 2W tactical voice system. There are several kinds of operational problems in the tactical telephone network where 2W telephone copper lines are installed hundreds of meters away from the PBX in a headquarter site. The reason is that it is difficult to install and maintain the 2W telephone copper cable in severe operational fields and to meet safety and stability operational requirements of the telephone line under lighting and electromagnetic environments. In order to solve these challenging demands, we proposed an efficient method that the 2W analog interface signals between a private PBX system and a 2W telephone is converted to Ethernet messages using the optical Ethernet data communication network already deployed in the tactical weapon system. Thus, it is not necessary to install an additional optic cable for the ethernet telephone line and to maintain the private PSTN 2W telephone network. Also it provides safe and secure telecommunication operation under lightning and electromagnetic environments.&lt;br/&gt;This paper presents the conversion protocol from 2W telephone signals over Ethernet interface between PBX systems and 2W telephones, the mutual exchange protocol of ethernet messages between two converters, and the rule to process analog signal interface. Finally, we demonstrate that the proposed technique can provide a feasible solution in the tactical weapon system by analyzing its performance and experimental results such as the bandwidth of 2W telephone ethernet network and the transmission latency of voice signal, and the stability of optic ethernet voice network along with the ethernet data network.

Analog ghost hidden in 2D random binary patterns for free-space optical data transmission

We propose high-fidelity analog ghost diffraction and transmission through scattering media in free space using a series of 2D randomly-distributed binary patterns. The proposed method utilizes ghost diffraction to enable high-fidelity free-space optical transmission through scattering media. Any type of ghosts, e.g., analog signal, can be encoded into a series of 2D randomly-distributed binary patterns to serve as information carriers. After the generated 2D randomly-distributed binary patterns are sequentially embedded into spatial light modulator and are illuminated to propagate through scattering media in free space, a single-pixel detector is used to collect light intensity at the receiving end and high-fidelity signals can be retrieved without any complex post-processing algorithm. The proposed method possesses high robustness for high-fidelity free-space optical transmission through scattering media, and different wavelengths and different propagation distances can be flexibly used for free-space optical transmission. The method could open up an avenue towards many applications, e.g., free-space optical data transmission and communication.

Designing optical fields in inhomogeneous media.

Designing optical fields with predetermined properties in source-free inhomogeneous media has been a long-sought goal due to its potential utilization in many applications, such as optical trapping, micromachining, imaging, and data communications. Using ideas from the calculus of variations, we provide a general framework based on the Helmholtz equation to design optical fields with prechosen amplitude and phase inside an inhomogeneous medium. The generated field is guaranteed to be the closest physically possible rendition of the desired field. The developed analytical approach is then verified via different techniques, where the approach's validity is demonstrated by generating the desired optical fields in different inhomogeneous media.

Tailoring frequency combs through VCSEL polarization dynamics.

We investigate experimentally the nonlinear polarization dynamics of a VCSEL subject to optical injection of a frequency comb. By tuning the polarization of the injected comb to be orthogonal to that of the VCSEL, we demonstrate the generation of either a single polarization or a dual polarization frequency comb. The injection parameters (injected power and detuning frequency) are then used either to generate harmonics of the initial comb spacing or to increase the number of total output frequency lines up to 15 times the number of injected comb lines. Optimisation of the injection parameters yields a comb extending over 60 GHz for a comb spacing of 2 GHz with a carrier to noise ratio (CNR) of up to 60 dB. Our technique allows us to separately control the comb spacing, comb bandwidth, CNR and polarization. Our finding can be used for spectroscopy measurement and also for polarization division multiplexing in optical data communications.

Epitaxially Grown InP Micro-Ring Lasers.

In the near future, technological advances driven by the Fourth Industrial Revolution will boost the demand for integrated, power-efficient miniature lasers, which are important for optical data communications and advanced sensing applications. Although top-down fabricated III-V semiconductor micro-disk and micro-ring lasers have been shown to be efficient light sources, challenges such as etching-induced sidewall roughness and poor fabrication scalability have been limiting the potential for high-density on-chip integration. Here, we demonstrate InP micro-ring lasers fabricated with a highly scalable epitaxial growth technique. With an optimized cavity design, the optically pumped micro-ring lasers show efficient room-temperature lasing with a lasing threshold of around 50 μJ cm-2 per pulse. Remarkably, through comprehensive modeling of the micro-ring laser, we demonstrate lasing mode engineering experimentally by tuning the vertical ring height. Our work is a major step toward realizing the high-density monolithic integration of III-V miniature lasers on submicrometer-scale optoelectronic devices.