Tunable Single-, Dual- and Multi-wavelength Fibre Lasers by Using Twin Core Fibre-based Filters

Tunable fibre lasers draw intensive attention because their emission wavelength can be systematically tuned within a certain spectral range, which allows using a single laser source instead of several sources. This is convenient and cost-effective for many applications in a range of fields, such as telecom, material processing, microscopy, medicine and imaging and so on. The laser wavelength can be tuned in a certain range of wavelength by inserting wavelength-selective elements into the laser’s optical cavity. This chapter describes the twin core fibre (TCF)-based filters, which work as the wavelength-selective element. They are introduced into the ring cavity to implement tunable single-, dual- and multi-wavelength fibre lasers. First, we deduced the coupled-mode theory of TCF-based filter. Second, we experimentally and numerically characterized the optical properties of TCF-based filters including free spectral range, polarization dependence, strain effect and bending effect. Finally, we investigated three tunable fibre lasers which operate at single-, dual- and multi-wavelengths, respectively. The operation mechanism of the fibre lasers mainly involved the elastic-optic effect, polarization hole burning effect and non-linear optical loop mirror. We emphasized the tuning mechanism and the tuning characteristics of the tunable fibre lasers

Multi-channel mode converters based on in-line fiber modal interferometer

A modal interferometer was proposed to realize multi-channel mode conversion in two mode fiber. The near-filed pattern confirmed the LP01 mode was converted into LP11 mode at the destructive wavelengths. The mode conversion was realized at 20-channels in the C+L wavelength band with conversion efficiency up to 99.5% and insertion loss lower than 0.6 dB

Label-free glucose biosensor based on enzymatic graphene oxide-functionalized tilted fiber grating

A label-free biosensor based on graphene oxide (GO) and glucose oxidase (GOD) functionalized tilted fiber grating (TFG) with large tilted angle is proposed for low concentration glucose detection. Taking advantages of sufficient binding sites of the GO with oxygen-containing groups, the enzymes (GOD) are covalently immobilized onto GO-deposited TFG via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxyl succinimide cross-liner. Surface characterizations with optical microscopy, scanning electron microscopy, Raman and infrared spectroscopy provide detailed assessments and evidences about the homogeneity of GO deposition and the effectiveness of enzyme modification. Through the specific catalysis reaction of GOD on the glucose, a considerable refractive index change in local microenvironment around the TFG results in the resonant wavelength shifts of cladding modes. The detection results of the low-concentration glucose demonstrate that the resonant wavelength has a linear response to the glucose concentration in the range of 0–8 mM with a response coefficient of ∼0.24 nm/mM, showing an enhanced sensitivity and bio-selectivity compared with the pristine TFG. The miniaturized size and remote label-free sensing capacity of the proposed device permit a multitude of opportunities for single-point measurement in harsh conditions and hard-to-reach spaces, presenting a promising candidate for label-free glucose detection for disease diagnosis, pharmaceutical research and bioengineering applications

Graphene-induced unique polarization tuning properties of excessively tilted fiber grating

By exploiting the polarization-sensitive coupling effect of graphene with the optical mode, we investigate the polarization modulation properties of a hybrid waveguide of graphene-integrated excessively tilted fiber grating (Ex-TFG). The theoretical analysis and experimental results demonstrate that the real and imaginary parts of complex refractive index of fewlayer graphene exhibit different effects on transverse electric (TE) and transverse magnetic (TM) cladding modes of the Ex-TFG, enabling stronger absorption in the TE mode and more wavelength shift in the TM mode. Furthermore, the surrounding refractive index can modulate the complex optical constant of graphene and then the polarization properties of the hybrid waveguide, such as resonant wavelength and peak intensity. Therefore, the unique polarization tuning property induced by the integration of the graphene layer with Ex-TFG may endow potential applications in all-in-one fiber modulators, fiber lasers, and biochemical sensors

Theoretical and experimental analysis of excessively tilted fiber gratings

We have theoretically and experimentally investigated the dual-peak feature of tilted fiber gratings with excessively tilted structure (named as Ex-TFGs). We have explained the dual-peak feature by solving eigenvalue equations for TM0m and TE0m of a circular waveguide, in which the TE (transverse electric) and TM (transverse magnetic) core modes are coupled into TE and TM cladding modes, respectively. Meanwhile, in the experiment, we have verified that one of the dual peaks at the shorter wavelength is due to the TM mode coupling whereas the other one at the longer wavelength arises from TE mode coupling when a linearly polarized light launched into the Ex-TFG. We have also investigated the peak separation of TE and TM cladding mode for different surrounding medium refractive indexes (SRI), revealed that the dual peaks separation is decreasing as increasing of SRI, which agrees very well with the theoretical analysis results

Comprehensive Characteristics of Summer Deep Convection over Tibetan Plateau and Its South Slope from the Global Precipitation Measurement Core Observatory

Observational data from the Global Precipitation Measurement (GPM) Core Observatory during four summers (2014–2017) has been used to investigate deep convection systems (DCSs) over the Tibetan Plateau (TP) and its south slope (SS). The frequency, geographical distribution diurnal variation, and vertical structure of DCSs over the TP and SS are compared among these two regions. The frequency of DCSs over the SS (0.98%) was far higher than over the TP (0.15%), suggesting that stronger DCSs occur to the east and south of the TP. The maximum number of DCS occurred in July and August. A clear diurnal variation in DCS was found over the whole region, DCSs over the TP and SS both have a greatest amplitude in the afternoon. The probability of DCSs from 1200 to 1800 local time (LT) was 76.3% and 44.1% over TP and SS respectively, whereas the probability of DCSs being generated from 2200 (LT) to 0600 on the next day LT was 0.03% and 33.1% over the TP and SS respectively. There was a very low frequency of DCSs over the TP during the night. Five special echo top heights were used to investigate the vertical structure of DCSs. DCSs over the TP were both weaker and smaller than those over the SS