Photonic lanterns are made by adiabatically merging several single-mode cores into one multimode core. They provide low-loss interfaces between single-mode and multimode systems, where the precise optical mapping between cores and individual modes is unimportant.

The photonic lantern

The tilted fiber Bragg grating (TFBG) is a new kind of fiber-optic sensor that possesses all the advantages of well-established Bragg grating technology in addition to being able to excite cladding modes resonantly. This device opens up a multitude of opportunities for single-point sensing in hard-to-reach spaces with very controllable cross-sensitivities, absolute and relative measurements of various parameters, and an extreme sensitivity to materials external to the fiber without requiring the fiber to be etched or tapered. Over the past five years, our research group has been developing multimodal fiber-optic sensors based on TFBG in various shapes and forms, always keeping the device itself simple to fabricate and compatible with low-cost manufacturing. This paper presents a brief review of the principle, fabrication, characterization, and implementation of TFBGs, followed by our progress in TFBG sensors for mechanical and biochemical applications, including one-dimensional TFBG vibroscopes, accelerometers and micro-displacement sensors; two-dimensional TFBG vector vibroscopes and vector rotation sensors; reflective TFBG refractometers with in-fiber and fiber-to-fiber configurations; polarimetric and plasmonic TFBG biochemical sensors for in-situ detection of cell, protein and glucose.

[INVITED] Tilted fiber grating mechanical and biochemical sensors ☆

Digital coherent transceivers have revolutionized optical fiber communications due to their superior performance offered compared to intensity modulation and direct detection based alternatives. As systems employing digital coherent transceivers seek to approach their information theoretic capacity, the use of multilevel modulation formats combined with appropriate forward error correction becomes essential. Given this context, in this tutorial paper, we, therefore, explore the digital signal processing (DSP) utilized in a coherent transceiver with a focus on multilevel modulation formats. By way of an introduction, we open by discussing the photonic technology required to realize a coherent transceiver. After discussing this interface between the analog optical channel and the digital domain, the rest of the paper is focused on DSP. We begin by discussing algorithms that correct for imperfections in the optical to digital conversion, including IQ imbalance and timing skew. Next, we discuss channel equalization including means for their realization for both quasi-static and dynamic channel impairments. Synchronization algorithms that correct for the difference between the transmitter and receiver oscillators both optical and electrical are then discussed and issues associated with symbol decoding highlighted. For most of the cases, we start with polarization division multiplexed quadrature phase-shift keying format as a basis and then discuss the extension to allow for high-order multilevel formats. Finally, we conclude by discussing some of the open research challenges in the field.

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Digital Signal Processing for Coherent Transceivers Employing Multilevel Formats

A Strategic Review on Gallium Oxide Based Deep-Ultraviolet Photodetectors: Recent Progress and Future Prospects

Low-loss all-fiber photonic lantern (PL) mode multiplexers (MUXs) capable of selectively exciting the first six fiber modes of a multimode fiber (LP01, LP11a, LP11b, LP21a, LP21b, and LP02) are demonstrated. Fabrication of the spatial mode multiplexers was successfully achieved employing a combination of either six step or six graded index fibers of four different core sizes. Insertion losses of 0.2-0.3 dB and mode purities above 9 dB are achieved. Moreover, it is demonstrated that the use of graded index fibers in a PL eases the length requirements of the adiabatic tapered transition and could enable scaling to large numbers.

Six mode selective fiber optic spatial multiplexer.

We demonstrate the first few-mode-fiber based passive optical network, effectively utilizing mode multiplexing to eliminate combining loss for upstream traffic. Error-free performance has been achieved for 20-km low-crosstalk 3-mode transmission in a commercial GPON system carrying live Ethernet traffic. The alternative approach of low modal group delay is also analyzed with simulation results over 10 modes.

Time-division-multiplexed few-mode passive optical network.

High temperature vulcanized (HTV) silicone rubber (SiR), due to its excellent electrical insulation, elasticity and temperature resistance, is used in cable accessories for HVDC cables. The space charge, generated under dc stress in SiR, would distort the local electric field, further influencing the characteristics of partial discharge and accelerating the insulation aging. This paper tried to modify the space charge behaviors of HTV SiR composites by incorporating silicon carbide (SiC) particles into the polymer matrix with the filler content of 10, 30, 50, 100 wt%. The effects of the SiC particle content on the field-dependent conductivity and the space charge behaviors of HTV SiR were studied. The conductivities under different electric fields were measured by a three-electrode system at room temperature and the space charge behaviors were observed by the pulsed electro-acoustic (PEA) method. The results indicated that the conductivity of SiR/SiC composites was a nonlinear function of electric field when the content exceeds 30 wt%. With the increase of filler content, the threshold field for nonlinear conductivity decreased and the nonlinear coefficient increased. Meanwhile, the amount of space charges was dramatically reduced. It is suggested that the SiR/SiC composites with nonlinear conductivity can improve the dc conductivity under high electric field, thus raising the carrier mobility and suppressing the accumulation of space charge.

Field-dependent conductivity and space charge behavior of silicone rubber/SiC composites

Few-mode receivers exploiting parallelism of free space for space-division multiplexed (SDM) transmission, in which one free-space 90° optical hybrid can be shared among multiple SDM channels, are proposed and experimentally demonstrated. A $\text{2} \times \text{20 Gb}/\text{s}$ quadrature phase-shift keying two-mode mode-division multiplexed receiver has been demonstrated experimentally.

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Few-Mode SDM Receivers Exploiting Parallelism of Free Space

High temperature vulcanized (HTV) silicone rubber (SiR) is widely used as HVDC cable accessory insulation considering its outstanding electrical properties along with elasticity and thermal resistance. However surface charge caused by corona would accumulate on SiR surface and distort the local field which in consequence led to accessory premature discharge and insulation failure. This paper focused on the behavior of surface charge of SiR composites and managed to modify it by incorporating silicon carbide (SiC) particles into SiR matrix. The content of SiC varied from 0 to 100 % and its effects on the field-dependent conductivity and surface charge behaviors were discussed. The outcomes of experiments showed that the SiR/SiC composites performed a nonlinear conductivity as a function of electric field when the content of SiC exceeded 30 %. It was also noticed that this field-dependent conductivity became more obvious when the content of SiC increased. Meanwhile, the decay rate of surface charge was observed to rise with the increase of SiC content and voltage. It was confirmed that the SiR/SiC composites could accelerate the decay of surface charge and increase the dc conductivity under high voltage, resulting from the modification of the trap level distribution.

Surface charge behavior of silicone rubber/SiC composites with field-dependent conductivity

The addition of silicon carbide (SiC) micro powder into silicone rubber (SiR) matrix has been found to introduce a marked nonlinear conductivity into the resulting composites, without a degradation of the insulating properties under low electric field. Owing to these properties, the SiR-based nonlinear conductive composites have a potential use as the field grading material (FGM) layer in high voltage direct current (HVDC) cable accessories, which is employed to uniform electric field and suppress charge accumulation at the interface between crosslinked polyethylene (XLPE) and accessory insulation. In this paper, the space charge transportation and dissipation behaviors of SiC/SiR composites were investigated by pulsed electro-acoustic (PEA) test, in view of the effects of SiC filler size and crystal morphology. The results indicated that with the enhancement of nonlinear conductivity, the charge transportation and dissipation processes were accelerated. Compared with α-SiC particles, the β-SiC whiskers could provide a larger nonlinear conductivity. Meanwhile, the trap level distributions of SiC/SiR composites showed that the β-SiC whiskers introduced a shallower trap level at 0.81 eV than the α-SiC particles (0.85 eV). In addition, for the α-SiC particles filled composites, with the increase of the filler size from 0.45 μm to 5.0 μm, the shallow trap density increased, and the deep trap density decreased, resulting in an enhancement of space charge dissipation.

Zhiqun Yang

Papers

Time-division-multiplexed few-mode passive optical network.

Field-dependent conductivity and space charge behavior of silicone rubber/SiC composites

Few-Mode SDM Receivers Exploiting Parallelism of Free Space

Surface charge behavior of silicone rubber/SiC composites with field-dependent conductivity

Effects of crystal morphology on space charge transportation and dissipation of SiC/silicone rubber composites