A fluorometric optical fiber nanoprobe for copper(II) by using AgInZnS quantum dots.

An optical fiber nanoprobe is presented for fluorometric determination of copper(II). The method based on the use of water-dispersible AgInZnS quantum dots (QDs) deposited at the end of an optical fiber in a poly(vinyl alcohol) matrix. The fluorescnece of the QDs, best measured at excitation/emisssion wavelengths of 365/570 nm, is quenched by Cu(II) due to both static and electron transfer from the QDs to Cu(II). This is experimentally confirmed by photoluminescence and UV-vis absorption spectra, and measurement of luminescence lifetimes. The probe is highly selective and possesses a linear detection range that extends from 2.5 to 800 nM. Graphical abstractSchematic representation of an optical fiber nanoprobe based on hydrophilic AgInZnS quantum dots for fluorometric determination of copper(II). The fluorescence is quenched by Cu(II) due to static quenching and dynamic quenching. It has a detection range of 2.5-800 nM.

Distributed vibration measurement based on a coherent multi-slope-assisted BOTDA with a large dynamic range.

We propose a novel technique to enhance the dynamic range of a coherent slope-assisted Brillouin optical time domain analysis. A multi-tone probe and a reference wave are launched into the fiber under test (FUT); after interacting with the pump pulse, the Brillouin gain, as well as the Brillouin phase shift of each tone, can be demodulated simultaneously. In light of this, the strain information can be determined by the Brillouin phase-gain ratio of each tone. In the experiment, a three-tone probe with a 60 MHz interval is used; effective measurement frequency span larger than 180 MHz is verified in a ∼2 km single-mode fiber with 2.5 m spatial resolution and 1.5 kHz sampling rate to strain. A vibration signal with 41 Hz frequency and 2546 µÎµ amplitude is successfully demodulated.

Nitrogen doped graphene quantum dots as a fluorescent probe for mercury(II) ions.

A highly selective fluorescent probe for Hg2+ is reported. It consists of nitrogen doped graphene quantum dots (NGQDs) that are nearly spherical in shape, have an average diameter of 2.7 nm and excitation-independent emission. The blue fluorescence of the NGQDs (with maximum excitation/emission at 378/447 nm) is quenched by Hg2+ due to both dynamic and static quenching. The probe has a wide detection range (2.5 µM - 800 µM) and a limit of detection of 2.5 µM. The dynamic and static quenching constants are 417 M-1 and 63500 M-1, respectively. The probe was used to quantfy Hg2+ in spiked real water samples with satisfactory results. Graphical abstract ᅟ.

Polarization independent fast BOTDA based on pump frequency modulation and cyclic coding.

We propose and experimentally demonstrate a scheme of polarization independent fast Brillouin optical time domain analysis (F-BOTDA) based on pump frequency modulation and cyclic coding. The Brillouin gain spectrum (BGS) is reconstructed by fast scanning frequency of the pump using an arbitrary waveform generator (AWG). To realize long range distributed dynamic strain sensing, polarization diversity technique and cyclic coding are employed to eliminate polarization fading and enhance the signal-to-noise ratio (SNR). Based on this configuration, the need of trace averaging is avoided, sensing speed of 440 Hz is achieved over ~2 km single mode fiber with 50 scanning frequencies and a spatial resolution of 1.5 m. Vibration events up to 40 Hz are successfully identified.

Distributed fiber sparse-wideband vibration sensing by sub-Nyquist additive random sampling.

The round-trip time of the light pulse limits the maximum detectable vibration frequency response range of phase-sensitive optical time domain reflectometry (ϕ-OTDR). Unlike the uniform laser pulse interval in conventional ϕ-OTDR, we randomly modulate the pulse interval so that an equivalent sub-Nyquist additive random sampling (sNARS) is realized for every sensing point of the long interrogation fiber. For a ϕ-OTDR system with 10 km sensing length, the sNARS method is optimized by theoretical analysis and Monte Carlo simulation, and the experimental results verify that a wideband sparse signal can be identified and reconstructed. Such a method can broaden the vibration frequency response range of ϕ-OTDR, which is of great significance in sparse-wideband-frequency vibration signal detection.

Tens of hertz narrow-linewidth laser based on stimulated Brillouin and Rayleigh scattering.

We proposed and demonstrated a linewidth compression method of a laser based on stimulated Brillouin scattering (SBS) and a Rayleigh backscattering structure (RBS). The relationship between the output SBS laser linewidth and the input pump linewidth was studied theoretically and experimentally. It is shown that the narrower linewidth of the pump laser leads to the narrower bandwidth of the SBS gain and, finally, the bandwidth of the SBS will tend to its intrinsic value as the linewidth of a pump laser narrower than 10 kHz; then the linewidth of an SBS fiber ring laser would tend to 200 Hz. In order to further reduce its linewidth with low cost, RBS and a simple dual-cavity feedback structure were added and, finally, a ∼75 Hz narrow-linewidth laser with a side-mode suppression ratio of 70 dB was obtained.