Opportunities and Challenges for Long-Distance Transmission in Hollow-Core Fibres

Anti-resonant hollow-core fiber of the Nested Antiresonant Nodeless type (NANF) has been showing a steady decrease in loss over the last few years, gradually approaching that of standard Single-Mode Fiber (SMF). It already by far outperforms SMF as to non-linear effects, which are three to four orders of magnitude lower in NANF than in SMF. Theoretical predictions and experimental evidence also hint at a much wider usable bandwidth than SMF, potentially amounting to several tens of THz. Propagation speed is 50% faster, a key feature in certain contexts. In this paper we investigate the potential impact of possible future high-performance NANF on long-haul optical communication systems, assuming NANF continues on its current steady path towards better performance. We look at the system throughput in different long-haul scenarios, addressing links of various length, from 100~km to 4,000~km, and different NANF optical bandwidths, loss and total launch power. We compare such throughput with a benchmark state-of-the-art SMF Raman-amplified C+L system. We found that NANF might enable relative throughput gains vs.~the benchmark on the order of 1.5x to 5x, at reasonable NANF and system parameter values. We also study the problem of the impact of NANF Inter-Modal-Interference (IMI) on system performance and show that a value of -60~dB/km, close to the currently best reported values, is low enough to have no substantial harmful effect. We finally look at a more long-term scenario in which NANF loss gets below that of SMF and we show that in this context repeterless or even completely amplifierless systems might be possible, delivering 300-400 Tb/s per NANF, over 200 to 300~km distances. The system simplification and ease of wideband exploitation implied by these systems might prove quite attractive especially in densely populated regions where inter-node distances are modest. While several technological hurdles remain towards NANF systems becoming practical contenders, in our opinion NANF appears to have the potential to become an attractive and possibly disruptive alternative to conventional solid-core silica fibers

Ultrawideband Systems and Networks: Beyond C + L-Band

In the evolution of optical networks, enhancement of spectral efficiency (SE) enhancement has been the most cost-efficient and thus the main driver for capacity enhancementincrease for decades. As a result, the development of optical transport systems has been focused on the C -and L -bands, where silica optical fiber exhibits the lowest attenuation, and erbium-doped fiber amplifiers provide an efficient solution forto compensatinge for the optical loss. With a gradual maturity in the SE growth, however, the extension of the optical bandwidth beyond the C + L -band is expected to play a significant role in the future capacity upgrades of optical networks and, thus, attracting increasing research interests. In this article, we discuss the merits and challenges of ultrawideband optical transport systems and networks beyond conventional bands

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques

Roadmap on spatiotemporal light fields

Spatiotemporal sculpturing of light pulse with ultimately sophisticated structures represents the holy grail of the human everlasting pursue of ultrafast information transmission and processing as well as ultra-intense energy concentration and extraction. It also holds the key to unlock new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are always treated as spatiotemporally separable wave packet as solution of the Maxwell's equations. In the past decade, however, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This roadmap intends to highlight the recent advances in the creation and control of increasingly complex spatiotemporally sculptured pulses, from spatiotemporally separable to complex nonseparable states, with diverse geometric and topological structures, presenting a bird's eye viewpoint on the zoology of spatiotemporal light fields and the outlook of future trends and open challenges.Comment: This is the version of the article before peer review or editing, as submitted by an author to Journal of Optics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Dense wavelength division multiplexing at 2 μm for future optical communications

The internet is ubiquitous in our lives today, enabling instantaneous access to information, increased international collaboration, and even real-time remote surgery. It has changed how we socialise and how we see the world. From 2017 to 2022, global internet traffic is forecasted to triple, creating severe pressure on optical communication systems. Delivering the capacity required to support this traffic presents significant challenges in terms of system design, device performance, and optical fibre capabilities. Novel solutions are needed now, if we are to meet the demands of the near future. While solutions to increase system capacity and bandwidth efficiency of optical communication systems at 1.55 µm is widely discussed in the literature, shifting transmission to the 2 µm wavelength window could open possibilities to new discoveries in optical components, improved bandwidth efficiency, innovative optical fibres, and other applications transcending beyond optical communications. This thesis explores opening the 2 µm transmission window for optical communications. The focus of this work investigates the feasibility of implementing dense wavelength division multiplexing (DWDM) systems at 2 µm, with key enabling technologies developed recently. Strained III-V materials can produce foundry-compatible 2 µm lasers and detectors. Hollow-core photonic band gap fibres (HC-PBGFs) can guide 2 µm light through air, offering potentially lower losses, reduced latency and higher power-handling capabilities (in comparison to standard silica fibre). Also, thulium doped fibre amplifiers (TDFAs) could offer bandwidth of up to ~30 THz in the 2 µm waveband (~double that of EDFAs at 1.55 µm). Contributions of this thesis include the demonstration of a 2 µm DWDM system with channel spacing of 100 GHz and system capacity above 100 Gbit/s, for the first time in this new transmission window. Further increasing the capacity of 2 µm DWDM systems requires improving the spectral efficiency, which can be accomplished by reducing the spacing between channels. While 50 GHz channel spacing is shown to be achievable with current technologies in the transmitter, insufficient filtering can be a barrier for implementation in the receiver. With this in mind, novel filtering technologies are required and optical injection locking (OIL) is investigated as a possible filtering solution. The first study of OIL using two slotted Fabry-Perot lasers at 2 µm is demonstrated, achieving a stable OIL bandwidth of ~7 GHz and OIL-induced single mode operation over a 15 GHz range

High spectral efficiency transmission using optical frequency combs

Modern long-haul optical communication systems transmit data on all available single-mode fiber dimensions, time, polarization, wavelength, phase and amplitude. Powerful digital signal processing and forward error correction has pushed the per-channel throughput towards its theoretical limits and the bandwidth is limited by the erbium-doped fiber amplifiers. Maximizing the spectral efficiency (SE), i.e. the throughput normalized to bandwidth, is therefore of indisputable importance. Even more so in optical networks as large routing guard-bands drastically reduce the SE of traditional WDM systems. Flex-grid networks with optical superchannels can overcome this limitation. Superchannels consist of multiple tightly packed WDM channels routed as a unit. A comb-based superchannel is formed by encoding independent information onto lines from an optical frequency comb, a multi-wavelength light source fully determined by its center frequency and line spacing. This thesis studies the generation, transmission and detection of comb-based superchannels. Focus is on profiting from unique frequency comb properties to realize systems with capabilities beyond that of conventional systems using arrays of independent lasers. Digital, analog and optical processing schemes are proposed, and combined, to increase the system SE. Superchannel modulation is investigated and a scheme capable of encoding independent information onto the lines from a frequency comb in a single waveguide structure is demonstrated. By combining overhead-optimized pilot-based DSP with a 22GHz-spaced soliton microcomb, superchannel transmission with record SE for distances up to 3000km is realized, closing the performance gap between chip-scale and bulk-optic combs in optical communications. The use of two optical pilot tones (PTs) to phase-lock a transmitter and receiver comb pair is studied, realizing self-homodyne detection of a 50x20Gbaud PM-64QAM superchannel with 4% pilot overhead. The PT gains are furthermore analyzed and a complexity-performance trade-off using a single PT and low complexity DSP is proposed. The scheme is used to demonstrate 12bits/s/Hz SE over the full C-band using 3x50xGBaud PM-256QAM superchannels and DSP-complexity reduction at distances exceeding 1000km is shown. Finally, a comb-enabled multi-channel joint equalization scheme capable of mitigating inter-channel crosstalk and thereby minimizing the SE loss from spectral guard bands is demonstrated

Miniaturized Silicon Photodetectors

Silicon (Si) technologies provide an excellent platform for the design of microsystems where photonic and microelectronic functionalities are monolithically integrated on the same substrate. In recent years, a variety of passive and active Si photonic devices have been developed, and among them, photodetectors have attracted particular interest from the scientific community. Si photodiodes are typically designed to operate at visible wavelengths, but, unfortunately, their employment in the infrared (IR) range is limited due to the neglectable Si absorption over 1100 nm, even though the use of germanium (Ge) grown on Si has historically allowed operations to be extended up to 1550 nm. In recent years, significant progress has been achieved both by improving the performance of Si-based photodetectors in the visible range and by extending their operation to infrared wavelengths. Near-infrared (NIR) SiGe photodetectors have been demonstrated to have a “zero change” CMOS process flow, while the investigation of new effects and structures has shown that an all-Si approach could be a viable option to construct devices comparable with Ge technology. In addition, the capability to integrate new emerging 2D and 3D materials with Si, together with the capability of manufacturing devices at the nanometric scale, has led to the development of new device families with unexpected performance. Accordingly, this Special Issue of Micromachines seeks to showcase research papers, short communications, and review articles that show the most recent advances in the field of silicon photodetectors and their respective applications

Wireless Terahertz Communications: Optoelectronic Devices and Signal Processing

Novel THz device concepts and signal processing schemes are introduced and experimentally confirmed. Record-high data rates are achieved with a simple envelope detector at the receiver. Moreover, a THz communication system using an optoelectronic receiver and a photonic local oscillator is shown for the first time, and a new class of devices for THz transmitters and receivers is investigated which enables a monolithic co-integration of THz components with advanced silicon photonic circuits