Zhiqun Yang

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

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.

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

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.

Experimental demonstration of adaptive recursive least square frequency-domain equalization for long-distance mode-division multiplexed transmission

We demonstrate experimentally adaptive recursive-least-square frequency-domain equalization (RLS-FDE) for mode-division-multiplexing. The RLS-FDE algorithm improves convergence speed as transmission distance increases. For MDM transmission over a 1,000-km few-mode fiber, the convergence speed was increased by 17.5 times in comparison with LMS-FDE.

Rayleigh backscattering in few-mode optical fibers

We measure inter-modal Rayleigh backscattering in few-mode optical fibers and compare the experimental results with simulation results obtained from newly-developed analytical formulae. Excellent agreement between theory and experiment allows investigation of mode-coupling dynamics using OTDR.

Amplified Spontaneous Emission and Rayleigh Scattering in Few-Mode Fiber Raman Amplifiers

A theoretical model of noise—amplified spontaneous emission and Rayleigh backscattering—in few-mode fiber Raman amplifiers is presented in this letter. Based on this model, the equalization of the signal modal gain determines the equalization of the noise modal gain. The model can also be used to predict the mode-dependent optical signal-noise ratio. The theoretical results of this model are consistent with reported experimental results.

Nonlinear conductivity and space charge behaviors of SiR/SiC composites

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, furtherly 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. The effects of the SiC particle content on the field-dependent conductivity and the space charge behaviors of HTV SiR were studied. 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 suppressing the accumulation of space charge.

MIMO-free space-division-multiplexing for data center applications

6×10 Gb/s polarization- and mode-multiplexed transmission without MIMO equalization for data center applications is experimentally demonstrated, over a 200-m elliptical core with two PANDA stress rods few-mode fiber (EP-FMF). This fiber simultaneously breaks the degeneracies of space and polarization in a mode group. A large effective index difference (Δneff=6.6×10-4 ) between adjacent modes among 6-modes is achieved over the entire C band, which translates to lower crosstalk than previous fiber designs. A polarization crosstalk of <-16 dB over 200 m and loss less than 0.63 dB/km were achieved.

Surface charge behaviors of SiR/SiC composites with nonlinear conductivity

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 tried to modify the surface charge behaviors of HTV SiR composites by incorporating silicon carbide (SiC) particles with different filler size and crystal morphology The influences of SiC filler size and crystal morphology on the dc conductivity and surface charge behavior of HTV SiR composites were studied. The results indicated that, for the α-SiC particles filled composites, with the increase of the filler size from 0.45 μm to 5.0 μm, the fleld-dependent conductivity and surface potential decay (SPD) is enhanced, due to an increase of shallow trap density and a decrease of deep trap density. Compared with α-SiC particles, the β-SiC whiskers could provide a larger nonlinear conductivity, resulting in an acceleration of surface charge dissipation.

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

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.

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

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.