A fiber bundle structure with uniform transmission characteristics for high-density astronomical optical cables

Transmission efficiency(TE) and focal ratio degradation(FRD) are two important parameters for evaluating the quality of an optical fiber system used for astronomy. Compared to TE, the focal ratio is more easily influenced by external factors, such as bending or stress. Optical cables are widely implemented for multi-object telescopes and integral field units(IFUs). The design and fabrication process of traditional optical cables seldom considers the requirements of astronomical applications. In this paper, we describe a fiber bundle structure as the basic unit for miniaturized high-density FASOT-IFU optical cables,instead of the micro-tube structure in stranded cables. Seven fibers with hexagonal arrangement were accurately positioned by ultraviolet(UV)-curing acrylate to form the bundle. The coating diameter of a fiber is0.125 mm, and the outer diameter of the bundle is 0.58 mm. Compared with the 0.8 mm micro-tube structure of a traditional stranded cable, the outer diameter of the fiber bundle was reduced by 27.5%. Fiber paste was filled into the bundle to reduce stress between the fibers. We tested the output focal ratio(OFR) in95% of the encircled energy(EE95) of the fibers in the bundle under different conditions. With the incident focal ratio F/8, the maximum difference of OFR is 0.6. In particular, when the incident focal ratio is F/5,the maximum difference of OFR is only 0.1. The jacket formed by the UV-curing acrylate can withstand a certain stress of less than 1.38 N mm-1. The fiber bundle can maintain uniform emitting characteristics with a bending radius of 7.5 cm and with tension less than 6 N. The test results show that the structure of the fiber bundle can be used as a basic unit for miniaturized high-density astronomical optical cables.

Facile Preparation of Al2O3 Hollow Microspheres Via a Urea-mediated Precipitation Process

Al2O hollow microspheres without noticeable aggregation have been prepared via a facile templating route with urea-mediated precipitation. The precipitation process is different from the surfaceadsorption method which is confined to the adsorption capacity of the template surface. TEM and SEM images indicate that most of these Al2O hollow microspheres with shell thickness of tens of nanometers and diameters in a narrow range of 100-200 nm consist of a shell of closely packed nanoparticles. The optimal amount of H2O and EtOH are 40 and 120 mL, respectively. The specific surface area, average pore size and pore volume of the Al2O hollow microspheres (calcinated at 600 °C) are 328.52 m2/g, 17.496 nm and 1.985 cm'/g, respectively. As the calcination temperature increases from 600 to 1 100 °C, the phase composition changes from y-Al2O3 to &-AI2O3 and a-Al2O3, and the surface morphology appears to change from a relatively rough surface formed by nanoparticles to a smooth surface formed by lamellar, which lead to the closure of pore channels and the reduction of specific surface.

1-Pbps orbital angular momentum fibre-optic transmission

Space-division multiplexing(SDM),as a main candidate for future ultra-high capacity fibre-optic communications,needs to address limitations to its scalability imposed by computation-intensive multi-input multi-output(MIMO)digital signal processing(DSP)required to eliminate the crosstalk caused by optical coupling between multiplexed spatial channels.By exploiting the unique propagation characteristics of orbital angular momentum(OAM)modes in ring core fibres(RCFs),a system that combines SDM and C+L band dense wavelength-division multiplexing(DWDM)in a 34 km 7-core RCF is demonstrated to transport a total of 24960 channels with a raw(net)capacity of 1.223(1.02)Peta-bit s−1(Pbps)and a spectral efficiency of 156.8(130.7)bit s−1 Hz−1.Remarkably for such a high channel count,the system only uses fixed-size 4×4 MIMO DSP modules with no more than 25 time-domain taps.Such ultra-low MIMO complexity is enabled by the simultaneous weak coupling among fibre cores and amongst non-degenerate OAM mode groups within each core that have a fixed number of 4 modes.These results take the capacity of OAM-based fibre-optic communications links over the 1 Pbps milestone for the first time.They also simultaneously represent the lowest MIMO complexity and the 2nd smallest fibre cladding diameter amongst reported few-mode multicore-fibre(FM-MCF)SDM systems of>1 Pbps capacity.We believe these results represent a major step forward in SDM transmission,as they manifest the significant potentials for further up-scaling the capacity per optical fibre whilst keeping MIMO processing to an ultra-low complexity level and in a modularly expandable fashion.

All-fiber generation of arbitrary cylindrical vector beams on the first-order Poincare sphere

We propose a linear mapping relationship between the polarization of the fundamental mode and the cylindrical vector(CV)modes on the first-order Poincaresphere(FOPS)in fiber.The new method is based on the fourdimensional complex Jones matrices in terms of the linearly polarized mode bases.With our theoretical model,an all-fiber approach to generate arbitrary CV beams on the FOPS is proposed theoretically and verified experimentally.In the experiment,through the combination of a mode converter and a two-segment cascaded few-mode fiber with fixed stresses,it is possible to generate all CV modes on the FOPS by only adjusting the polarization of the fundamental mode.The Stokes parameters of the output light are measured to verify our scheme,which shows good agreement with the theoretical prediction.The method may provide a convenient way to generate CV beams and evolve the polarization states in any path on the FOPS,which is expected to have potential applications in encoding information and quantum computation.