Water quality and health risk assessment of the water bodies in the Yamdrok-tso basin, southern Tibetan Plateau.

Water resources in good quality guarantee the primary condition for the maintenance and development of the natural ecosystem and human society. Water quality status and health risk of the lake water bodies in the national nature reserve, the Yamdrok-tso basin, in the southern Tibetan Plateau are assessed by 25 water parameters including 12 heavy metal(loid)s. Results reveal that the lake water bodies possess relatively high pH (9.68), high concentrations of F (1.66 mg/L), Cu (13.92 µg/L), As (41.60 µg/L), Pb (26.69 µg/L), and U (19.53 µg/L), and a low value of dissolved oxygen (19.30%). The pollution indices (heavy metal pollution index of 0.88-22.88, heavy metal evaluation index of 0.18-3.75, and the degree of contamination of -8.82 to -5.25) demonstrate that the lake water bodies are in a low pollution level with respect to heavy metal(loid)s. The evaluation of water quality based on the fuzzy comprehensive assessment method suggests that 75.56% of the water samples meet the regulation of the China National Standard for water resources in national nature reserves. Health risk assessment shows that potential hazards exist on this region when the residents under long-term exposure to the lake water through oral and dermal pathways, of which children and adults are mostly exposed to As and F for non-carcinogenic and As for carcinogenic risks, especially for children. Results of this study contribute to targeted water resources management in the national nature reserves.

Integrated high responsivity photodetectors based on graphene/glass hybrid waveguide.

We propose and fabricate an integrated graphene/glass hybrid photodetector (PD) with high responsivity and broad spectral bandwidth. The glass straight waveguide enables high absorption of the evanescent light of transverse magnetic (TM) mode propagating parallel to the single layer of graphene. It is based on the mechanism of light-induced change in conductance. As a result, a responsivity as high as 0.72 A/W at a low bias voltage of -0.1 V for a wide wavelength range from 1510 to 1630 nm is experimentally obtained. The proposed graphene/glass hybrid PD could find important applications in graphene-based photonic integrated circuits.

Modified rod-in-tube for high-NA tellurite glass fiber fabrication: materials and technologies.

In this paper, we report the whole fabrication process for high-numerical aperture (NA) tellurite glass fibers from material preparation to preform fabrication, and eventually, fiber drawing. A tellurite-based high-NA (0.9) magneto-optical glass fiber was drawn successfully and characterized. First, matchable core and cladding glasses were fabricated and matched in terms of physical properties. Second, a uniform bubble-free preform was fabricated by means of a modified rod-in-tube technique. Finally, the fiber drawing process was studied and optimized. The high-NA fibers (∅(core), 40-50 µm and ∅(cladding), 120-130 µm) so obtained were characterized for their geometrical and optical properties.

Trade-off between optical modulation amplitude and modulation bandwidth of silicon micro-ring modulators.

An analytic model is developed to study the dynamic response of carrier-depletion silicon ring modulators. Its validity is confirmed by a detailed comparison between the modeled and the measured small signal frequency response of a practical device. The model is used to investigate how to maximize the optical modulation amplitude (OMA) and how the OMA could be traded for the bandwidth by tuning the coupling strength and the operation wavelength. Our calculation shows that for a ring modulator with equal RC time constant and photon lifetime, if its operation wavelength shifts from the position of the maximum OMA towards the direction that is away from the resonance, the 3dB modulation bandwidth increases ~2.1 times with a penalty of 3 dB to the OMA.

Microring-based ratio-metric wavelength monitor on silicon.

An integrated dual-ring ratio-metric wavelength monitor (DR-RMWM) with ultra-high wavelength resolution and compact size is demonstrated. Two microrings are used as the edge filters and designed to achieve an "X-type" spectral response in a particular wavelength range. It is fabricated with the CMOS-compatible fabrication process on the silicon-on-insulator. The measured wavelength resolution is 5 pm in a 1.2 nm wide wavelength range. By tuning the resonant wavelength thermally, the functional wavelength range can be shifted. The DR-RMWM can find applications in wavelength monitoring systems, especially the on-chip systems.

Long-wave infrared 1 × 2 MMI based on air-gap beneath silicon rib waveguides.

The undercut long-wave infrared (LWIR) waveguide components with air-gap beneath are analyzed and fabricated on the Si-wafer with simple manufacturing process. A 1 × 2 multimode interference (MMI) splitter based on this structure is presented and measured under the 10.6 µm wavelength experimental setup. The uniformity of the MMI fabricated is 0.76 dB. The relationship among the output power, slab thickness and air-gap width is also fully discussed. Furthermore, undercut straight waveguides based on SOI platform are fabricated for propagation loss evaluation. Ways to reduce the loss are discussed either.

Enhancement of nonreciprocal phase shift by using nanoscale air gap.

A novel magneto-optical (MO) waveguide with a nanoscale air gap (NAG) is presented. By using an NAG, the nonreciprocal phase shift (NPS) of the MO waveguide can be enhanced. A linear transition model is proposed to theoretically explain the NPS of the MO waveguide with the NAG. Simulation result shows that when the NAG thickness is less than 10 nm, the MO waveguide exhibits an NPS more than four times that of the MO waveguide without the NAG.

Fano-resonance-based Mach-Zehnder optical switch employing dual-bus coupled ring resonator as two-beam interferometer.

A kind of Mach-Zehnder optical switch with a dual-bus coupled ring resonator as a two-beam interferometer is proposed and investigated. The analysis based on the transfer matrix method shows that a sharp asymmetric Fano line shape can be generated in the transmission spectra of such a configuration, which can be used to significantly reduce the phase change required for switching. Meanwhile, it can also be found that complete extinctions can be achieved in both switching states if the structural parameters are carefully chosen and the phase bias is properly set. Through tuning the phase difference between the arms of the Mach-Zehnder interferometer, complete extinction can be easily kept within a large range of the ring-bus coupling ratios in the OFF state. By properly modulating the phase change in the ring waveguide, the shift of the resonant frequency and the asymmetry of the transmission spectra can be controlled to finally enable optical switching with a high extinction ratio, even complete extinction, in the ON state. The switching functionality is verified by the finite-difference time-domain simulation.

Novel dispersive and focusing device configuration based on curved waveguide grating (CWG).

Configuration of a novel compact dispersive and focusing device based on a Curved Waveguide Grating (CWG) is presented, which is essentially an integrated optic wavelength demultiplexer consisting of a curved stripe waveguide with tilted grating, a slab waveguide adjacent to it, and a set of output waveguides locate on focal line of the curved waveguide. Underlying wavelength demultiplexing mechanism of CWG is theoretically illustrated by employing the Fourier optics approach. Analysis shows that device based on CWG possesses fine wavelength resolution, compact configuration, and potentially low cost as well, which make it a promising wavelength demultiplexer, or a network performance monitor, in DWDM optical networks.